Nuclear power (updated)

From Wikipedia, the free encyclopedia

The 1200 MWe Leibstadt Nuclear Power Plant in Switzerland. The boiling water reactor (BWR), located inside the dome capped cylindrical structure, is dwarfed in size by its cooling tower. The station produces a yearly average of 25 million kilowatt-hours per day, sufficient to power a city the size of Boston.

 

The Palo Verde Nuclear Generating Station, the largest in the United States with 3 pressurized water reactors (PWRs), is situated in the Arizona desert. It uses sewage from cities as its cooling water in 9 squat mechanical draft cooling towers. Its total spent fuel inventory, produced since 1986, is contained in dry cask storage cylinders located between the artificial body of water and the electrical switchyard.

 

U.S. nuclear powered ships: (top to bottom) cruisers USS Bainbridge, USS Long Beach, and USS Enterprise, the first nuclear-powered aircraft carrier. Picture taken in 1964 during a record setting voyage of 26,540 nmi (49,152 km) around the world in 65 days without refueling. Crew members are spelling out Einstein's mass-energy equivalence formula E = mc² on the flight deck.

 

Nuclear power is the use of nuclear reactions that release nuclear energy to generate heat, which most frequently is then used in steam turbines to produce electricity in a nuclear power plant. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion. Presently, the vast majority of electricity from nuclear power is produced by nuclear fission of uranium and plutonium. Nuclear decay processes are used in niche applications such as radioisotope thermoelectric generators. The possibility of generating electricity from nuclear fusion is still at a research phase with no commercial applications. This article mostly deals with nuclear fission power for electricity generation. 

Nuclear power is one of the leading low carbon power generation methods of producing electricity. In terms of total life-cycle greenhouse gas emissions per unit of energy generated, nuclear power has emission values comparable or lower than renewable energy. From the beginning of its commercialization in the 1970s, nuclear power prevented about 1.84 million air pollution-related deaths and the emission of about 64 billion tonnes of carbon dioxide equivalent that would have otherwise resulted from the burning of fossil fuels in thermal power stations.

Civilian nuclear power supplied 2,488 terawatt hours (TWh) of electricity in 2017, equivalent to about 10% of global electricity generation. As of April 2018, there are 449 civilian fission reactors in the world, with a combined electrical capacity of 394 gigawatt (GW). Additionally, there are 58 reactors under construction and 154 reactors planned, with a combined capacity of 63 GW and 157 GW, respectively. Over 300 more reactors are proposed. Most of reactors under construction are of generation III reactor design, with the majority in Asia.

There is a social debate about nuclear power. Proponents, such as the World Nuclear Association and Environmentalists for Nuclear Energy, contend that nuclear power is a safe, sustainable energy source that reduces carbon emissions. Opponents, such as Greenpeace and NIRS, contend that nuclear power poses many threats to people and the environment. 

Some accidents have occurred in nuclear power plants. These include the Chernobyl disaster in the Soviet Union in 1986, the Fukushima Daiichi nuclear disaster in Japan in 2011, and the more contained Three Mile Island accident in the United States in 1979. There have also been some nuclear submarine accidents. In terms of lives lost per unit of energy generated, nuclear reactors have caused the lowest number of fatalities per unit of energy generated when compared to the other major energy producing methods. Coal, petroleum, natural gas and hydroelectricity each have caused a greater number of fatalities per unit of energy, due to air pollution and accidents.

Collaboration on research and development towards greater efficiency, safety and recycling of spent fuel in future Generation IV reactors presently includes Euratom and the co-operation of more than 10 permanent countries globally.

History

Origins

The Nuclear binding energy of all natural elements in the periodic table. Higher values translate into more tightly bound nuclei and greater nuclear stability. Iron (Fe) is the end product of nucleosynthesis within the core of hydrogen fusing stars. The elements surrounding iron are the fission products of the fissionable actinides (e.g. uranium). Except for iron, all other elemental nuclei have in theory the potential to be nuclear fuel, and the greater distance from iron the greater nuclear potential energy that could be released.

 

In 1932 physicist Ernest Rutherford discovered that when lithium atoms were "split" by protons from a proton accelerator, immense amounts of energy were released in accordance with the principle of mass–energy equivalence. However, he and other nuclear physics pioneers Niels Bohr and Albert Einstein believed harnessing the power of the atom for practical purposes anytime in the near future was unlikely, with Rutherford labeling such expectations "moonshine."

The same year, his doctoral student James Chadwick discovered the neutron, which was immediately recognized as a potential tool for nuclear experimentation because of its lack of an electric charge. Experimentation with bombardment of materials with neutrons led Frédéric and Irène Joliot-Curie to discover induced radioactivity in 1934, which allowed the creation of radium-like elements at much less the price of natural radium. Further work by Enrico Fermi in the 1930s focused on using slow neutrons to increase the effectiveness of induced radioactivity. Experiments bombarding uranium with neutrons led Fermi to believe he had created a new, transuranic element, which was dubbed hesperium.

In 1938, German chemists Otto Hahn and Fritz Strassmann, along with Austrian physicist Lise Meitner and Meitner's nephew, Otto Robert Frisch, conducted experiments with the products of neutron-bombarded uranium, as a means of further investigating Fermi's claims. They determined that the relatively tiny neutron split the nucleus of the massive uranium atoms into two roughly equal pieces, contradicting Fermi. This was an extremely surprising result: all other forms of nuclear decay involved only small changes to the mass of the nucleus, whereas this process—dubbed "fission" as a reference to biology—involved a complete rupture of the nucleus. Numerous scientists, including Leó Szilárd, who was one of the first, recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. Once this was experimentally confirmed and announced by Frédéric Joliot-Curie in 1939, scientists in many countries (including the United States, the United Kingdom, France, Germany, and the Soviet Union) petitioned their governments for support of nuclear fission research, just on the cusp of World War II, for the development of a nuclear weapon.

First nuclear reactor

In the United States, where Fermi and Szilárd had both emigrated, the discovery of the nuclear chain reaction led to the creation of the first man-made reactor, known as Chicago Pile-1, which achieved criticality on December 2, 1942. This work became part of the Manhattan Project, a massive secret U.S. government military project to make enriched uranium and by building large production reactors to produce (breed) plutonium for use in the first nuclear weapons. The United States would test an atom bomb in July 1945 with the Trinity test, and eventually two such weapons were used in the atomic bombings of Hiroshima and Nagasaki. 

The first light bulbs ever lit by electricity generated by nuclear power at EBR-1 at Argonne National Laboratory-West, December 20, 1951.

 

In August 1945, the first widely distributed account of nuclear energy, in the form of the pocketbook The Atomic Age, discussed the peaceful future uses of nuclear energy and depicted a future where fossil fuels would go unused. Nobel laureate Glenn Seaborg, who later chaired the Atomic Energy Commission, is quoted as saying "there will be nuclear powered earth-to-moon shuttles, nuclear powered artificial hearts, plutonium heated swimming pools for SCUBA divers, and much more".

In the same month, with the end of the war, Seaborg and others would file hundreds of initially classifed patents, most notably Eugene Wigner and Alvin Weinberg's Patent #2,736,696, on a conceptual light water reactor (LWR) that would later become the United States' primary reactor for naval propulsion and later take up the greatest share of the commercial fission-electric landscape.

The United Kingdom, Canada, and the USSR proceeded to research and develop nuclear energy over the course of the late 1940s and early 1950s. Electricity was generated for the first time by a nuclear reactor on December 20, 1951, at the EBR-I experimental station near Arco, Idaho, which initially produced about 100 kW.

In 1953, American President Dwight Eisenhower gave his "Atoms for Peace" speech at the United Nations, emphasizing the need to develop "peaceful" uses of nuclear power quickly. This was followed by the 1954 Amendments to the Atomic Energy Act which allowed rapid declassification of U.S. reactor technology and encouraged development by the private sector.

Early years

The launching ceremony of the USS Nautilus January 1954

The first organization to develop nuclear power was the U.S. Navy, with the S1W reactor for the purpose of propelling submarines and aircraft carriers. The first nuclear-powered submarine, USS Nautilus, was put to sea in January 1954. The trajectory of civil reactor design was heavily influenced by Admiral Hyman G. Rickover, who with Weinberg as a close advisor, selected the PWR/Pressurized Water Reactor design, in the form of a 10 MW reactor for the Nautilus, a decision that would result in the PWR receiving a government commitment to develop, an engineering lead that would result in a lasting impact on the civilian electricity market in the years to come. The United States Navy Nuclear Propulsion design and operation community, under Rickover's style of attentive management retains a continuing record of zero reactor accidents (defined as the uncontrolled release of fission products to the environment resulting from damage to a reactor core). with the U.S. Navy fleet of nuclear-powered ships, standing at some 80 vessels as of 2018.

On June 27, 1954, the USSR's Obninsk Nuclear Power Plant, based on what would become the prototype of the RBMK reactor design, became the world's first nuclear power plant to generate electricity for a power grid, producing around 5 megawatts of electric power.

On July 17, 1955 the BORAX III reactor, the prototype to later Boiling Water Reactors, became the first to generate electricity for an entire community, the town of Arco, Idaho. A motion picture record of the demonstration, of supplying some 2 megawatts(2 MW) of electricity, was presented to the United Nations, Where at the "First Geneva Conference", the world's largest gathering of scientists and engineers, met to explore the technology in that year. In 1957 EURATOM was launched alongside the European Economic Community (the latter is now the European Union). The same year also saw the launch of the International Atomic Energy Agency (IAEA). 

The Calder Hall nuclear power station in the United Kingdom was the world's first commercial nuclear power station. It was connected to the national power grid on 27 August 1956 and officially revealed in a ceremony by Queen Elizabeth II on 17 October 1956. In common with a number of other Generation I nuclear reactors, the plant had the dual purpose of producing electrical power and plutonium-239, the latter for the nascent nuclear weapons program in Britain.

 

The 60 MWe Shippingport Atomic Power Station in Shippingport, Pennsylvania, opened in 1957, originating from a cancelled nuclear-powered aircraft carrier contract the Pressurized water reactor design became the first commercial reactor in the United States, its early adoption, a case of technological lock-in, and familiarity amongst retired naval personnel, established the PWR as the predominant civilian reactor design, that it still retains today in the US.

 

The world's first "commercial nuclear power station", Calder Hall at Windscale, England, was opened in 1956 with an initial capacity of 50 MW (later 200 MW), it was the first of a fleet of dual-purpose MAGNOX reactors, though officially code-named PIPPA(Pressurized Pile Producing Power and Plutonium) by the UKAEA to denote the plant's dual commercial and military role.

The U.S. Army, nuclear power program, formally commenced in 1954. Under its management, the 2 megawatt SM-1, at Fort Belvoir, Virginia, was the first in the United States to supply electricity in an industrial capacity to the commercial grid (VEPCO), in April 1957.

The first commercial nuclear station to become operational in the United States was the 60 MW Shippingport Reactor (Pennsylvania, in December 1957.

The 3 MW SL-1 was a U.S. Army experimental nuclear power reactor at the National Reactor Testing Station in eastern Idaho, derived from the Borax Boiling water reactor(BWR) design, it first achieving operational criticality/connection to the grid in 1958. In 1961 During a manual removal of a control rod, it underwent a steam explosion and meltdown, which killed its three operators. The event was eventually rated at 4 on the seven-level INES scale. 

The Soviet submarine K-27's liquid metal cooled reactor accident in 1968 resulted in 9 fatalities and 83 other injuries.

Development and early opposition to nuclear power

Number of generating and under construction civilian fission-electric reactors, over the period 1960 to 2015.

 

The total global installed nuclear capacity initially rose relatively quickly, rising from less than 1 gigawatt (GW) in 1960 to 100 GW in the late 1970s, and 300 GW in the late 1980s. Since the late 1980s worldwide capacity has risen much more slowly, reaching 366 GW in 2005. Between around 1970 and 1990, more than 50 GW of capacity was under construction (peaking at over 150 GW in the late 1970s and early 1980s) — in 2005, around 25 GW of new capacity was planned. More than two-thirds of all nuclear plants ordered after January 1970 were eventually cancelled. A total of 63 nuclear units were canceled in the United States between 1975 and 1980.

In 1972 Alvin Weinberg, co-inventor of the light water reactor design (the most common nuclear reactors today) was fired from his job at Oak Ridge National Laboratory by the Nixon administration, "at least in part" over his raising of concerns about the safety and wisdom of ever larger scaling-up of his design, above the power rating of ~ 500 MWe, in a loss of coolant accident-scenario, the particular combination of compact solid-fuel cores and the decay heat generated from ~ 500 MW is beyond the capabilities of passive/natural convection cooling to prevent a rapid fuel rod melt-down and resulting in then far reaching fission product pluming. While considering the LWR, well suited at sea for the submarine and naval fleet, Weinberg did not show complete support for its use by utilities on land at the power output that they were interested in for supply scale reasons, and would request for the development and funding of his team's molten salt reactor, a design with greater inherent safety in this scenario and with that an envisioned greater economic growth potential in the market of large-scale civilian electricity generation.

In 1976 Idaho National Laboratory began a test program focused on LWR reactors under various accident scenarios, with the aim of understanding the event progression and mitigating steps necessary to respond to a failure of one or more of the disparate systems, with much of the back-up safety equipment and nuclear regulations arising from these series of investigations, heavily influencing the design of LWRs for the civilian market.

During the 1970s and 1980s rising economic costs (related to extended construction times largely due to regulatory changes and pressure-group litigation) and falling fossil fuel prices made nuclear power plants then under construction less attractive. In the 1980s in the U.S. and 1990s in Europe, the flat electric grid growth and electricity liberalization also made the addition of large new baseload energy generators economically unattractive. 

Electricity production in France has been dominated by fission-electric stations since the early 1980s with a large portion of the electricity exported to neighboring Germany etc. today.

 

  thermofossil

  hydroelectric

  nuclear

  Other renewables

The 1973 oil crisis had a significant effect on countries, such as France and Japan, which had relied more heavily on oil for electric generation (39% and 73% respectively) to invest in nuclear power. The french plan, known as the Messmer plan, was for the complete independence from oil, with an envisaged construction of 80 reactors by 1985 and 170 by 2000. France would construct 25 fission-electric stations, installing 56 mostly PWR design reactors over the next 15 years, though foregoing the 100 reactors initially charted in 1973, for the 1990s. As of 2018 over 70% of french electricity, is generated from these 56 reactors, the highest percentage by any nation in the world.

Some local opposition to nuclear power emerged in the early 1960s, and in the late 1960s some members of the scientific community began to express their concerns. These anti-nuclear concerns related to nuclear accidents, nuclear proliferation, nuclear terrorism and radioactive waste disposal. In the early 1970s, there were large protests about a proposed nuclear power plant in Wyhl, Germany. The project was cancelled in 1975 the anti-nuclear success at Wyhl inspired opposition to nuclear power in other parts of Europe and North America. By the mid-1970s anti-nuclear activism gained a wider appeal and influence, and nuclear power began to become an issue of major public protest. In some countries, the nuclear power conflict "reached an intensity unprecedented in the history of technology controversies". In May 1979, an estimated 70,000 people, including then governor of California Jerry Brown, attended a march against nuclear power in Washington, D.C. Anti-nuclear power groups emerged in every country that has had a nuclear power program.

Three Mile Island and Chernobyl

The abandoned town of Pripyat with the Chernobyl plant and the Chernobyl New Safe Confinement arch in the distance, prior to it moving into place and retaining the hazardous dust generated during the disassembly process, on site.

According to some commentators, the 1979 accident at Three Mile Island, and the 1986 Chernobyl disaster played a major part in the reduction in the number of new plant constructions in many countries, although the public policy organization, the Brookings Institution states that new nuclear units, at the time of publishing in 2006, had not been built in the United States because of soft demand for electricity, cost overruns on nuclear plants due to regulatory issues and construction delays. By the end of the 1970s it became clear that nuclear power would not grow nearly as dramatically as once believed. Eventually, more than 120 reactor orders in the United States were ultimately cancelled and the construction of new reactors ground to a halt. A cover story in the February 11, 1985, issue of Forbes magazine commented on the overall failure of the U.S. nuclear power program, saying it "ranks as the largest managerial disaster in business history".

In 1982, occurring amongst a backdrop of ongoing protests directed at the construction of the first commercial scale breeder reactor in France, a later member of the Swiss Green Party fired five RPG-7 rocket-propelled grenades at the still under construction containment building of the Superphenix reactor. While 2 warheads hit and caused minor damage to the reinforced concrete outer shell. It was the first time protests reached such heights. After examination of the damage, the prototype fast breeder reactor started and operated for over a decade.

A simplified diagram of the major differences between the most common nuclear reactor design, the Light water reactor and the Chernobyl RBMK design. 1. In "red", the use of a graphite moderator in a water cooled reactor. 2. A positive steam void coefficient that made the power excursion possible, which blew the reactor vessel. 3. The control rods were very slow, taking 18–20 seconds to be deployed. With the control rods having graphite tips that moderated and therefore increased the fission rate in the beginning of the rod insertion. 4. No reinforced containment building.

 

Unlike the Three Mile Island accident, the much more serious Chernobyl accident did not increase regulations affecting Western reactors since the Chernobyl reactors were of the problematic RBMK design only used in the Soviet Union, for example lacking "robust" containment buildings. Over 10 RBMK reactors are still in use today. However, changes were made in both the reactors themselves (use of a safer enrichment of uranium) and in the control system (prevention of disabling safety systems), amongst other things, to reduce the possibility of a duplicate accident.

An international organization to promote safety awareness and professional development on operators in nuclear facilities, the World Association of Nuclear Operators (WANO), was created as a direct outcome of the 1986 Chernobyl accident. The organization was created with the intent to share and grow the adoption of nuclear safety technology, where before there was an atmosphere of cold war secrecy. 

Numerous countries, including Austria (1978), Sweden (1980) and Italy (1987) (influenced by Chernobyl) have voted in referendums to oppose or phase out nuclear power.

Nuclear renaissance

Olkiluoto 3 under construction in 2009. It was the first EPR, a modernized PWR design with a core catcher, to start construction but problems with workmanship and supervision have created costly delays which led to an inquiry by the Finnish nuclear regulator STUK.^[77] Allowing the later started Taishan Nuclear EPR project to reach first connection to a national grid, in 2018. In December 2012, Areva estimated that the full cost of Olkiluoto will be about €8.5 billion, or almost three times the original delivery price of €3 billion and it will not be connected to the grid until the 2020s.

Since about 2001 the term nuclear renaissance has been used to refer to a possible nuclear power industry revival, driven by rising fossil fuel prices and new concerns about meeting greenhouse gas emission limits. With the energy source, as of 2010, an established alternative, providing two thirds(2/3) of the twenty seven nation European Union's low-carbon electricity. However since commercial nuclear energy began in the mid-1950s, 2008 was the first year that no new nuclear power plant was connected to the grid, although two were connected in 2009. In 2009, Petteri Tiippana, the director of STUK's nuclear power plant division, told the BBC that it was difficult to deliver a Generation III reactor project on schedule because builders were not used to working to the exacting standards required on nuclear construction sites, since so few new reactors had been built in recent years.

In 2018 the MIT Energy Initiative study on the future of nuclear energy concluded that together with the strong suggestion that government should financially support development and demonstration of new Generation IV nuclear technologies, for a world wide renaissance to commence, a global standardization of regulations needs to take place, with a move towards serial manufacturing of standardized units akin to the other complex engineering field of aircraft and aviation. At present it is common for each country to demand bespoke changes to the design to satisfy varying national regulatory bodies, often to the benefit of domestic engineering supply firms. The report goes on to note that the most cost-effective projects have been built with multiple (up to six) reactors per site using a standardized design, with the same component suppliers and construction crews working on each unit, in a continuous work flow.

Fukushima Daiichi Nuclear Disaster

Following the Tōhoku earthquake on 11 March 2011, one of the largest earthquakes ever recorded, and a subsequent tsunami off the coast of Japan, the Fukushima Daiichi Nuclear Power Plant suffered multiple core meltdowns due to failure of the emergency cooling system for lack of electricity supply. This resulted in the most serious nuclear accident since the Chernobyl disaster. 

The Fukushima Daiichi nuclear accident prompted a re-examination of nuclear safety and nuclear energy policy in many countries and raised questions among some commentators over the future of the renaissance. Germany approved plans to close all its reactors by 2022, Italian nuclear energy plans in 2009, ended when Italy banned the generation, but not consumption of, nuclear electricity in a June 2011 referendum. China, Switzerland, Israel, Malaysia, Thailand, United Kingdom, and the Philippines reviewed their nuclear power programs.

In 2011 the International Energy Agency halved its prior estimate of new generating capacity to be built by 2035. Nuclear power generation had the biggest ever fall year-on-year in 2012, with nuclear power plants globally producing 2,346 TWh of electricity, a drop of 7% from 2011. This was caused primarily by the majority of Japanese reactors remaining offline that year and the permanent closure of eight reactors in Germany.

Post-Fukushima

The Fukushima Daiichi nuclear accident sparked controversy about the importance of the accident and its effect on nuclear's future. The crisis prompted countries with nuclear power to review the safety of their reactor fleet and reconsider the speed and scale of planned nuclear expansions. In 2011, The Economist opined that nuclear power "looks dangerous, unpopular, expensive and risky", and that "it is replaceable with relative ease and could be forgone with no huge structural shifts in the way the world works". Earth Institute Director Jeffrey Sachs disagreed, claiming combating climate change would require an expansion of nuclear power. Investment banks were also critical of nuclear soon after the accident.

In September 2011, German engineering giant Siemens announced it will withdraw entirely from the nuclear industry as a response to the Fukushima accident.

By contrast, the Associated Press and Reuters reported in 2011 the suggestion that the safety and survival of the younger Onagawa Nuclear Power Plant, the closest reactor facility to the epicenter and on the coast, demonstrate that it is possible for nuclear facilities to withstand the greatest natural disasters. The Onagawa plant was also said to show that nuclear power can retain public trust, with the surviving residents of the town of Onagawa taking refuge in the gymnasium of the nuclear facility following the destruction of their town.

Following an IAEA inspection in 2012, the agency stated that "The structural elements of the [Onagawa] NPS (nuclear power station) were remarkably undamaged given the magnitude of ground motion experienced and the duration and size of this great earthquake,”.

In February 2012, the United States Nuclear Regulatory Commission approved the construction of two additional reactors at the Vogtle Electric Generating Plant, the first reactors to be approved in over 30 years since the Three Mile Island accident. In October 2016, Watts Bar 2 became the first new United States reactor to enter commercial operation since 1996.

In 2013 Japan signed a deal worth $22 billion, in which Mitsubishi Heavy Industries would build four modern Atmea reactors for Turkey. In August 2015, following 4 years of near zero fission-electricity generation, Japan began restarting its nuclear reactors, after safety upgrades were completed, beginning with Sendai Nuclear Power Plant.

By 2015, the IAEA's outlook for nuclear energy had become more promising. "Nuclear power is a critical element in limiting greenhouse gas emissions," the agency noted, and "the prospects for nuclear energy remain positive in the medium to long term despite a negative impact in some countries in the aftermath of the [Fukushima-Daiichi] accident...it is still the second-largest source worldwide of low-carbon electricity. And the 72 reactors under construction at the start of last year were the most in 25 years." According to the World Nuclear Association, as of 2015 the global trend was for new nuclear power stations coming online to be balanced by the number of old plants being retired. Eight new grid connections were completed by China in 2015.

Future of the industry

The Hanul Nuclear Power Plant in South Korea, presently the second largest in the world by output, with six operating power reactors. Two additional indigenously designed APR-1400 generation-III reactors are under construction. Korea exported the APR design to the United Arab Emirates, were four of these reactors are under construction at Barakah nuclear power plant.

 

As of 2018, there are over 150 nuclear reactors planned including 50 under construction. However, while investment on upgrades of existing plant and life-time extensions continues, investment in new nuclear is declining, reaching a 5-year-low in 2017. 

In 2015, the International Energy Agency reported that the Fukushima accident had a strongly negative effect on nuclear power, yet nuclear power prospects are positive in the medium to long term mainly thanks to new construction in Asia. In 2016, the U.S. Energy Information Administration projected for its “base case” that world nuclear power generation would increase from 2,344 terawatt hours (TWh) in 2012 to 4,501 TWh in 2040. Most of the predicted increase was expected to be in Asia.

The future of nuclear power varies greatly between countries, depending on government policies. Some countries, many of them in Europe, such as Germany, Belgium, and Lithuania, have adopted policies of nuclear power phase-out. At the same time, some Asian countries, such as China and India, have committed to rapid expansion of nuclear power. Many other countries, such as the United Kingdom and the United States, have policies in between. Japan generated about 30% of its electricity from nuclear power before the Fukushima accident. In 2015 the Japanese government committed to the aim of restarting its fleet of 40 reactors by 2030 after safety upgrades, and to finish the construction of the Generation III Ōma Nuclear Power Plant. This would mean that approximately 20% of electricity would come from nuclear power by 2030. As of 2018, some reactors have restarted commercial operation following inspections and upgrades with new regulations. While South Korea has a large nuclear power industry, the new government in 2017, partly influenced by a large anti-nuclear movement, committed to halting nuclear development and to gradually phase out nuclear power as reactors that are now operating or under construction close after 40 years of operations.

The Generation IV roadmap. Nuclear Energy Systems Deployable no later than 2030 and offering significant advances in sustainability, safety and reliability, and economics.

The nuclear power industry in some western nations have a history of construction delays, cost overruns, plant cancellations, and nuclear safety issues, despite significant government subsidies and support. These problems are related to very strict safety requirements, uncertain regulatory environment, slow rate of construction, and large stretches of time with no nuclear construction and consequent loss of know-how. Commentators therefore argue that new nuclear is impractical in western countries because of popular opposition, regulatory uncertainty, soft demand for multiple reactor units and high costs. Recent financial problems of western nuclear companies, most prominently the bankruptcy of Westinghouse in March 2017 because of US$9 billion of losses from nuclear construction projects in the United States, suggest a shift to the East, as the dominant exporter and designer of nuclear fuel and reactors.

The greatest new build activity is occurring in Asian countries like South Korea, India and China. In March 2016, China had 30 reactors in operation, 24 under construction and plans to build more.

Blue light from Cherenkov radiation being produced near the core of the Fission powered Advanced Test Reactor. A facility taking part in the Advanced Fuel Cycle Initiative, to transmute certain actinides into fuel, that would be able to be used in commercial light water reactors, reducing a number of the security hazards of, what is all presently considered "waste".

 

In 2016 the BN-800 sodium cooled fast reactor in Russia, began commercial electricity generation, while plans for a BN-1200 were initially conceived the future of the fast reactor program in Russia awaits the results from MBIR, an under construction multi-loop Generation IV research facility for testing the chemically more inert lead, lead-bismuth and gas coolants, it will similarly run on recycled MOX (mixed uranium and plutonium oxide) fuel. An on-site pyrochemical processing, closed fuel-cycle facility, is planned, to recycle the spent fuel/"waste" and reduce the necessity for a growth in uranium mining and exploration. In 2017 the manufacture program for the reactor commenced with the facility open to collaboration under the "International Project on Innovative Nuclear Reactors and Fuel Cycle", it has a construction schedule, that includes an operational start in 2020. As planned, it will be the world's most-powerful research reactor.

In the United States, licenses of almost half of the operating nuclear reactors have been extended to 60 years. The U.S. NRC and the U.S. Department of Energy have initiated research into Light water reactor sustainability which is hoped will lead to allowing extensions of reactor licenses beyond 60 years, provided that safety can be maintained, to increase energy security and preserve low-carbon generation sources. Research into nuclear reactors that can last 100 years, known as Centurion Reactors, is being conducted.

According to the World Nuclear Association, globally during the 1980s one new nuclear reactor started up every 17 days on average, and in the year 2015 it was estimated that this rate could in theory eventually increase to one every 5 days, although no plans exist for that.

Nuclear power station

Play media

An animation of a Pressurized water reactor in operation.

Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants convert the energy released from the nucleus of an atom via nuclear fission that takes place in a nuclear reactor. When a neutron hits the nucleus of a uranium-235 or plutonium atom, it can split the nucleus into two smaller nuclei. The reaction is called nuclear fission. The fission reaction releases energy and neutrons. The released neutrons can hit other uranium or plutonium nuclei, causing new fission reactions, which release more energy and more neutrons. This is called a chain reaction. The reaction rate is controlled by control rods that absorb excess neutrons. The controllability of nuclear reactors depends on the fact that a small fraction of neutrons resulting from fission are delayed. The time delay between the fission and the release of the neutrons slows down changes in reaction rates and gives time for moving the control rods to adjust the reaction rate.

A fission nuclear power plant is generally composed of a nuclear reactor, in which the nuclear reactions generating heat take place; a cooling system, which removes the heat from inside the reactor; a steam turbine, which transforms the heat in mechanical energy; an electric generator, which transform the mechanical energy into electrical energy.

Life cycle of nuclear fuel

The nuclear fuel cycle begins when uranium is mined, enriched, and manufactured into nuclear fuel, (1) which is delivered to a nuclear power plant. After usage in the power plant, the spent fuel is delivered to a reprocessing plant (2) or to a final repository (3) for geological disposition. In reprocessing 95% of spent fuel can potentially be recycled to be returned to usage in a power plant (4).

A nuclear reactor is only part of the fuel life-cycle for nuclear power. The process starts with mining. Uranium mines are underground, open-pit, or in-situ leach mines. In any case, the uranium ore is extracted, usually converted into a stable and compact form such as yellowcake, and then transported to a processing facility. Here, the yellowcake is converted to uranium hexafluoride, which is then generally enriched using various techniques. Some reactor designs can also use natural uranium without enrichment. The enriched uranium, containing more than the natural 0.7% uranium-235, is generally used to make rods of the proper composition and geometry for the particular reactor that the fuel is destined for. In modern light-water reactors the fuel rods will spend about 3 operational cycles (typically 6 years total now) inside the reactor, generally until about 3% of their uranium has been fissioned, then they will be moved to a spent fuel pool where the short lived isotopes generated by fission can decay away. After about 5 years in a spent fuel pool the spent fuel is radioactively and thermally cool enough to handle, and it can be moved to dry storage casks or reprocessed.

Conventional fuel resources

Proportions of the isotopes uranium-238 (blue) and uranium-235 (red) found in natural uranium and in enriched uranium for different applications. Light water reactors use 3-5% enriched uranium, while CANDU reactors work with natural uranium.

Uranium is a fairly common element in the Earth's crust: it is approximately as common as tin or germanium, and is about 40 times more common than silver. Uranium is present in trace concentrations in most rocks, dirt, and ocean water, but is generally economically extracted only where it is present in high concentrations. As of 2011 the world's known resources of uranium, economically recoverable at the arbitrary price ceiling of 130 USD/kg, were enough to last for between 70 and 100 years.

The OECD's red book of 2011 said that conventional uranium resources had grown by 12.5% since 2008 due to increased exploration, with this increase translating into greater than a century of uranium available if the rate of use were to continue at the 2011 level. In 2007, the OECD estimated 670 years of economically recoverable uranium in total conventional resources and phosphate ores assuming the then-current use rate.

Light water reactors make relatively inefficient use of nuclear fuel, mostly fissioning only the very rare uranium-235 isotope. Nuclear reprocessing can make this waste reusable. Newer Generation III reactors also achieve a more efficient use of the available resources than the generation II reactors which make up the vast majority of reactors worldwide. With a pure fast reactor fuel cycle with a burn up of all the Uranium and actinides (which presently make up the most hazardous substances in nuclear waste), there is an estimated 160,000 years worth of Uranium in total conventional resources and phosphate ore at the price of 60–100 US$/kg.

Unconventional fuel resources

Unconventional uranium resources also exist. Uranium is naturally present in seawater at a concentration of about 3 micrograms per liter, with 4.5 billion tons of uranium considered present in seawater at any time. In 2012 it was estimated that this fuel source could be extracted at 10 times the current price of uranium.

In 2014, with the advances made in the efficiency of seawater uranium extraction, it was suggested that it would be economically competitive to produce fuel for light water reactors from seawater if the process was implemented at large scale. Uranium extracted on an industrial scale from seawater would constantly be replenished by both river erosion of rocks and the natural process of uranium dissolved from the surface area of the ocean floor, both of which maintain the solubility equilibria of seawater concentration at a stable level. Some commentators have argued that this strengthens the case for Nuclear power to be considered a renewable energy.

Breeding

As opposed to light water reactors which use uranium-235 (0.7% of all natural uranium), fast breeder reactors use uranium-238 (99.3% of all natural uranium). In 2006 it was estimated that with seawater extraction, there was likely some five billion years' worth of uranium-238 for use in these power plants.

Breeder technology has been used in several reactors, but the high cost of reprocessing fuel safely, at 2006 technological levels, requires uranium prices of more than US$200/kg before becoming justified economically. Breeder reactors are however being pursued as they have the potential to burn up all of the actinides in the present inventory of nuclear waste while also producing power and creating additional quantities of fuel for more reactors via the breeding process.

As of 2017, there are two breeders producing commercial power, BN-600 reactor and the BN-800 reactor, both in Russia. The BN-600, with a capacity of 600 MW, was built in 1980 in Beloyarsk and is planned to produce power until 2025. The BN-800 is an updated version of the BN-600, and started operation in 2014. The Phénix breeder reactor in France was powered down in 2009 after 36 years of operation.

A nuclear fuel rod assembly bundle being inspected before entering a reactor.

Both China and India are building breeder reactors. The Indian 500 MWe Prototype Fast Breeder Reactor is in the commissioning phase, with plans to build five more by 2020. The China Experimental Fast Reactor began operating in 2011.

Another alternative to fast breeders is thermal breeder reactors that use uranium-233 bred from thorium as fission fuel in the thorium fuel cycle. Thorium is about 3.5 times more common than uranium in the Earth's crust, and has different geographic characteristics. This would extend the total practical fissionable resource base by 450%. India's three-stage nuclear power program features the use of a thorium fuel cycle in the third stage, as it has abundant thorium reserves but little uranium.

Nuclear waste

The lifecycle of fuel in the present US system

The most important waste stream from nuclear power plants is spent nuclear fuel. It is primarily composed of unconverted uranium as well as significant quantities of transuranic actinides (plutonium and curium, mostly). In addition, about 3% of it is fission products from nuclear reactions. The actinides (uranium, plutonium, and curium) are responsible for the bulk of the long-term radioactivity, whereas the fission products are responsible for the bulk of the short-term radioactivity.

High-level radioactive waste

Typical composition of UOx LWR fuel, before and after approximately 3 years of fission service in the once-thru fuel cycle. Thermal reactors, which presently constitute the majority of the world fleet, cannot burn up the reactor grade plutonium that is generated efficiently, limiting the effective useful fuel life to a few years at most. Reactors in Europe and Asia are permitted to burn later refined MOX fuel, though the burnup is similarly not complete.

 

In the years outside a reactor, the activity of spent UOx fuel, in reference to the activity of natural uranium ore. Together with the varied plutonium isotopes that are generated, the minor actinides constitute the primary hazard following the relatively rapid decay of the fission products after approximately 300 years.

 

Following interim storage in a spent fuel pool, the bundles of used fuel rod assemblies of a typical nuclear power station are often stored on site in the likes of the eight dry cask storage vessels pictured above.^[180] At Yankee Rowe Nuclear Power Station, which generated 44 billion kilowatt hours of electricity over its lifetime, its complete spent fuel inventory is contained within sixteen casks.

 

During power production, high-level radioactive waste is generated that requires treatment, management and disposal. The technical issues in accomplishing this are considerable, due to the extremely long periods some particularly mobile, albeit mildly radioactive wastes, remain potentially hazardous to living organisms, namely the long-lived fission products, Technetium-99 (half-life 220,000 years) and Iodine-129 (half-life 15.7 million years), which dominate the true-waste stream in radioactivity after the more intensely radioactive short-lived fission products have decayed into stable elements, after approximately 300 years. To successfully isolate this waste from the biosphere, some variation of a synroc treatment and permanent storage is commonly suggested.

The most concerning isotopes found in spent fuel that does not undergo reprocessing, are the medium-lived transuranic elements, which are led by reactor grade plutonium (half-life 24,000 years). While in the US, spent fuel in its entirety is legally classified as a nuclear waste and is treated similarly, in other countries it is considered a fuel.

Some proposed reactor designs, such as the American Integral Fast Reactor and the Molten salt reactor can more completely use or burnup the spent reactor grade plutonium fuel and other minor actinides, generated from light water reactors, as under the designed fast fission spectrum, these elements are more likely to fission and produce the aforementioned fission products in their place. This offers a potentially more attractive alternative to deep geological disposal.

Another possibility is the use of thorium. While still containing similar fission products, used thorium fuel, owing to its starting point below uranium on the periodic table, builds up much less transuranic elements and therefore spent thorium fuel, is less concerning from a radiotoxic and security standpoint.

Low-level radioactive waste

The nuclear industry also produces a large volume of low-level radioactive waste in the form of contaminated items like clothing, hand tools, water purifier resins, and (upon decommissioning) the materials of which the reactor itself is built. Low-level waste can be stored on-site until radiation levels are low enough to be disposed as ordinary waste, or it can be sent to a low-level waste disposal site.

Waste relative to other types

In countries with nuclear power, radioactive wastes account for less than 1% of total industrial toxic wastes, much of which remains hazardous for long periods. Overall, nuclear power produces far less waste material by volume than fossil-fuel based power plants. Coal-burning plants are particularly noted for producing large amounts of toxic and mildly radioactive ash due to concentrating naturally occurring metals and mildly radioactive material from the coal. A 2008 report from Oak Ridge National Laboratory concluded that coal power actually results in more radioactivity being released into the environment than nuclear power operation, and that the population effective dose equivalent, or dose to the public from radiation from coal plants is 100 times as much as from the operation of nuclear plants. Although coal ash is much less radioactive than spent nuclear fuel on a weight per weight basis, coal ash is produced in much higher quantities per unit of energy generated, and this is released directly into the environment as fly ash, whereas nuclear plants use shielding to protect the environment from radioactive materials, for example, in dry cask storage vessels.

Waste disposal

The placement of waste containers, generated during US cold war activities, underground at the WIPP facility. The facility is seen as a potential demonstration, for later civilian generated spent fuel, or constituents of it.

Disposal of nuclear waste is often said to be the among the most problematic aspects of the industry. Presently, waste is mainly stored at individual reactor sites and there are over 430 locations around the world where radioactive material continues to accumulate. Some experts suggest that centralized underground repositories which are well-managed, guarded, and monitored, would be a vast improvement. There is an "international consensus on the advisability of storing nuclear waste in deep geological repositories", with the lack of movement of nuclear waste in the 2 billion year old natural nuclear fission reactors in Oklo, Gabon being cited as "a source of essential information today."

Most waste packaging, small-scale experimental fuel recycling chemistry and radiopharmaceutical refinement is conducted within remote-handled Hot cells.

There are no commercial scale purpose built underground high-level waste repositories in operation. However, in Finland the Onkalo spent nuclear fuel repository is under construction as of 2015. The Waste Isolation Pilot Plant (WIPP) in New Mexico has been taking nuclear waste since 1999 from production reactors, but as the name suggests is a research and development facility. In 2014 a radiation leak caused by violations in the use of chemically reactive packaging brought renewed attention to the need for quality control management, along with some initial calls for more R&D into the alternative methods of disposal for radioactive waste and spent fuel. In 2017, the facility was formally reopened after three years of investigation and cleanup, with the resumption of new storage taking place later that year.

Reprocessing

Separation of uranium and plutonium from spent nuclear fuel by the 1940s-1950s PUREX method.^[207] While variations of this wet-chemical process are employed to separate reactor grade plutonium(RGPu) for later re-use as MOX fuel. The process is controversial as it can be used to produce chemically pure WGPu.

 

A frequently proposed alternative fuel-cycle, as incorporated in the proposed 1990s Integral fast reactor(IFR), In which the RGPu is not isolated, rather all actinides are "electro-won" from the "true waste" of fission products in spent fuel. Mixed with the gamma and alpha emitting actinides, species that "self-protect" the RGPu in numerous possible theft scenarios. The fuel cycle is not limited to the depicted IFR/sodium-fast-reactor class, it may be combined with the "Stable salt reactor". Animations of the pyroprocessing technology are also available.

 

Today's thermal reactors primarily run on the once-through fuel cycle though they can reuse once-thru reactor-grade plutonium to a limited degree in the form of mixed-oxide or MOX fuel. This is a routine commercial practice in most countries outside the United States, as it increases the sustainability of nuclear fission and lowers the volume of high level nuclear waste. Reprocessing can potentially recover up to 95% of the remaining uranium and plutonium in spent nuclear fuel, putting it into new mixed oxide fuel. This produces a reduction in long term radioactivity within the remaining waste, since this is largely short-lived fission products, and reduces its volume by over 90%. Reprocessing of civilian fuel from power reactors is currently done in Europe, Russia, Japan, and India. The full potential of reprocessing had not been achieved as of 2013 because it requires breeder reactors, which were not commercially available at that time.

Nuclear reprocessing reduces the volume of high-level waste, but does not reduce the fission products that are the primary residual heat generating and radioactive substances, thus still requiring geological waste repository management. Reprocessing has been politically controversial because of the potential to contribute to nuclear proliferation, vulnerability to nuclear terrorism and because of its high cost compared to the once-through fuel cycle.

In the United States, spent nuclear fuel is currently all treated as waste. A major recommendation of the Blue Ribbon Commission on America's Nuclear Future was that "the United States should undertake...one or more permanent deep geological facilities for the safe disposal of spent fuel and high-level nuclear waste".

In France, the La Hague reprocessing facility has operated commercially since 1976. The plant is responsible for half the world's reprocessing as of 2010 treating spent nuclear fuel from France, Japan, Germany, Belgium, Switzerland, Italy, Spain and the Netherlands, it produces new fuel, with the non-recyclable part of the radioactive waste eventually sent back to the user nation. Up to 2015, more than 32,000 tonnes of spent nuclear fuel had been reprocessed, with 70% of that from France, 17% from Germany and 9% from Japan. When initially opened La Hague received criticism from Greenpeace, though more recently they have ceased attempting to criticize the fuel reprocessing facility on technical grounds, having succeeded at performing the process without serious incidents that have been frequent at other such facilities around the world. In the past, the antinuclear movement argued that reprocessing would not be technically or economically feasible.

Nuclear decommissioning

The financial costs of every nuclear power plant continues for some time after the facility has finished generating its last useful electricity. Once no longer economically viable, nuclear reactors and uranium enrichment facilities are generally decommissioned, returning the facility and its parts to a safe enough level to be entrusted for other uses, such as greenfield status. After a cooling-off period that may last decades, reactor core materials are dismantled and cut into small pieces to be packed in containers for interim storage or transmutation experiments. 

In the United States a Nuclear Waste Policy Act and Nuclear Decommissioning Trust Fund is legally required, with utilities banking 0.1 to 0.2 cents/kWh during operations to fund future decommissioning. They must report regularly to the Nuclear Regulatory Commission (NRC) on the status of their decommissioning funds. About 70% of the total estimated cost of decommissioning all U.S. nuclear power reactors has already been collected (on the basis of the average cost of $320 million per reactor-steam turbine unit).

In the United States in 2011, there are 13 reactors that had permanently shut down and are in some phase of decommissioning. With Connecticut Yankee Nuclear Power Plant and Yankee Rowe Nuclear Power Station having completed the process in 2006–2007, after ceasing commercial electricity production circa 1992. The majority of the 15 years, was used to allow the station to naturally cool-down on its own, which makes the manual disassembly process both safer and cheaper. Decommissioning at nuclear sites which have experienced a serious accident are the most expensive and time-consuming.

Installed capacity and electricity production

Share of electricity produced by nuclear power in the world

 

The status of nuclear power globally (click image for legend)

 

Net electrical generation by source and growth from 1980 to 2010. (Brown) – fossil fuels. (Red) – Fission. (Green) – "all renewables". In terms of energy generated between 1980 and 2010, the contribution from fission grew the fastest.

 

The rate of new construction builds for civilian fission-electric reactors essentially halted in the late 1980s, with the effects of accidents having a chilling effect. Increased capacity factor realizations in existing reactors was primarily responsible for the continuing increase in electrical energy produced during this period. The halting of new builds c. 1985, resulted in greater fossil fuel generation, see above graph.

 

Electricity generation trends in the top five fission-energy producing countries (US EIA data)

 

Nuclear fission power stations, excluding the contribution from naval nuclear fission reactors, provided 11% of the world's electricity in 2012, somewhat less than that generated by hydro-electric stations at 16%. Since electricity accounts for about 25% of humanity's energy usage with the majority of the rest coming from fossil fuel reliant sectors such as transport, manufacture and home heating, nuclear fission's contribution to the global final energy consumption was about 2.5%. This is a little more than the combined global electricity production from wind, solar, biomass and geothermal power, which together provided 2% of global final energy consumption in 2014.

In addition, there were approximately 140 naval vessels using nuclear propulsion in operation, powered by about 180 reactors.

Nuclear power's share of global electricity production has fallen from 16.5% in 1997 to about 10% in 2017, in large part because the economics of nuclear power have become more difficult.

Regional differences in the use of nuclear power are large. The United States produces the most nuclear energy in the world, with nuclear power providing 19% of the electricity it consumes, while France produces the highest percentage of its electrical energy from nuclear reactors—80% as of 2006. In the European Union as a whole nuclear power provides 30% of the electricity. Nuclear power is the single largest low-carbon electricity source in the United States, and accounts for two-thirds of the European Union's low-carbon electricity. Nuclear energy policy differs among European Union countries, and some, such as Austria, Estonia, Ireland and Italy, have no active nuclear power stations. In comparison, France has a large number of nuclear reactors in use and nuclear power supplied over 70% of its electricity in 2017.

Many military and some civilian (such as some icebreakers) ships use nuclear marine propulsion. A few space vehicles have been launched using nuclear reactors: 33 reactors belong to the Soviet RORSAT series and one was the American SNAP-10A. 

International research is continuing into additional uses of process heat such as hydrogen production (in support of a hydrogen economy), for desalinating sea water, and for use in district heating systems.

Economics

The Ikata Nuclear Power Plant, a pressurized water reactor that cools by utilizing a secondary coolant heat exchanger with a large body of water, an alternative cooling approach to large cooling towers.

The economics of new nuclear power plants is a controversial subject, since there are diverging views on this topic, and multibillion-dollar investments depend on the choice of an energy source. Nuclear power plants typically have high capital costs for building the plant, but low fuel costs. Comparison with other power generation methods is strongly dependent on assumptions about construction timescales and capital financing for nuclear plants as well as the future costs of fossil fuels and renewables as well as for energy storage solutions for intermittent power sources. On the other hand, measures to mitigate global warming, such as a carbon tax or carbon emissions trading, may favor the economics of nuclear power.

Analysis of the economics of nuclear power must also take into account who bears the risks of future uncertainties. To date all operating nuclear power plants were developed by state-owned or regulated electric utility monopolies Many countries have now liberalized the electricity market where these risks, and the risk of cheaper competitors emerging before capital costs are recovered, are borne by plant suppliers and operators rather than consumers, which leads to a significantly different evaluation of the economics of new nuclear power plants.

Nuclear power plants, though capable of some grid-load following, are typically run as much as possible to keep the cost of the generated electrical energy as low as possible, supplying mostly base-load electricity.

Internationally the price of nuclear plants rose 15% annually in 1970–1990. With PWR stations, having total costs in 2012 of about $96 per megawatt hour (MWh), most of which involves capital construction costs, compared with solar power at $130 per MWh, and natural gas at the low end at $64 per MWh. The Fukushima Daiichi nuclear disaster, is expected to increase the costs of operating and new LWR power stations, due to increased requirements for on-site spent fuel management and elevated design basis threats.

Accidents, attacks and safety

Nuclear reactors have three unique characteristics that affect their safety, as compared to other power plants. Firstly, intensely radioactive materials are present in a nuclear reactor. Their release to the environment could be hazardous. Secondly, the fission products, which make up most of the intensely radioactive substances in the reactor, continue to generate a significant amount of decay heat even after the fission chain reaction has stopped. If the heat cannot be removed from the reactor, the fuel rods may overheat and release radioactive materials. Thirdly, a rapid increase of the reactor power is possible if the chain reaction cannot be controlled in certain reactor designs. These three characteristics have to be taken into account when designing nuclear reactors.

Many reactor designs include intrinsic features that in the event of a rapid increase of the reactor power or an increase in temperature, will cause a decrease in the fission rate. With most common western reactors now in service and under design review by government regulators, are designs which use natural feedback mechanisms to prevent an uncontrolled increase of the fission reactor power-level, principally the nuclear engineering reliance on a negative Void coefficient of reactivity, a form of Passive nuclear safety. The chain reaction can also be manually stopped completely by inserting control rods into the reactor core. While again depending on the reactor design, a mixture of both passive and active Nuclear reactor safety systems, the most well known being the Essential service coolant system, operate to remove decay heat. Alongside this, a number of further defense in depth (nuclear engineering) principles are relied upon, such as physical containment barriers.

Accidents

Following the 2011 Fukushima Daiichi nuclear disaster, the world's worst nuclear accident since 1986, 50,000 households were displaced after radiation leaked into the air, soil and sea. Radiation checks led to bans of some shipments of vegetables and fish.

 

Some serious nuclear and radiation accidents have occurred. The severity of nuclear accidents is generally classified using the International Nuclear Event Scale (INES) introduced by the International Atomic Energy Agency (IAEA). The scale ranks anomalous events or accidents on a scale from 0 (a deviation from normal operation that pose no safety risk) to 7 (a major accident with widespread effects). There have been 3 accidents of level 5 or higher in the civilian nuclear power industry, two of which, the Chernobyl accident and the Fukushima accident, are ranked at level 7. 

The Chernobyl accident in 1986 caused approximately 50 deaths from direct and indirect effects, and some temporary serious injuries. The future predicted mortality from cancer increases, is usually estimated at some 4000 in the decades to come. With a higher number of the routinely treatable Thyroid cancer, set to be the only type of causal cancer, likely to be seen in future large studies.

The Fukushima Daiichi nuclear accident was caused by the 2011 Tohoku earthquake and tsunami. The accident has not caused any radiation related deaths, but resulted in radioactive contamination of surrounding areas. The difficult Fukushima disaster cleanup will take 40 or more years, and is expected to cost tens of billions of dollars. The Three Mile Island accident in 1979 was a smaller scale accident, rated at INES level 5. There were no direct or indirect deaths caused by the accident.

According to Benjamin K. Sovacool, fission energy accidents ranked first among energy sources in terms of their total economic cost, accounting for 41 percent of all property damage attributed to energy accidents. Another analysis presented in the international journal Human and Ecological Risk Assessment found that coal, oil, Liquid petroleum gas and hydroelectric accidents (primarily due to the Banqiao dam burst) have resulted in greater economic impacts than nuclear power accidents.

Nuclear power works under an insurance framework that limits or structures accident liabilities in accordance with the Paris convention on nuclear third-party liability, the Brussels supplementary convention, the Vienna convention on civil liability for nuclear damage and the Price-Anderson Act in the United States. It is often argued that this potential shortfall in liability represents an external cost not included in the cost of nuclear electricity; but the cost is small, amounting to about 0.1% of the levelized cost of electricity, according to a CBO study. These beyond-regular-insurance costs for worst-case scenarios are not unique to nuclear power, as hydroelectric power plants are similarly not fully insured against a catastrophic event such as the Banqiao Dam disaster, where 11 million people lost their homes and from 30,000 to 200,000 people died, or large dam failures in general. As private insurers base dam insurance premiums on limited scenarios, major disaster insurance in this sector is likewise provided by the state.

Safety

In terms of lives lost per unit of energy generated, nuclear power has caused fewer accidental deaths per unit of energy generated than all other major sources of energy generation. Energy produced by coal, petroleum, natural gas and hydropower has caused more deaths per unit of energy generated due to air pollution and energy accidents. This is found when comparing the immediate deaths from other energy sources to both the immediate nuclear related deaths from accidents and also including the latent, or predicted, indirect cancer deaths from nuclear energy accidents. When the combined immediate and indirect fatalities from nuclear power and all fossil fuels are compared, including fatalities resulting from the mining of the necessary natural resources to power generation and to air pollution, the use of nuclear power has been calculated to have prevented about 1.8 million deaths between 1971 and 2009, by reducing the proportion of energy that would otherwise have been generated by fossil fuels, and is projected to continue to do so. Following the 2011 Fukushima nuclear disaster, it has been estimated that if Japan had never adopted nuclear power, accidents and pollution from coal or gas plants would have caused more lost years of life.

Forced evacuation from a nuclear accident may lead to social isolation, anxiety, depression, psychosomatic medical problems, reckless behavior, even suicide. Such was the outcome of the 1986 Chernobyl nuclear disaster in Ukraine. A comprehensive 2005 study concluded that "the mental health impact of Chernobyl is the largest public health problem unleashed by the accident to date". Frank N. von Hippel, an American scientist, commented on the 2011 Fukushima nuclear disaster, saying that a disproportionate radiophobia, or "fear of ionizing radiation could have long-term psychological effects on a large portion of the population in the contaminated areas". A 2015 report in Lancet explained that serious impacts of nuclear accidents were often not directly attributable to radiation exposure, but rather social and psychological effects. Evacuation and long-term displacement of affected populations created problems for many people, especially the elderly and hospital patients. In January 2015, the number of Fukushima evacuees was around 119,000, compared with a peak of around 164,000 in June 2012.

Attacks and sabotage

Terrorists could target nuclear power plants in an attempt to release radioactive contamination into the community. The United States 9/11 Commission has said that nuclear power plants were potential targets originally considered for the September 11, 2001 attacks. An attack on a reactor's spent fuel pool could also be serious, as these pools are less protected than the reactor core. The release of radioactivity could lead to thousands of near-term deaths and greater numbers of long-term fatalities.

In the United States, the NRC carries out "Force on Force" (FOF) exercises at all Nuclear Power Plant (NPP) sites at least once every three years. In the United States, plants are surrounded by a double row of tall fences which are electronically monitored. The plant grounds are patrolled by a sizeable force of armed guards.

Insider sabotage is also a threat because insiders can observe and work around security measures. Successful insider crimes depended on the perpetrators' observation and knowledge of security vulnerabilities. A fire caused 5–10 million dollars worth of damage to New York's Indian Point Energy Center in 1971. The arsonist turned out to be a plant maintenance worker. Some reactors overseas have also reported varying levels of sabotage by workers.

Nuclear proliferation

United States and USSR/Russian nuclear weapons stockpiles, 1945–2006. The Megatons to Megawatts Program was the main driving force behind the sharp reduction in the quantity of nuclear weapons worldwide since the cold war ended. However, without an increase in nuclear reactors and greater demand for fissile fuel, the cost of dismantling has dissuaded Russia from continuing their disarmament.

 

Many technologies and materials associated with the creation of a nuclear power program have a dual-use capability, in that they can be used to make nuclear weapons if a country chooses to do so. When this happens a nuclear power program can become a route leading to a nuclear weapon or a public annex to a "secret" weapons program. The concern over Iran's nuclear activities is a case in point.

As of April 2012 there were thirty one countries that have civil nuclear power plants, of which nine have nuclear weapons, with the vast majority of these nuclear weapons states having first produced weapons, before commercial fission electricity stations. Moreover, the re-purposing of civilian nuclear industries for military purposes would be a breach of the Non-proliferation treaty, of which 190 countries adhere to. 

A fundamental goal for global security is to minimize the nuclear proliferation risks associated with the expansion of nuclear power. The Global Nuclear Energy Partnership is an international effort to create a distribution network in which developing countries in need of energy would receive nuclear fuel at a discounted rate, in exchange for that nation agreeing to forgo their own indigenous develop of a uranium enrichment program. The France-based Eurodif/European Gaseous Diffusion Uranium Enrichment Consortium is a program that successfully implemented this concept, with Spain and other countries without enrichment facilities buying a share of the fuel produced at the French controlled enrichment facility, but without a transfer of technology. Iran was an early participant from 1974, and remains a shareholder of Eurodif via Sofidif. 

A 2009 United Nations report said that:

...the revival of interest in nuclear power could result in the worldwide dissemination of uranium enrichment and spent fuel reprocessing technologies, which present obvious risks of proliferation as these technologies can produce fissile materials that are directly usable in nuclear weapons.

On the other hand, power reactors can also reduce nuclear weapons arsenals when military grade nuclear materials are reprocessed to be used as fuel in nuclear power plants. The Megatons to Megawatts Program, the brainchild of Thomas Neff of MIT, is the single most successful non-proliferation program to date. Up to 2005, the Megatons to Megawatts Program had processed $8 billion of high enriched, weapons grade uranium into low enriched uranium suitable as nuclear fuel for commercial fission reactors by diluting it with natural uranium. This corresponds to the elimination of 10,000 nuclear weapons. For approximately two decades, this material generated nearly 10 percent of all the electricity consumed in the United States (about half of all U.S. nuclear electricity generated) with a total of around 7 trillion kilowatt-hours of electricity produced. Enough energy to energize the entire United States electric grid for about two years. In total it is estimated to have cost $17 billion, a "bargain for US ratepayers", with Russia profiting $12 billion from the deal. Much needed profit for the Russian nuclear oversight industry, which after the collapse of the Soviet economy, had difficulties paying for the maintenance and security of the Russian Federations highly enriched uranium and warheads.

The Megatons to Megawatts Program was hailed as a major success by anti-nuclear weapon advocates as it has largely been the driving force behind the sharp reduction in the quantity of nuclear weapons worldwide since the cold war ended. However without an increase in nuclear reactors and greater demand for fissile fuel, the cost of dismantling and down blending has dissuaded Russia from continuing their disarmament. As of 2013 Russia appears to not be interested in extending the program.

Environmental impact

A 2000s survey of direct Life-cycle greenhouse-gas emissions of energy sources, the indirect or total global warming potential impacts of each energy source, in terms of Carbon dioxide equivalent figures, remain under continued investigation.

Carbon emissions

A 2008 meta analysis of 103 studies, published by Benjamin K. Sovacool, estimated that the mean value of CO₂ emissions for nuclear power over the lifecycle of a plant was 66.08 g/kW·h, this 2008 study by Sovacool garnered controversy by comparing statistical mean with median values, to suggest various renewable energy sources were "two to seven times more effective at combating climate change", by presenting median values of "9–32 g/kW·h" for the latter. A similar 2012 meta-analysis by Yale University arrived at a different conclusion, with the median value, depending on which reactor design was analyzed, ranging from 11 to 25 g/kW·h of total life cycle nuclear power CO_2e emissions, a figure comparable, and in some instances lower, than a number of renewable energy generation methods.

 

Nuclear power is one of the leading low carbon power generation methods of producing electricity, and in terms of total life-cycle greenhouse gas emissions per unit of energy generated, has emission values comparable to or lower than renewable energy. A 2014 analysis of the carbon footprint literature by the Intergovernmental Panel on Climate Change (IPCC) reported that the embodied total life-cycle emission intensity of fission electricity has a median value of 12 g CO₂eq/kWh which is the lowest out of all commercial baseload energy sources. This is contrasted with coal and fossil gas at 820 and 490 g CO₂ eq/kWh. From the beginning of fission-electric power station commercialization in the 1970s, nuclear power prevented the emission of about 64 billion tonnes of carbon dioxide equivalent that would have otherwise resulted from the burning of fossil fuels in thermal power stations.

Radiation

The variation in a person's absorbed natural background radiation, averages 2.4 mSv/a globally but frequently varies between 1 mSv/a and 13 mSv/a depending in most part on the geology a person resides upon. According to the United Nations (UNSCEAR), regular NPP/nuclear power plant operations including the nuclear fuel cycle, increases this amount to 0.0002 millisieverts (mSv) per year of public exposure as a global average. The average dose from operating NPPs to the local populations around them is less than 0.0001 mSv/a.The average dose to those living within 50 miles of a coal power plant is over three times this dose, 0.0003 mSv/a.

As of a 2008 report, Chernobyl resulted in the most affected surrounding populations and male recovery personnel receiving an average initial 50 to 100 mSv over a few hours to weeks, while the remaining global legacy of the worst nuclear power plant accident in average exposure is 0.002 mSv/a and is continually dropping at the decaying rate, from the initial high of 0.04 mSv per person averaged over the entire populace of the Northern Hemisphere in the year of the accident in 1986.

Comparison with renewable energy

While the majority of installed renewable energy across the world is currently in the form of hydro power and an approximate doubling of present world hydro capacity is considered possible in developing nations, in western nations by contrast, the economically feasible geography for new hydropower is lacking, with every geographically suitable area largely exploited. With the growing apparent negative externalities from the use of fossil fuels, a need for alternatives that could be implemented on a large scale has resulted in an ongoing debate on the relative benefits of nuclear power compared to conventional "new renewable" energy sources for the generation of low-carbon electricity. These "new renewable" proponents, suggest that with government supports the expanding of Wind and Solar energy is feasible and could eliminate the need for nuclear power.

Nuclear powered aircraft carriers, presently require as depicted, jet-fuel Replenishment at sea operations, by expensive Replenishment oilers. The Naval Research Laboratory team led by Heather Willauer has developed a process that is designed to use the ample electrical power onboard carriers, for alternative in-situ snythesis of jet-fuel, from its chemical building blocks. Extracting the carbon dioxide (CO₂) in seawater, in tandem with hydrogen (H₂) and recombining the two into long chain hydrocarbon liquids. Writing in the Journal of Renewable Sustainable Energy, in 2012, Willauer estimated that the carbon neutral jet fuel for Navy and Marine aviation, could be synthesized from seawater in quantities up to 100,000 US gal (380,000 L) per day, at a cost of three to six U.S. dollars per gallon. The U.S. Navy is expected to deploy the technology some time in the 2020s.

 

Nuclear power proponents argue that conventional renewable energy sources, wind and solar do not offer the scalability necessary for a large scale decarbonization of the electric grid, mainly due to their intermittency and note that wherever implementations of these "new renewables" are attempted in industrialized nations, such as the "energy transition" in Germany and Denmark, an invariable increase in the extraction, reliance and use of fossil fuels, occurs. With a number of commentators questioning if the promotion of "new renewables", is simply a way for the fossil fuel industry to ensure its position in the world energy market, by the backdoor. These commentators point, in support of the assessment, to the expansion of the coal burning Lippendorf Power Station in Germany and in 2015 the opening of a large, 1730 MW coal burning power station in Moorburg, the only such coal burning facility of its kind to commence operations, in Western Europe in the 2010s. With Germany now committed to coal mining and burning until well into 2050, widely missing its 2020 emission reduction target. While in Denmark, before being pressured to step down in 2018, former chair of the Danish Council on Climate Change, was directly requested not to publish criticisms of the Danish governments "unambitious" decarbonizing policy, after it became apparent that the 2020 emission reduction target, would similarly, not be met.

Several studies suggest that it might be theoretically possible to cover a majority of world energy generation with new renewable sources. The Intergovernmental Panel on Climate Change (IPCC) has said that if governments were supportive, renewable energy supply could account for close to 80% of the world's energy use by 2050 by subsidizing renewables with 1% of global GDP, equating to $1 trillion annually.

Nuclear power by contrast is proposed as a tested and practical way to implement a 100% low-carbon energy infrastructure, as opposed to renewable sources. Analysis in 2015 by professor and chair of Environmental Sustainability Barry W. Brook and his colleagues on the topic of replacing fossil fuels entirely, from the electric grid of the world, has determined that at the historically modest and proven-rate at which nuclear energy was added to and replaced fossil fuels in France and Sweden during each nation's building programs in the 1980s, nuclear energy could displace or remove fossil fuels from the electric grid completely within 10 years, "allow[ing] the world to meet the most stringent greenhouse-gas mitigation targets.".

In a similar analysis, Brook had earlier determined that 50% of all global energy, that is not solely electricity, but transportation synthetic fuels etc. could be generated within approximately 30 years, if the global nuclear fission build rate was identical to each of these nation's already proven installation rates in units of installed nameplate capacity, GW per year, per unit of global GDP (GW/year/$). This is in contrast to the conceptual studies for a 100% renewable energy world, which would require an orders of magnitude more costly global investment per year, which has no historical precedent, along with far greater land that would have to be devoted to the wind, wave and solar projects, and the inherent assumption that humanity will use less, and not more, energy in the future. As Brook notes, the "principal limitations on nuclear fission are not technical, economic or fuel-related, but are instead linked to complex issues of societal acceptance, fiscal and political inertia, and inadequate critical evaluation of the real-world constraints facing [the other] low-carbon alternatives."

EROEI	energy sources in 2013
3.5	Biomass (corn)
3.9	Solar PV (Germany)
16	Wind (E-66 turbine)
19	Solar thermal CSP (desert)
28	Fossil gas in a CCGT
30	Coal
49	Hydro (medium-sized dam)
75	Nuclear (in a PWR)

According to a transatlantic collaborative research paper on Energy return on energy Invested (EROEI), conducted by six analysts led by German academic D. Weißbach, and described as "...the most extensive overview so far based on a careful evaluation of available Life Cycle Assessments". Published in the peer reviewed journal Energy in 2013. The uncorrected for their intermittency("unbuffered") EROEI for each energy source analyzed is as depicted in the attached table at right. While the buffered (corrected for their intermittency) EROEI stated in the paper, for all low-carbon power sources, with the exception of the only two baseload energy supplying systems of biomass and nuclear, were lowered considerably due to weather variations, a reduction of EROEI proportional to how reliant the other systems are on the manufacture of back-up energy sources, a reliance necessary in order to approximate the steady electrical supply characteristics of a biomass or nuclear facility.

Nuclear power stations require orders of magnitude less materials per unit of energy generated. An analysis by researchers at Uppsala University in 2016 returned the conclusion that due to the much greater dependence on materials for renewable energy construction, while a theoretical expansion of "new renewables" to supply a future 50% of global final energy demand is often conceptualized, the physical expansion of the crucial mining and raw material extraction rates, would be un-sustainable.

A number of other studies similarly report that solar and wind energy are presently not cost-effective compared to nuclear power. The Brookings Institution published The Net Benefits of Low and No-Carbon Electricity Technologies in 2014 which states, after performing an energy and emissions cost analysis, that "The net benefits of new nuclear, hydro, and natural gas combined cycle plants far outweigh the net benefits of new wind or solar plants", with the most cost effective low carbon power technology being determined to be nuclear power.

Several studies suggest that wind and solar power have costs that are comparable or lower than nuclear power, when considering price per kWh. The cost of constructing established nuclear power reactor designs has followed an increasing trend due to frequently changing regulations, public protests and delaying court cases whereas the levelized cost of electricity (LCOE) is declining for wind and solar power. In 2010 a report from Solar researchers at Duke University suggested that solar power may be already cheaper than new nuclear power plants. However they state that if government aid/financial subsidies were removed for solar power, the crossover point would be delayed by years. Data from the U.S. Energy Information Administration (EIA) in 2011 estimated that in 2016, solar will have a levelized cost of electricity almost twice as expensive as nuclear (21¢/kWh for solar, 11.39¢/kWh for nuclear), and wind somewhat less expensive than nuclear (9.7¢/kWh). However, the EIA has also cautioned that levelized costs of intermittent sources such as wind and solar are not directly comparable to costs of "dispatchable" sources (those that can be adjusted to meet demand), as intermittent sources need costly large-scale back-up power supplies for when the weather changes.

A 2010 study by the Global Subsidies Initiative compared global relative energy subsidies, or government financial aid for the deployment of different energy sources. Results show that fossil fuels receive about 1 U.S. cents per kWh of energy they produce, nuclear energy receives 1.7 cents / kWh, renewable energy (excluding hydroelectricity) receives 5.0 cents / kWh and biofuels receive 5.1 cents / kWh in subsidies.

Nuclear power is comparable to, and in some cases lower, than many renewable energy sources in terms of lives lost per unit of electricity delivered. However, as opposed to renewable energy, conventional designs for nuclear reactors produce a smaller volume of manufacture and operations related waste, most notably, the intensely radioactive spent fuel that needs to be stored or reprocessed. A nuclear plant also needs to be disassembled and removed and much of the disassembled nuclear plant needs to be stored as low level nuclear waste for a few decades.

In an EU wide 2018 assessment of progress in reducing greenhouse gas emissions per capita, France and Sweden were the only two large industrialized nations within the EU to receive a positive rating, every other country received a "poor" to "very poor" grade.

Debate on nuclear power

The nuclear power debate concerns the controversy which has surrounded the deployment and use of nuclear fission reactors to generate electricity from nuclear fuel for civilian purposes. The debate about nuclear power peaked during the 1970s and 1980s, when it "reached an intensity unprecedented in the history of technology controversies", in some countries.

Proponents of nuclear energy contend that nuclear power is a sustainable energy source that reduces carbon emissions and increases energy security by decreasing dependence on imported energy sources. Proponents claim that nuclear power produces virtually no conventional air pollution, such as greenhouse gases and smog, in contrast to the main alternative of fossil-fuel power stations. Nuclear power can produce base-load power unlike many renewables which are intermittent energy sources lacking large-scale and cheap ways of storing energy. M. King Hubbert saw oil as a resource that would run out, and proposed nuclear energy as a replacement energy source. Proponents claim that the risks of storing waste are small and can be further reduced by using the latest technology in newer reactors, and the operational safety record in the Western world is excellent when compared to the other major kinds of power plants.

Opponents believe that nuclear power poses many threats to people and the environment. These threats include the problems of processing, transport and storage of radioactive nuclear waste, the risk of nuclear weapons proliferation and terrorism, as well as health risks and environmental damage from uranium mining. They also contend that reactors themselves are enormously complex machines where many things can and do go wrong; and there have been serious nuclear accidents. Critics do not believe that the risks of using nuclear fission as a power source can be fully offset through the development of new technology. In years past, they also argued that when all the energy-intensive stages of the nuclear fuel chain are considered, from uranium mining to nuclear decommissioning, nuclear power is neither a low-carbon nor an economical electricity source.

Arguments of economics and safety are used by both sides of the debate.

Use in space

The loading of the Plutonium-238 based MMRTG into the Mars Curiosity rover. Assembled in a Hot cell at Idaho National Laboratory

 

The Multi-mission radioisotope thermoelectric generator (MMRTG), used in several space missions such as the Curiosity Mars rover

 

Both fission and fusion appear promising for space propulsion applications, generating higher mission velocities with less reaction mass. This is due to the much higher energy density of nuclear reactions: some 7 orders of magnitude (10,000,000 times) more energetic than the chemical reactions which power the current generation of rockets. 

Radioactive decay has been used on a relatively small scale (few kW), mostly to power space missions and experiments by using radioisotope thermoelectric generators such as those developed at Idaho National Laboratory.

Research

Advanced fission reactor designs

Generation IV roadmap from Argonne National Laboratory

 

Current fission reactors in operation around the world are second or third generation systems, with most of the first-generation systems having been already retired. Research into advanced generation IV reactor types was officially started by the Generation IV International Forum (GIF) based on eight technology goals, including to improve nuclear safety, improve proliferation resistance, minimize waste, improve natural resource utilization, the ability to consume existing nuclear waste in the production of electricity, and decrease the cost to build and run such plants. Most of these reactors differ significantly from current operating light water reactors, and are generally not expected to be available for commercial construction before 2030.

One disadvantage of any new reactor technology is that safety risks may be greater initially as reactor operators have little experience with the new design. Nuclear engineer David Lochbaum has explained that almost all serious nuclear accidents have occurred with what was at the time the most recent technology. He argues that "the problem with new reactors and accidents is twofold: scenarios arise that are impossible to plan for in simulations; and humans make mistakes". As one director of a U.S. research laboratory put it, "fabrication, construction, operation, and maintenance of new reactors will face a steep learning curve: advanced technologies will have a heightened risk of accidents and mistakes. The technology may be proven, but people are not".

Hybrid nuclear fusion-fission

Hybrid nuclear power is a proposed means of generating power by use of a combination of nuclear fusion and fission processes. The concept dates to the 1950s, and was briefly advocated by Hans Bethe during the 1970s, but largely remained unexplored until a revival of interest in 2009, due to delays in the realization of pure fusion. When a sustained nuclear fusion power plant is built, it has the potential to be capable of extracting all the fission energy that remains in spent fission fuel, reducing the volume of nuclear waste by orders of magnitude, and more importantly, eliminating all actinides present in the spent fuel, substances which cause security concerns.

Nuclear fusion

Schematic of the ITER tokamak under construction in France.

Nuclear fusion reactions have the potential to be safer and generate less radioactive waste than fission. These reactions appear potentially viable, though technically quite difficult and have yet to be created on a scale that could be used in a functional power plant. Fusion power has been under theoretical and experimental investigation since the 1950s. 

Several experimental nuclear fusion reactors and facilities exist. The largest and most ambitious international nuclear fusion project currently in progress is ITER, a large tokamak under construction in France. ITER is planned to pave the way for commercial fusion power by demonstrating self-sustained nuclear fusion reactions with positive energy gain. Construction of the ITER facility began in 2007, but the project has run into many delays and budget overruns. The facility is now not expected to begin operations until the year 2027 – 11 years after initially anticipated. A follow on commercial nuclear fusion power station, DEMO, has been proposed. There are also suggestions for a power plant based upon a different fusion approach, that of an inertial fusion power plant. 

Fusion powered electricity generation was initially believed to be readily achievable, as fission-electric power had been. However, the extreme requirements for continuous reactions and plasma containment led to projections being extended by several decades. In 2010, more than 60 years after the first attempts, commercial power production was still believed to be unlikely before 2050.

Late Heavy Bombardment

From Wikipedia, the free encyclopedia

Artist's impression of the Moon during the Late Heavy Bombardment (above) and today (below).

The Late Heavy Bombardment (abbreviated LHB and also known as the lunar cataclysm) is an event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. During this interval, a disproportionately large number of asteroids are theorized to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars.

The Late Heavy Bombardment happened after the Earth and other rocky planets had formed and accreted most of their mass, but still quite early in Earth's history.

Evidence for the LHB derives from lunar samples brought back by the Apollo astronauts. Isotopic dating of Moon rocks implies that most impact melts occurred in a rather narrow interval of time. Several hypotheses attempt to explain the apparent spike in the flux of impactors (i.e. asteroids and comets) in the inner Solar System, but no consensus yet exists. The Nice model, popular among planetary scientists, postulates that the giant planets underwent orbital migration and in doing so, scattered objects in the asteroid and/or Kuiper belts into eccentric orbits, and into the path of the terrestrial planets. Other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga, and that the apparent clustering of impact-melt ages near this time is an artifact of sampling materials retrieved from a single large impact basin. They also note that the rate of impact cratering could differ significantly between the outer and inner zones of the Solar System.

Evidence for a cataclysm

The main piece of evidence for a lunar cataclysm comes from the radiometric ages of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are believed to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter. The Apollo 15, 16, and 17 landing sites were chosen as a result of their proximity to the Imbrium, Nectaris, and Serenitatis basins, respectively. 

The apparent clustering of ages of these impact melts, between about 3.8 and 4.1 Ga, led to postulation that the ages record an intense bombardment of the Moon. They called it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on stratigraphic grounds. At the time, the conclusion was considered controversial. 

As more data has become available, particularly from lunar meteorites, this theory, while still controversial, has gained in popularity. The lunar meteorites are believed to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the feldspathic lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9 Ga. Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9 Ga.

Dating of howardite, eucrite and diogenite (HED) meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga. The 3.4–4.1 Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode reveal that the volume of impact melt increases 100–1,000 times as the impact velocity increases from the current asteroid belt average of 5 km/s to 10 km/s. Impact velocities above 10 km/s require very high inclinations or the large eccentricities of asteroids on planet crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment. If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, Caloris, is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.

Criticisms of the cataclysm hypothesis

While the cataclysm hypothesis has recently gained in popularity, particularly among dynamicists who have identified possible causes for such a phenomenon, the cataclysm hypothesis is still controversial and based on debatable assumptions. Two criticisms are that (1) the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, and (2) that the lack of impact melt rocks older than about 4.1 Ga is related to all such samples having been pulverized, or their ages being reset. 

The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been commonly attributed to having been derived from the closest basin, it has been argued that a large portion of these might instead be derived from the Imbrium basin. The Imbrium impact basin is the youngest and largest of the multi-ring basins found on the central nearside of the Moon, and quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites. According to this alternative hypothesis, the cluster of impact melt ages near 3.9 Ga simply reflects material being collected from a single impact event, Imbrium, and not several. Additional criticism also argues that the age spike at 3.9 Ga identified in 40Ar/39Ar dating could also be produced by an episodic early crust formation followed by partial 40Ar losses as the impact rate declined.

A second criticism concerns the significance of the lack of impact melt rocks older than about 4.1 Ga. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their ages have all been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could all have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods. Latest reinterpretation of crater statistics suggests that the flux on the Moon and on Mars may have been lower in general. Thus, the recorded crater population can be explained without any peak in the earliest bombardment of the inner Solar System.

Geological consequences on Earth

If a cataclysmic cratering event truly occurred on the Moon, the Earth would have been affected as well. Extrapolating lunar cratering rates to Earth at this time suggests that the following number of craters would have formed:

• 22,000 or more impact craters with diameters >20 km (12 mi),
• about 40 impact basins with diameters about 1,000 km (620 mi),
• several impact basins with diameters about 5,000 km (3,100 mi),

Before the formulation of the LHB theory, geologists generally assumed that the Earth remained molten until about 3.8 Ga. This date could be found in many of the oldest-known rocks from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, uranium–lead dating of zircons. As no older rocks could be found, it was generally assumed that the Earth had remained molten until this date, which defined the boundary between the earlier Hadean and later Archean eons. Nonetheless, more recently, in 1999, the oldest known rock on Earth was dated to be 4.031 ± 0.003 billion years old, and is part of the Acasta Gneiss of the Slave Craton in northwestern Canada.

Older rocks could be found, however, in the form of asteroid fragments that fall to Earth as meteorites. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6 Ga, which is assumed to be the time when the first solids formed in the protoplanetary disk around the then-young Sun. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of the Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and the slow cooling of the Earth into a solid body as the gravitational potential energy of accretion was released.

Later calculations showed that the rate of collapse and cooling depends on the size of the rocky body. Scaling this rate to an object of Earth mass suggested very rapid cooling, requiring only 100 million years. The difference between measurement and theory presented a conundrum at the time.

The LHB offers a potential explanation for this anomaly. Under this model, the rocks dating to 3.8 Ga solidified only after much of the crust was destroyed by the LHB. Collectively, the Acasta Gneiss in the North American cratonic shield and the gneisses within the Jack Hills portion of the Narryer Gneiss Terrane in Western Australia are the oldest continental fragments on Earth, yet they appear to post-date the LHB. The oldest mineral yet dated on Earth, a 4.404 Ga zircon from Jack Hills, predates this event, but it is likely a fragment of crust left over from before the LHB, contained within a much younger (~3.8 Ga old) rock.

The Jack Hills zircon led to something of a revolution in our understanding of the Hadean eon. Older references generally show that Hadean Earth had a molten surface with prominent volcanos. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek Hades. Zircon dating suggested, albeit controversially, that the Hadean surface was solid, temperate, and covered by acidic oceans. This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see Cool early Earth).

Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in Greenland were a relic of organic matter. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3.8 Ga old, and others suggesting a more "modest" 3.6 Ga. In either case it was a very short time for abiogenesis to have taken place, and if Schidlowski was correct, arguably too short a time. The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible; life either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the Hadean. Recent studies suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3.85 Ga, suggesting the latter possibility is the most likely answer. More recent studies have found no evidence for the isotopically light carbon ratios that were the basis for the original claims.

More recently, a similar study of Jack Hills rocks shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the University of Münster studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4.25 Ga. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life.

Three-dimensional computer models developed in May 2009 by a team at the University of Colorado at Boulder postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of the Earth would have been sterilized, hydrothermal vents below the Earth's surface could have incubated life by providing a sanctuary for heat-loving microbes.

In April 2014, scientists reported finding evidence of the largest terrestrial meteor impact event to date near the Barberton Greenstone Belt. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37 to 58 kilometres (23 to 36 miles) wide. The crater from this event, if it still exists, has not yet been found.

Possible causes

Giant-planet migration

Simulation showing outer planets and planetesimal belt: a) Early configuration, before Jupiter (green) and Saturn (orange) reach 2:1 resonance b) Scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue) c) After ejection of planetesimals by planets.

 

In the Nice model the Late Heavy Bombardment is the result of a dynamical instability in the outer Solar System. The original Nice model simulations by Gomes et al. began with the Solar System's giant planets in a tight orbital configuration surrounded by a rich trans-Neptunian belt. Objects from this belt stray into planet crossing orbits causing the orbits of the planets to migrate over several hundred million years. Jupiter and Saturn's orbits drift apart slowly until they cross a 2:1 orbital resonance causing the eccentricities of their orbits to increase. The orbits of the planets become unstable and Uranus and Neptune are scattered onto wider orbits that disrupt the outer belt, causing a bombardment of comets as they enter planet crossing orbits. Interactions between the objects and the planets also drive a faster migration of Jupiter and Saturn's orbits. This migration causes resonances to sweep through the asteroid belt, increasing the eccentricities of many asteroids until they enter the inner Solar System and impact the terrestrial planets.

The Nice model has undergone some modification since its initial publication. The giant planets now begin in a multi-resonant configuration due an early gas-driven migration through the protoplanetary disk. Interactions with the trans-Neptunian belt allow their escape from the resonances after several hundred million years. The encounters between planets that follow include one between an ice giant and Saturn the that propels the ice giant onto a Jupiter-crossing orbit followed by an encounter with Jupiter which drives the ice giant outward. This jumping-Jupiter scenario quickly increases the separation of Jupiter and Saturn, limiting the effects of resonance sweeping on the asteroids and the terrestrial planets. While this is required to preserve the low eccentricities of the terrestrial planets and avoid leaving the asteroid belt with too many high eccentricity asteroids, it also reduces the fraction of asteroids removed from the main asteroid belt, leaving a now nearly depleted inner band of asteroids as the primary source of the impactors of the LHB. The ice giant is often ejected following its encounter with Jupiter leading some to propose that the Solar System began with five giant planets. Recent works, however, have found that impacts from this inner asteroid belt would be insufficient to explain the formation of ancient impact spherule beds and the lunar basins, and that the asteroid belt was probably not the source of the Late Heavy Bombardment.

Late Uranus/Neptune formation

According to one planetesimal simulation of the establishment of the planetary system, the outermost planets Uranus and Neptune formed very slowly, over a period of several billion years. Harold Levison and his team have also suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion. This "late appearance" of these planets has therefore been suggested as a different reason for the LHB. However, recent calculations of gas-flows combined with planetesimal runaway growth in the outer Solar System imply that Jovian planets formed extremely rapidly, on the order of 10 My, which does not support this explanation for the LHB.

Planet V hypothesis

The Planet V hypothesis posits that a fifth terrestrial planet created the Late Heavy Bombardment when its meta-stable orbit entered the inner asteroid belt. The hypothetical fifth terrestrial planet, Planet V, had a mass less than half of Mars and originally orbited between Mars and the asteroid belt. Planet V's orbit became unstable due to perturbations from the other inner planets causing it to intersect the inner asteroid belt. After close encounters with Planet V, many asteroids entered Earth-crossing orbits producing the Late Heavy Bombardment. Planet V was ultimately lost, likely plunging into the Sun. In numerical simulations, an uneven distribution of asteroids, with the asteroids heavily concentrated toward the inner asteroid belt, has been shown to be necessary to produce the LHB via this mechanism. An alternate version of this hypothesis in which the lunar impactors are debris resulting from Planet V impacting Mars, forming the Borealis Basin, has been proposed to explain a low number of giant lunar basins relative to craters and a lack of evidence of cometary impactors.

Disruption of Mars-crossing asteroid

A hypothesis proposed by Matija Ćuk posits that the last few basin-forming impacts were the result of the collisional disruption of a large Mars-crossing asteroid. This Vesta-sized asteroid was a remnant of a population which initially was much larger than the current main asteroid belt. Most of the pre-Imbrium impacts would have been due to these Mars-crossing objects, with the early bombardment extending until 4.1 billion years ago. A lull in basin-forming impacts then followed during which the lunar magnetic field decayed. Then roughly 3.9 billion years ago a catastrophic impact disrupted the Vesta-sized asteroid radically increasing the population of Mars-crossing objects. Many of these objects then evolved onto Earth-crossing orbits producing a spike in the lunar impact rate during which the last few lunar impact basins are formed. Ćuk points to the weak or absent residual magnetism of the last few basins and a change in the size-frequency distribution of craters which formed during this late bombardment as evidence supporting this hypothesis. The timing and the cause of the change in the size-frequency distribution of craters is controversial.

Other potential sources

A number of other possible sources of the Late Heavy Bombardment have been investigated. Among these are additional Earth satellites orbiting independently or as lunar trojans, planetesimals left over from the formations of the terrestrial planets, Earth or Venus co-orbitals, and the breakup of a large main belt asteroid. Additional Earth satellites on independent orbits were shown to be quickly captured into resonances during the Moon's early tidally-driven orbital expansion and were lost or destroyed within in a few million years Lunar trojans were found to be destabilized within 100 million years by a solar resonance when the Moon reached 27 Earth radii. Planetesimals left over from the formation of the terrestrial planets were shown to be depleted too rapidly due to collisions and ejections to form the last lunar basins. The long-term stability of primordial Earth or Venus co-orbitals (trojans or objects with horseshoe orbits) in conjunction with the lack of current observations indicate that they were unlikely to have been common enough to contribute to the LHB. Producing the LHB from the collisional disruption of a main belt asteroid was found to require at minimum a 1,000–1,500 km parent body with the most favorable initial conditions. Debris produced by collisions among inner planets, now lost, has also been proposed as a source of the LHB.

Exosystem with possible Late Heavy Bombardment

Evidence has been found for Late Heavy Bombardment-like conditions around the star Eta Corvi.