Nuclear renaissance

From Wikipedia, the free encyclopedia

 

For miniatures wargame, see Nuclear Renaissance (wargame).

 

Timeline of commissioned and decommissioned nuclear capacity since 1970

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.

In the 2009 World Energy Outlook, the International Energy Agency stated that:

A nuclear renaissance is possible but cannot occur overnight. Nuclear projects face significant hurdles, including extended construction periods and related risks, long licensing processes and manpower shortages, plus long‐standing issues related to waste disposal, proliferation and local opposition. The financing of new nuclear power plants, especially in liberalized markets, has always been difficult and the financial crisis seems almost certain to have made it even more so. The huge capital requirements, combined with risks of cost overruns and regulatory uncertainties, make investors and lenders very cautious, even when demand growth is robust.

The World Nuclear Association reported that nuclear electricity generation in 2012 was at its lowest level since 1999.

In 2015:

• Ten new reactors were connected to the grid, the highest number since 1990, but expanding Asian nuclear programs are balanced by retirements of aging plants and nuclear reactor phase-outs.
• Seven reactors were permanently shut down.
• 441 operational reactors had a worldwide net capacity of 382,855 megawatts of electricity. However, some reactors are classified as operational, but are not producing any power.
• 67 new nuclear reactors were under construction, including four EPR units. The first two EPR projects, in Finland and France, were meant to lead a nuclear renaissance but both are facing costly construction delays. Construction commenced on two Chinese EPR units in 2009 and 2010. The Chinese units were to start operation in 2014 and 2015, but the Chinese government halted construction because of safety concerns.

March 2017 saw a setback for nuclear renaissance when producer of the AP1000 reactor Westinghouse Electric Company filed for Chapter 11 bankruptcy protection. Four months later the bankruptcy together with delays and cost overruns caused cancellation of the two AP1000 reactors under construction at the Virgil C. Summer Nuclear Generating Station.

History

In 2009 annual generation of nuclear power has been on a slight downward trend since 2007, decreasing 1.8% in 2009 to 2558 TWh with nuclear power meeting 13–14% of the world's electricity demand. A major factor in the decrease has been the prolonged repair of seven large reactors at the Kashiwazaki-Kariwa Nuclear Power Plant in Japan following the Niigata-Chuetsu-Oki earthquake.

In March 2011 the nuclear accidents at Japan's Fukushima I Nuclear Power Plant and shutdowns at other nuclear facilities raised questions among some commentators over the future of the renaissance. Platts has reported that "the crisis at Japan's Fukushima nuclear plants has prompted leading energy-consuming countries to review the safety of their existing reactors and cast doubt on the speed and scale of planned expansions around the world". In 2011 Siemens exited the nuclear power sector following the Fukushima disaster and subsequent changes to German energy policy, and supported the German government's planned energy transition to renewable energy technologies. China, Germany, Switzerland, Israel, Malaysia, Thailand, United Kingdom, Italy and the Philippines have reviewed their nuclear power programs. Indonesia and Vietnam still plan to build nuclear power plants. Countries such as Australia, Austria, Denmark, Greece, Ireland, Latvia, Liechtenstein, Luxembourg, Portugal, Israel, Malaysia, New Zealand, and Norway remain opposed to nuclear power. Following the Fukushima I nuclear accidents, the International Energy Agency halved its estimate of additional nuclear generating capacity built by 2035.

The World Nuclear Association has reported that “nuclear power generation suffered its biggest ever one-year fall through 2012 as the bulk of the Japanese fleet remained offline for a full calendar year”. Data from the International Atomic Energy Agency showed that nuclear power plants globally produced 2346 TWh of electricity in 2012 – seven per cent less than in 2011. The figures illustrate the effects of a full year of 48 Japanese power reactors producing no power during the year. The permanent closure of eight reactor units in Germany was also a factor. Problems at Crystal River, Fort Calhoun and the two San Onofre units in the USA meant they produced no power for the full year, while in Belgium Doel 3 and Tihange 2 were out of action for six months. Compared to 2010, the nuclear industry produced 11% less electricity in 2012.

As of July 2013, "a total of 437 nuclear reactors were operating in 30 countries, seven fewer than the historical maximum of 444 in 2002. Since 2002, utilities have started up 28 units and disconnected 36 including six units at the Fukushima Daiichi nuclear power plant in Japan. The 2010 world reactor fleet had a total nominal capacity of about 370 gigawatts (or thousand megawatts). Despite seven fewer units operating in 2013 than in 2002, the capacity is still about 7 gigawatts higher". The numbers of new operative reactors, final shutdowns and new initiated constructions according to International Atomic Energy Agency (IAEA) in 2010 are as follows:

Overview

A total of 72 reactors were under construction at the beginning of 2014, the highest number in 25 years. Several of the under construction reactors are carry over from earlier eras; some are partially completed reactors on which work has resumed (e.g., in Argentina); some are small and experimental (e.g., Russian floating reactors); and some have been on the IAEA's “under construction” list for years (e.g., in India and Russia). Reactor projects in Eastern Europe are essentially replacing old Soviet reactors shut down due to safety concerns. Most of the 2010 activity ― 30 reactors ― is taking place in four countries: China, India, Russia and South Korea. Turkey, the United Arab Emirates and Iran are the only countries that are currently building their first power reactors, Iran's construction began decades ago.

Eight German nuclear power reactors (Biblis A and B, Brunsbuettel, Isar 1, Kruemmel, Neckarwestheim 1, Philippsburg 1 and Unterweser) were permanently shutdown on August 6, 2011, following the Japanese Fukushima nuclear disaster.

Various barriers to a nuclear renaissance have been suggested. These include: unfavourable economics compared to other sources of energy, slowness in addressing climate change, industrial bottlenecks and personnel shortages in the nuclear sector, and the contentious issue of what to do with nuclear waste or spent nuclear fuel. There are also concerns about more nuclear accidents, security, and nuclear weapons proliferation.

New reactors under construction in Finland and France, which were meant to lead a nuclear renaissance, have been delayed and are running over-budget. China has 22 new reactors under construction, and there are also a considerable number of new reactors being built in South Korea, India, and Russia. At the same time, at least 100 older and smaller reactors will "most probably be closed over the next 10–15 years". So the expanding nuclear programs in Asia are balanced by retirements of aging plants and nuclear reactor phase-outs.

A study by UBS, reported on April 12, 2011, predicts that around 30 nuclear plants may be closed worldwide, with those located in seismic zones or close to national boundaries being the most likely to shut. The analysts believe that 'even pro-nuclear countries such as France will be forced to close at least two reactors to demonstrate political action and restore the public acceptability of nuclear power', noting that the events at Fukushima 'cast doubt on the idea that even an advanced economy can master nuclear safety'. In September 2011, German engineering giant Siemens announced it will withdraw entirely from the nuclear industry, as a response to the Fukushima nuclear disaster in Japan.

The 2011 World Energy Outlook report by the International Energy Agency stated that having "second thoughts on nuclear would have far-reaching consequences" and that a substantial shift away from nuclear power would boost demand for fossil fuels, putting additional upward pressure on the price of energy, raising additional concerns about energy security, and making it more difficult and expensive to combat climate change. The reports suggests that the consequences would be most severe for nations with limited local energy resources and which have been planning to rely heavily on nuclear power for future energy security, and that it would make it substantially more challenging for developing economies to satisfy their rapidly increasing demand for electricity.

John Rowe, chair of Exelon (the largest nuclear power producer in the US), has said that the nuclear renaissance is "dead". He says that solar, wind and cheap natural gas have significantly reduced the prospects of coal and nuclear power plants around the world. Amory Lovins says that the sharp and steady cost reductions in solar power has been a "stunning market success".

In 2013 the analysts at the investment research firm Morningstar, Inc. concluded that nuclear power was not a viable source of new power in the West. On nuclear renaissance they wrote:

The economies of scale experienced in France during its initial build-out and the related strength of supply chain and labor pool were imagined by the dreamers who have coined the term ‘nuclear renaissance’ for the rest of the world. But outside of China and possibly South Korea this concept seems a fantasy, as should become clearer examining even theoretical projections for new nuclear build today.

Economics

Nuclear power plants are large construction projects with very high up-front costs. The cost of capital is also steep due to the risk of construction delays and obstructing legal action. The large capital cost of nuclear power has been a key barrier to the construction of new reactors around the world, and the economics have recently worsened, as a result of the global financial crisis. As the OECD's Nuclear Energy Agency points out, "investors tend to favor less capital intensive and more flexible technologies". This has led to a large increase in the use of natural gas for base-load power production, often using more sophisticated combined cycle plants.

Accidents and safety

Following the 2011 Japanese Fukushima nuclear disaster, authorities shut down the nation's 54 nuclear power plants. As of 2013, the Fukushima site remains highly radioactive, with some 160,000 evacuees still living in temporary housing, and some land will be unfarmable for centuries. The difficult cleanup job will take 40 or more years, and cost tens of billions of dollars.

Major nuclear reactor accidents include Three Mile Island accident (1979), Chernobyl disaster (1986), and Fukushima (2011). A report in Lancet says that the effects of these accidents on individuals and societies are diverse and enduring. Relatively few immediate deaths have occurred, but nuclear-related fatalities are mostly in the hazardous uranium mining industry, which supplies fuel to nuclear reactors. There are also physical health problems directly attributable to radiation exposure, as well as psychological and social effects. The Fukushima accident forced more than 80,000 residents to evacuate from neighborhoods around the crippled nuclear plant. Evacuation and long-term displacement create severe health-care problems for the most vulnerable people, such as hospital inpatients and elderly people.

Charles Perrow, in his book Normal accidents says that multiple and unexpected failures are built into complex and tightly coupled systems, such as nuclear power plants. Such accidents often involve operator error and are unavoidable and cannot be designed around. Since the terrorist attacks of September 11, 2001, there has been heightened concern that nuclear power plants may be targeted by terrorists or criminals, and that nuclear materials may be purloined for use in nuclear or radiological weapons.

Nevertheless, newer reactor designs intended to provide increased safety have been developed over time. The next nuclear plants to be built will likely be Generation III or III+ designs, and a few are being built in Japan. However, safety risks may be the greatest when nuclear systems are the newest, and operators have less experience with them. Nuclear engineer David Lochbaum explained that almost all serious nuclear accidents 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".

Controversy

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

In 2008 there were reports of a revival of the anti-nuclear movement in Germany and protests in France during 2004 and 2007. Also in 2008 in the United States, there were protests about, and criticism of, several new nuclear reactor proposals and later some objections to license renewals for existing nuclear plants.

Public opinion

Global public support for energy sources, based on a survey by Ipsos (2011) taken 2 months after the Fukushima Disaster.

In 2005, the International Atomic Energy Agency presented the results of a series of public opinion surveys in the Global Public Opinion on Nuclear Issues report. Majorities of respondents in 14 of the 18 countries surveyed believe that the risk of terrorist acts involving radioactive materials at nuclear facilities is high, because of insufficient protection. While majorities of citizens generally support the continued use of existing nuclear power reactors, most people do not favour the building of new nuclear plants, and 25% of respondents feel that all nuclear power plants should be closed down. Stressing the climate change benefits of nuclear energy positively influences 10% of people to be more supportive of expanding the role of nuclear power in the world, but there is still a general reluctance to support the building of more nuclear power plants. After the Fukushima Disaster, Civil Society Institute (CSI) found out that 58 percent of the respondents indicated less support of using nuclear power in the United States. Two-thirds of the respondents said they would protest the construction of a nuclear reactor within 50 miles of their homes.

There was little support across the world for building new nuclear reactors, a 2011 poll for the BBC indicates. The global research agency GlobeScan, commissioned by BBC News, polled 23,231 people in 23 countries from July to September 2011, several months after the Fukushima nuclear disaster. In countries with existing nuclear programmes, people are significantly more opposed than they were in 2005, with only the UK and US bucking the trend. Most believe that boosting energy efficiency and renewable energy can meet their needs.

By region and country

Africa

As of March 2010, ten African nations had begun exploring plans to build nuclear reactors.

South Africa (which has two nuclear power reactors), however, removed government funding for its planned new PBMRs in 2010.

America

United States

George W. Bush signing the Energy Policy Act of 2005, which was designed to promote US nuclear reactor construction, through incentives and subsidies, including cost-overrun support up to a total of $2 billion for six new nuclear plants.

Between 2007 and 2009, 13 companies applied to the Nuclear Regulatory Commission for construction and operating licenses to build 30 new nuclear power reactors in the United States. However, the case for widespread nuclear plant construction was eroded due to abundant natural gas supplies, slow electricity demand growth in a weak US economy, lack of financing, and uncertainty following the Fukushima nuclear disaster. Many license applications for proposed new reactors were suspended or cancelled. Only a few new reactors will enter service by 2020. These will not be the cheapest energy options available, but they are an attractive investment for utilities because the government mandates that taxpayers pay for construction in advance. In 2013, four aging, uncompetitive, reactors were permanently closed: San Onofre 2 and 3 in California, Crystal River 3 in Florida, and Kewaunee in Wisconsin. Vermont Yankee, in Vernon, is scheduled to close in 2014, following many protests. New York State is seeking to close Indian Point Energy Center, in Buchanan, 30 miles from New York City.

Neither climate change abatement, nor the Obama Administration's endorsement of nuclear power with $18.5 billion in loan guarantees, have been able to propel nuclear power in the US past existing obstacles. The Fukushima nuclear disaster has not helped either.

As of 2014, the U.S. nuclear industry began a new lobbying effort, hiring three former senators — Evan Bayh, a Democrat; Judd Gregg, a Republican; and Spencer Abraham, a Republican — as well as William M. Daley, a former staffer to President Obama. The initiative is called Nuclear Matters, and it has begun a newspaper advertising campaign.

Locations of new US reactors and their scheduled operating dates are:

• Tennessee, Watts Bar unit 2 in operation since October 2016
• Georgia, Vogtle Electric unit 3 planned to be operational 2021, unit 4 planned to be operational 2022

On 29 March 2017, parent company Toshiba placed Westinghouse Electric Company in Chapter 11 bankruptcy because of US$9 billion of losses from its nuclear reactor construction projects. The projects responsible for this loss are mostly the construction of four AP1000 reactors at Vogtle in Georgia and V. C. Summer in South Carolina. The U.S. government had given $8.3 billion of loan guarantees on the financing of the four nuclear reactors being built in the U.S. The plans at V. C. Summer have been cancelled, whereas construction at Vogtle continues. Peter A. Bradford, former U.S. Nuclear Regulatory Commission member, commented "They placed a big bet on this hallucination of a nuclear renaissance".

Asia

As of 2008, the greatest growth in nuclear generation was expected to be in China, Japan, South Korea and India.

As of early 2013 China had 17 nuclear reactors operating and 32 under construction, with more planned. "China is rapidly becoming self-sufficient in reactor design and construction, as well as other aspects of the fuel cycle." However, according to a government research unit, China must not build "too many nuclear power reactors too quickly", in order to avoid a shortfall of fuel, equipment and qualified plant workers.

Following the Fukushima disaster, many are questioning the mass roll-out of new plants in India, including the World Bank, the Indian Environment Minister, Jairam Ramesh, and the former head of the country's nuclear regulatory body, A. Gopalakrishnan. The massive Jaitapur Nuclear Power Project is the focus of concern – "931 hectares of farmland will be needed to build the reactors, land that is now home to 10,000 people, their mango orchards, cashew trees and rice fields". Fishermen in the region say their livelihoods will be wiped out.

South Korea is exploring nuclear projects with a number of nations.

Australia

Australia is a major producer of uranium, which it exports as uranium oxide to nuclear power generating nations. Australia has a single research reactor at Lucas Heights, but does not generate electricity via nuclear power. As of 2015, the majority of the nation's uranium mines are in South Australia, where a Nuclear Fuel Cycle Royal Commission is investigating the opportunities and costs of expanding the state's role in the nuclear fuel cycle. As of January 2016, new nuclear industrial development (other than the mining of uranium) is prohibited by various acts of federal and state legislation. The Federal government will consider the findings of the South Australian Royal Commission after it releases its findings in 2016.

Canada

With several CANDU reactors facing closure selected plants will be completely refurbished between 2016 and 2026, extending their operation beyond 2050.

Europe

EDF has said its third-generation EPR Flamanville 3 project (seen here in 2010) will be delayed until 2018, due to "both structural and economic reasons," and the project's total cost has climbed to EUR 11 billion in 2012. Similarly, the cost of the EPR being built at Olkiluoto, Finland has escalated dramatically, and the project is well behind schedule. The initial low cost forecasts for these megaprojects exhibited "optimism bias".

On 18 October 2010 the British government announced eight locations it considered suitable for future nuclear power stations. This has resulted in public opposition and protests at some of the sites. In March 2012, two of the big six power companies announced they would be pulling out of developing new nuclear power plants. The decision by RWE npower and E.ON follows uncertainty over nuclear energy following the Fukushima nuclear disaster last year. The companies will not proceed with their Horizon project, which was to develop nuclear reactors at Wylfa in North Wales and at Oldbury-on-Severn in Gloucestershire. Their decision follows a similar announcement by Scottish and Southern Electricity last year. Analysts said the decision meant the future of UK nuclear power could now be in doubt.

The 2011 Japanese Fukushima nuclear disaster has led some European energy officials to "think twice about nuclear expansion". Switzerland has abandoned plans to replace its old nuclear reactors and will take the last one offline in 2034. Anti-nuclear opposition intensified in Germany. In the following months the government decided to shut down eight reactors immediately (August 6, 2011) and to have the other nine off the grid by the end of 2022. Renewable energy in Germany is believed to be able to compensate for much of the loss. In September 2011 Siemens, which had been responsible for constructing all 17 of Germany's existing nuclear power plants, announced that it would exit the nuclear sector following the Fukushima disaster and the subsequent changes to German energy policy. Chief executive Peter Loescher has supported the German government's planned energy transition to renewable energy technologies, calling it a "project of the century" and saying Berlin's target of reaching 35% renewable energy sources by 2020 was feasible.

On October 21, 2013, EDF Energy announced that an agreement had been reached regarding new nuclear plants to be built on the site of Hinkley Point C. EDF Group and the UK Government agreed on the key commercial terms of the investment contract. The final investment decision is still conditional on completion of the remaining key steps, including the agreement of the EU Commission.

In February 2014, Amory Lovins commented that:

Britain's plan for a fleet of new nuclear power stations is … unbelievable ... It is economically daft. The guaranteed price [being offered to French state company EDF] is over seven times the unsubsidised price of new wind in the US, four or five times the unsubsidised price of new solar power in the US. Nuclear prices only go up. Renewable energy prices come down. There is absolutely no business case for nuclear. The British policy has nothing to do with economic or any other rational base for decision making.

Middle East

In December 2009 South Korea won a contract for four nuclear power plants to be built in the United Arab Emirates, for operation in 2017 to 2020.

On March 17, 2011, Israeli Prime Minister Benjamin Netanyahu stated that Israel was now unlikely to pursue civil nuclear energy.

Russia

In April 2010 Russia announced new plans to start building 10 new nuclear reactors in the next year.

Views and opinions

In June 2009, Mark Cooper from the Vermont Law School said: "The highly touted renaissance of nuclear power is based on fiction, not fact... There are numerous options available to meet the need for electricity in a carbon-constrained environment that are superior to building nuclear reactors".

In September 2009, Luc Oursel, chief executive of Areva Nuclear Plants (the core nuclear reactor manufacturing division of Areva) stated: "We are convinced about the nuclear renaissance". Areva has been hiring up to 1,000 people a month, "to prepare for a surge in orders from around the world". However, in June 2010, Standard & Poor's downgraded Areva's debt rating to BBB+ due to weakened profitability.

In 2010, Trevor Findlay from the Centre for International Governance Innovation stated that "despite some powerful drivers and clear advantages, a revival of nuclear energy faces too many barriers compared to other means of generating electricity for it to capture a growing market share to 2030".

In January 2010, the International Solar Energy Society stated that "... it appears that the pace of nuclear plant retirements will exceed the development of the few new plants now being contemplated, so that nuclear power may soon start on a downward trend. It will remain to be seen if it has any place in an affordable future world energy policy".

In March 2010, Steve Kidd from the World Nuclear Association said: "Proof of whether the mooted nuclear renaissance is merely 'industry hype' as some commentators suggest or reality will come over the next decade". In 2013 Kidd characterised the situation as a nuclear slowdown, requiring the industry to focus on better economics and improving public acceptance.

In August 2010, physicist Michael Dittmar stated that: "Nuclear fission's contribution to total electric energy has decreased from about 18 per cent a decade ago to about 14 per cent in 2008. On a worldwide scale, nuclear energy is thus only a small component of the global energy mix and its share, contrary to widespread belief, is not on the rise".

In March 2011, Alexander Glaser said: "It will take time to grasp the full impact of the unimaginable human tragedy unfolding after the earthquake and tsunami in Japan, but it is already clear that the proposition of a global nuclear renaissance ended on that day".

In 2011, Benjamin K. Sovacool said: "The nuclear waste issue, although often ignored in industry press releases and sponsored reports, is the proverbial elephant in the room stopping a nuclear renaissance".

 

Enriched uranium

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Enriched_uranium 

 

Proportions of uranium-238 (blue) and uranium-235 (red) found naturally versus enriched grades

Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written ²³⁵U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (²³⁸U with 99.2739–99.2752% natural abundance), uranium-235 (²³⁵U, 0.7198–0.7202%), and uranium-234 (²³⁴U, 0.0050–0.0059%). ²³⁵U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons.

Enriched uranium is a critical component for both civil nuclear power generation and military nuclear weapons. The International Atomic Energy Agency attempts to monitor and control enriched uranium supplies and processes in its efforts to ensure nuclear power generation safety and curb nuclear weapons proliferation.

There are about 2,000 tonnes of highly enriched uranium in the world, produced mostly for nuclear power, nuclear weapons, naval propulsion, and smaller quantities for research reactors.

The ²³⁸U remaining after enrichment is known as depleted uranium (DU), and is considerably less radioactive than even natural uranium, though still very dense and extremely hazardous in granulated form – such granules are a natural by-product of the shearing action that makes it useful for armor-penetrating weapons. Despite being mildly radioactive, depleted uranium is also an effective radiation shielding material.

Contents

• 1 Grades
  • 1.1 Reprocessed uranium (RepU)
  • 1.2 Low enriched uranium (LEU)
  • 1.3 Highly enriched uranium (HEU)
• 2 Enrichment methods
  • 2.1 Diffusion techniques
  • 2.2 Centrifuge techniques
  • 2.3 Laser techniques
  • 2.4 Other techniques
• 3 Separative work unit
• 4 Cost issues
• 5 Downblending
• 6 Global enrichment facilities
• 7 Codename
• 8 See also
• 9 References
• 10 External links

Grades

Uranium as it is taken directly from the Earth is not suitable as fuel for most nuclear reactors and requires additional processes to make it usable. Uranium is mined either underground or in an open pit depending on the depth at which it is found. After the uranium ore is mined, it must go through a milling process to extract the uranium from the ore.

This is accomplished by a combination of chemical processes with the end product being concentrated uranium oxide, which is known as "yellowcake", contains roughly 60% uranium whereas the ore typically contains less than 1% uranium and as little as 0.1% uranium.

After the milling process is complete, the uranium must next undergo a process of conversion, "to either uranium dioxide, which can be used as the fuel for those types of reactors that do not require enriched uranium, or into uranium hexafluoride, which can be enriched to produce fuel for the majority of types of reactors". Naturally-occurring uranium is made of a mixture of ²³⁵U and ²³⁸U. The ²³⁵U is fissile, meaning it is easily split with neutrons while the remainder is ²³⁸U, but in nature, more than 99% of the extracted ore is ²³⁸U. Most nuclear reactors require enriched uranium, which is uranium with higher concentrations of ²³⁵U ranging between 3.5% and 4.5% (although a few reactor designs using a graphite or heavy water moderator, such as the RBMK and CANDU, are capable of operating with natural uranium as fuel). There are two commercial enrichment processes: gaseous diffusion and gas centrifugation. Both enrichment processes involve the use of uranium hexafluoride and produce enriched uranium oxide.

A drum of yellowcake (a mixture of uranium precipitates)

Reprocessed uranium (RepU)

Reprocessed uranium (RepU) is a product of nuclear fuel cycles involving nuclear reprocessing of spent fuel. RepU recovered from light water reactor (LWR) spent fuel typically contains slightly more ²³⁵U than natural uranium, and therefore could be used to fuel reactors that customarily use natural uranium as fuel, such as CANDU reactors. It also contains the undesirable isotope uranium-236, which undergoes neutron capture, wasting neutrons (and requiring higher ²³⁵U enrichment) and creating neptunium-237, which would be one of the more mobile and troublesome radionuclides in deep geological repository disposal of nuclear waste.

Low enriched uranium (LEU)

Low enriched uranium (LEU) has a lower than 20% concentration of ²³⁵U; for instance, in commercial LWR, the most prevalent power reactors in the world, uranium is enriched to 3 to 5% ²³⁵U. High-assay LEU (HALEU) is enriched from 5–20%. Fresh LEU used in research reactors is usually enriched 12 to 19.75% ²³⁵U, the latter concentration is used to replace HEU fuels when converting to LEU.

Highly enriched uranium (HEU)

A billet of highly enriched uranium metal

Highly enriched uranium (HEU) has a 20% or higher concentration of ²³⁵U. The fissile uranium in nuclear weapon primaries usually contains 85% or more of ²³⁵U known as weapons-grade, though theoretically for an implosion design, a minimum of 20% could be sufficient (called weapon-usable) although it would require hundreds of kilograms of material and "would not be practical to design"; even lower enrichment is hypothetically possible, but as the enrichment percentage decreases the critical mass for unmoderated fast neutrons rapidly increases, with for example, an infinite mass of 5.4% ²³⁵U being required. For criticality experiments, enrichment of uranium to over 97% has been accomplished.

The very first uranium bomb, Little Boy, dropped by the United States on Hiroshima in 1945, used 64 kilograms of 80% enriched uranium. Wrapping the weapon's fissile core in a neutron reflector (which is standard on all nuclear explosives) can dramatically reduce the critical mass. Because the core was surrounded by a good neutron reflector, at explosion it comprised almost 2.5 critical masses. Neutron reflectors, compressing the fissile core via implosion, fusion boosting, and "tamping", which slows the expansion of the fissioning core with inertia, allow nuclear weapon designs that use less than what would be one bare-sphere critical mass at normal density. The presence of too much of the ²³⁸U isotope inhibits the runaway nuclear chain reaction that is responsible for the weapon's power. The critical mass for 85% highly enriched uranium is about 50 kilograms (110 lb), which at normal density would be a sphere about 17 centimetres (6.7 in) in diameter.

Later US nuclear weapons usually use plutonium-239 in the primary stage, but the jacket or tamper secondary stage, which is compressed by the primary nuclear explosion often uses HEU with enrichment between 40% and 80% along with the fusion fuel lithium deuteride. For the secondary of a large nuclear weapon, the higher critical mass of less-enriched uranium can be an advantage as it allows the core at explosion time to contain a larger amount of fuel. The ²³⁸U is not said to be fissile but still is fissionable by fast neutrons (>2 MeV) such as the ones produced during D-T fusion.

HEU is also used in fast neutron reactors, whose cores require about 20% or more of fissile material, as well as in naval reactors, where it often contains at least 50% ²³⁵U, but typically does not exceed 90%. The Fermi-1 commercial fast reactor prototype used HEU with 26.5% ²³⁵U. Significant quantities of HEU are used in the production of medical isotopes, for example molybdenum-99 for technetium-99m generators.

Enrichment methods

Isotope separation is difficult because two isotopes of the same element have nearly identical chemical properties, and can only be separated gradually using small mass differences. (²³⁵U is only 1.26% lighter than ²³⁸U). This problem is compounded because uranium is rarely separated in its atomic form, but instead as a compound (²³⁵UF₆ is only 0.852% lighter than ²³⁸UF₆). A cascade of identical stages produces successively higher concentrations of ²³⁵U. Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.

There are currently two generic commercial methods employed internationally for enrichment: gaseous diffusion (referred to as first generation) and gas centrifuge (second generation), which consumes only 2% to 2.5% as much energy as gaseous diffusion (at least a "factor of 20" more efficient). Some work is being done that would use nuclear resonance; however there is no reliable evidence that any nuclear resonance processes have been scaled up to production.

Diffusion techniques

Gaseous diffusion

Gaseous diffusion uses semi-permeable membranes to separate enriched uranium

Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (hex) through semi-permeable membranes. This produces a slight separation between the molecules containing ²³⁵U and ²³⁸U. Throughout the Cold War, gaseous diffusion played a major role as a uranium enrichment technique, and as of 2008 accounted for about 33% of enriched uranium production, but in 2011 was deemed an obsolete technology that is steadily being replaced by the later generations of technology as the diffusion plants reach their ends-of-life. In 2013, the Paducah facility in the US ceased operating, it was the last commercial ²³⁵U gaseous diffusion plant in the world.

Thermal diffusion

Thermal diffusion uses the transfer of heat across a thin liquid or gas to accomplish isotope separation. The process exploits the fact that the lighter ²³⁵U gas molecules will diffuse toward a hot surface, and the heavier ²³⁸U gas molecules will diffuse toward a cold surface. The S-50 plant at Oak Ridge, Tennessee was used during World War II to prepare feed material for the EMIS process. It was abandoned in favor of gaseous diffusion.

Centrifuge techniques

Gas centrifuge

A cascade of gas centrifuges at a U.S. enrichment plant

The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder's rotation creates a strong centripetal force so that the heavier gas molecules containing ²³⁸U move tangentially toward the outside of the cylinder and the lighter gas molecules rich in ²³⁵U collect closer to the center. It requires much less energy to achieve the same separation than the older gaseous diffusion process, which it has largely replaced and so is the current method of choice and is termed second generation. It has a separation factor per stage of 1.3 relative to gaseous diffusion of 1.005, which translates to about one-fiftieth of the energy requirements. Gas centrifuge techniques produce close to 100% of the world's enriched uranium.

Zippe centrifuge

Diagram of the principles of a Zippe-type gas centrifuge with U-238 represented in dark blue and U-235 represented in light blue

The Zippe centrifuge is an improvement on the standard gas centrifuge, the primary difference being the use of heat. The bottom of the rotating cylinder is heated, producing convection currents that move the ²³⁵U up the cylinder, where it can be collected by scoops. This improved centrifuge design is used commercially by Urenco to produce nuclear fuel and was used by Pakistan in their nuclear weapons program.

Laser techniques

Laser processes promise lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages. Several laser processes have been investigated or are under development. Separation of isotopes by laser excitation (SILEX) is well developed and is licensed for commercial operation as of 2012.

Atomic vapor laser isotope separation (AVLIS)

Atomic vapor laser isotope separation employs specially tuned lasers to separate isotopes of uranium using selective ionization of hyperfine transitions. The technique uses lasers tuned to frequencies that ionize ²³⁵U atoms and no others. The positively charged ²³⁵U ions are then attracted to a negatively charged plate and collected.

Molecular laser isotope separation (MLIS)

Molecular laser isotope separation uses an infrared laser directed at UF₆, exciting molecules that contain a ²³⁵U atom. A second laser frees a fluorine atom, leaving uranium pentafluoride, which then precipitates out of the gas.

Separation of isotopes by laser excitation (SILEX)

Separation of isotopes by laser excitation is an Australian development that also uses UF₆. After a protracted development process involving U.S. enrichment company USEC acquiring and then relinquishing commercialization rights to the technology, GE Hitachi Nuclear Energy (GEH) signed a commercialization agreement with Silex Systems in 2006. GEH has since built a demonstration test loop and announced plans to build an initial commercial facility. Details of the process are classified and restricted by intergovernmental agreements between United States, Australia, and the commercial entities. SILEX has been projected to be an order of magnitude more efficient than existing production techniques but again, the exact figure is classified. In August, 2011 Global Laser Enrichment, a subsidiary of GEH, applied to the U.S. Nuclear Regulatory Commission (NRC) for a permit to build a commercial plant. In September 2012, the NRC issued a license for GEH to build and operate a commercial SILEX enrichment plant, although the company had not yet decided whether the project would be profitable enough to begin construction, and despite concerns that the technology could contribute to nuclear proliferation.

Other techniques

Aerodynamic processes

Schematic diagram of an aerodynamic nozzle. Many thousands of these small foils would be combined in an enrichment unit.

 

The X-ray based LIGA manufacturing process was originally developed at the Forschungszentrum Karlsruhe, Germany, to produce nozzles for isotope enrichment.

Aerodynamic enrichment processes include the Becker jet nozzle techniques developed by E. W. Becker and associates using the LIGA process and the vortex tube separation process. These aerodynamic separation processes depend upon diffusion driven by pressure gradients, as does the gas centrifuge. They in general have the disadvantage of requiring complex systems of cascading of individual separating elements to minimize energy consumption. In effect, aerodynamic processes can be considered as non-rotating centrifuges. Enhancement of the centrifugal forces is achieved by dilution of UF₆ with hydrogen or helium as a carrier gas achieving a much higher flow velocity for the gas than could be obtained using pure uranium hexafluoride. The Uranium Enrichment Corporation of South Africa (UCOR) developed and deployed the continuous Helikon vortex separation cascade for high production rate low enrichment and the substantially different semi-batch Pelsakon low production rate high enrichment cascade both using a particular vortex tube separator design, and both embodied in industrial plant. A demonstration plant was built in Brazil by NUCLEI, a consortium led by Industrias Nucleares do Brasil that used the separation nozzle process. However all methods have high energy consumption and substantial requirements for removal of waste heat; none are currently still in use.

Electromagnetic isotope separation

Schematic diagram of uranium isotope separation in a calutron shows how a strong magnetic field is used to redirect a stream of uranium ions to a target, resulting in a higher concentration of uranium-235 (represented here in dark blue) in the inner fringes of the stream.

In the electromagnetic isotope separation process (EMIS), metallic uranium is first vaporized, and then ionized to positively charged ions. The cations are then accelerated and subsequently deflected by magnetic fields onto their respective collection targets. A production-scale mass spectrometer named the Calutron was developed during World War II that provided some of the ²³⁵U used for the Little Boy nuclear bomb, which was dropped over Hiroshima in 1945. Properly the term 'Calutron' applies to a multistage device arranged in a large oval around a powerful electromagnet. Electromagnetic isotope separation has been largely abandoned in favour of more effective methods.

Chemical methods

One chemical process has been demonstrated to pilot plant stage but not used for production. The French CHEMEX process exploited a very slight difference in the two isotopes' propensity to change valency in oxidation/reduction, using immiscible aqueous and organic phases. An ion-exchange process was developed by the Asahi Chemical Company in Japan that applies similar chemistry but effects separation on a proprietary resin ion-exchange column.

Plasma separation

Plasma separation process (PSP) describes a technique that makes use of superconducting magnets and plasma physics. In this process, the principle of ion cyclotron resonance is used to selectively energize the ²³⁵U isotope in a plasma containing a mix of ions. The French developed their own version of PSP, which they called RCI. Funding for RCI was drastically reduced in 1986, and the program was suspended around 1990, although RCI is still used for stable isotope separation.

Separative work unit

"Separative work" – the amount of separation done by an enrichment process – is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units that are so calculated as to be proportional to the total input (energy / machine operation time) and to the mass processed. Separative work is not energy. The same amount of separative work will require different amounts of energy depending on the efficiency of the separation technology. Separative work is measured in Separative work units SWU, kg SW, or kg UTA (from the German Urantrennarbeit – literally uranium separation work)

• 1 SWU = 1 kg SW = 1 kg UTA
• 1 kSWU = 1 tSW = 1 t UTA
• 1 MSWU = 1 ktSW = 1 kt UTA

Cost issues

In addition to the separative work units provided by an enrichment facility, the other important parameter to be considered is the mass of natural uranium (NU) that is needed to yield a desired mass of enriched uranium. As with the number of SWUs, the amount of feed material required will also depend on the level of enrichment desired and upon the amount of ²³⁵U that ends up in the depleted uranium. However, unlike the number of SWUs required during enrichment, which increases with decreasing levels of ²³⁵U in the depleted stream, the amount of NU needed will decrease with decreasing levels of ²³⁵U that end up in the DU.

For example, in the enrichment of LEU for use in a light water reactor it is typical for the enriched stream to contain 3.6% ²³⁵U (as compared to 0.7% in NU) while the depleted stream contains 0.2% to 0.3% ²³⁵U. In order to produce one kilogram of this LEU it would require approximately 8 kilograms of NU and 4.5 SWU if the DU stream was allowed to have 0.3% ²³⁵U. On the other hand, if the depleted stream had only 0.2% ²³⁵U, then it would require just 6.7 kilograms of NU, but nearly 5.7 SWU of enrichment. Because the amount of NU required and the number of SWUs required during enrichment change in opposite directions, if NU is cheap and enrichment services are more expensive, then the operators will typically choose to allow more ²³⁵U to be left in the DU stream whereas if NU is more expensive and enrichment is less so, then they would choose the opposite.

Downblending

The opposite of enriching is downblending; surplus HEU can be downblended to LEU to make it suitable for use in commercial nuclear fuel.

The HEU feedstock can contain unwanted uranium isotopes: ²³⁴U is a minor isotope contained in natural uranium; during the enrichment process, its concentration increases but remains well below 1%. High concentrations of ²³⁶U are a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history. HEU reprocessed from nuclear weapons material production reactors (with an ²³⁵U assay of approx. 50%) may contain ²³⁶U concentrations as high as 25%, resulting in concentrations of approximately 1.5% in the blended LEU product. ²³⁶U is a neutron poison; therefore the actual ²³⁵U concentration in the LEU product must be raised accordingly to compensate for the presence of ²³⁶U.

The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt% ²³⁵U may used as a blendstock to dilute the unwanted byproducts that may be contained in the HEU feed. Concentrations of these isotopes in the LEU product in some cases could exceed ASTM specifications for nuclear fuel, if NU, or DU were used. So, the HEU downblending generally cannot contribute to the waste management problem posed by the existing large stockpiles of depleted uranium. At present, 95 percent of the world's stocks of depleted uranium remain in secure storage.^{[citation needed]}

A major downblending undertaking called the Megatons to Megawatts Program converts ex-Soviet weapons-grade HEU to fuel for U.S. commercial power reactors. From 1995 through mid-2005, 250 tonnes of high-enriched uranium (enough for 10,000 warheads) was recycled into low-enriched-uranium. The goal is to recycle 500 tonnes by 2013. The decommissioning programme of Russian nuclear warheads accounted for about 13% of total world requirement for enriched uranium leading up to 2008.

The United States Enrichment Corporation has been involved in the disposition of a portion of the 174.3 tonnes of highly enriched uranium (HEU) that the U.S. government declared as surplus military material in 1996. Through the U.S. HEU Downblending Program, this HEU material, taken primarily from dismantled U.S. nuclear warheads, was recycled into low-enriched uranium (LEU) fuel, used by nuclear power plants to generate electricity.

Global enrichment facilities

The following countries are known to operate enrichment facilities: Argentina, Brazil, China, France, Germany, India, Iran, Japan, the Netherlands, North Korea, Pakistan, Russia, the United Kingdom, and the United States. Belgium, Iran, Italy, and Spain hold an investment interest in the French Eurodif enrichment plant, with Iran's holding entitling it to 10% of the enriched uranium output. Countries that had enrichment programs in the past include Libya and South Africa, although Libya's facility was never operational. Australia has developed a laser enrichment process known as SILEX, which it intends to pursue through financial investment in a U.S. commercial venture by General Electric. It has also been claimed that Israel has a uranium enrichment program housed at the Negev Nuclear Research Center site near Dimona.

Codename

During the Manhattan Project, weapons-grade highly enriched uranium was given the codename oralloy, a shortened version of Oak Ridge alloy, after the location of the plants where the uranium was enriched. The term oralloy is still occasionally used to refer to enriched uranium.

Breeder reactor

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Breeder_reactor 

 

Assembly of the core of Experimental Breeder Reactor I in Idaho, United States, 1951

A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile material, such as uranium-238 or thorium-232, that is loaded into the reactor along with fissile fuel. Breeders were at first found attractive because they made more complete use of uranium fuel than light water reactors, but interest declined after the 1960s as more uranium reserves were found, and new methods of uranium enrichment reduced fuel costs.

Fuel efficiency and types of nuclear waste

Fission probabilities of selected actinides, thermal vs. fast neutrons

Isotope	Thermal fission cross section	Thermal fission %	Fast fission cross section	Fast fission %
Th-232	nil	1 (non-fissile)	0.350 barn	3 (non-fissile)
U-232	76.66 barn	59	2.370 barn	95
U-233	531.2 barn	89	2.450 barn	93
U-235	584.4 barn	81	2.056 barn	80
U-238	11.77 microbarn	1 (non-fissile)	1.136 barn	11
Np-237	0.02249 barn	3 (non-fissile)	2.247 barn	27
Pu-238	17.89 barn	7	2.721 barn	70
Pu-239	747.4 barn	63	2.338 barn	85
Pu-240	58.77 barn	1 (non-fissile)	2.253 barn	55
Pu-241	1012 barn	75	2.298 barn	87
Pu-242	0.002557 barn	1 (non-fissile)	2.027 barn	53
Am-241	600.4 barn	1 (non-fissile)	0.2299 microbarn	21
Am-242m	6409 barn	75	2.550 barn	94
Am-243	0.1161 barn	1 (non-fissile)	2.140 barn	23
Cm-242	5.064 barn	1 (non-fissile)	2.907 barn	10
Cm-243	617.4 barn	78	2.500 barn	94
Cm-244	1.037 barn	4 (non-fissile)	0.08255 microbarn	33

Breeder reactors could, in principle, extract almost all of the energy contained in uranium or thorium, decreasing fuel requirements by a factor of 100 compared to widely used once-through light water reactors, which extract less than 1% of the energy in the uranium mined from the earth. The high fuel-efficiency of breeder reactors could greatly reduce concerns about fuel supply, energy used in mining and storage of radioactive waste. Adherents claim that with seawater uranium extraction, there would be enough fuel for breeder reactors to satisfy our energy needs for 5 billion years at 1983's total energy consumption rate, thus making nuclear energy effectively a renewable energy.

Nuclear waste became a greater concern by the 1990s. In broad terms, spent nuclear fuel has two main components. The first consists of fission products, the leftover fragments of fuel atoms after they have been split to release energy. Fission products come in dozens of elements and hundreds of isotopes, all of them lighter than uranium. The second main component of spent fuel is transuranics (atoms heavier than uranium), which are generated from uranium or heavier atoms in the fuel when they absorb neutrons but do not undergo fission. All transuranic isotopes fall within the actinide series on the periodic table, and so they are frequently referred to as the actinides.

The physical behavior of the fission products is markedly different from that of the transuranics. In particular, fission products do not themselves undergo fission, and therefore cannot be used for nuclear weapons. Furthermore, only seven long-lived fission product isotopes have half-lives longer than a hundred years, which makes their geological storage or disposal less problematic than for transuranic materials.

With increased concerns about nuclear waste, breeding fuel cycles became interesting again because they can reduce actinide wastes, particularly plutonium and minor actinides. Breeder reactors are designed to fission the actinide wastes as fuel, and thus convert them to more fission products.

After spent nuclear fuel is removed from a light water reactor, it undergoes a complex decay profile as each nuclide decays at a different rate. Due to a physical oddity referenced below, there is a large gap in the decay half-lives of fission products compared to transuranic isotopes. If the transuranics are left in the spent fuel, after 1,000 to 100,000 years, the slow decay of these transuranics would generate most of the radioactivity in that spent fuel. Thus, removing the transuranics from the waste eliminates much of the long-term radioactivity of spent nuclear fuel.

Today's commercial light water reactors do breed some new fissile material, mostly in the form of plutonium. Because commercial reactors were never designed as breeders, they do not convert enough uranium-238 into plutonium to replace the uranium-235 consumed. Nonetheless, at least one-third of the power produced by commercial nuclear reactors comes from fission of plutonium generated within the fuel. Even with this level of plutonium consumption, light water reactors consume only part of the plutonium and minor actinides they produce, and nonfissile isotopes of plutonium build up, along with significant quantities of other minor actinides.

Conversion ratio, break-even, breeding ratio, doubling time, and burnup

One measure of a reactor's performance is the "conversion ratio," defined as the ratio of new fissile atoms produced to fissile atoms consumed. All proposed nuclear reactors except specially designed and operated actinide burners experience some degree of conversion. As long as there is any amount of a fertile material within the neutron flux of the reactor, some new fissile material is always created. When the conversion ratio is greater than 1, it is often called the "breeding ratio."

For example, commonly used light water reactors have a conversion ratio of approximately 0.6. Pressurized heavy water reactors (PHWR) running on natural uranium have a conversion ratio of 0.8. In a breeder reactor, the conversion ratio is higher than 1. "Break-even" is achieved when the conversion ratio reaches 1.0 and the reactor produces as much fissile material as it uses.

The doubling time is the amount of time it would take for a breeder reactor to produce enough new fissile material to replace the original fuel and additionally produce an equivalent amount of fuel for another nuclear reactor. This was considered an important measure of breeder performance in early years, when uranium was thought to be scarce. However, since uranium is more abundant than thought in the early days of nuclear reactor development, and given the amount of plutonium available in spent reactor fuel, doubling time has become a less-important metric in modern breeder-reactor design.

"Burnup" is a measure of how much energy has been extracted from a given mass of heavy metal in fuel, often expressed (for power reactors) in terms of gigawatt-days per ton of heavy metal. Burnup is an important factor in determining the types and abundances of isotopes produced by a fission reactor. Breeder reactors, by design, have extremely high burnup compared to a conventional reactor, as breeder reactors produce much more of their waste in the form of fission products, while most or all of the actinides are meant to be fissioned and destroyed.

In the past, breeder-reactor development focused on reactors with low breeding ratios, from 1.01 for the Shippingport Reactor running on thorium fuel and cooled by conventional light water to over 1.2 for the Soviet BN-350 liquid-metal-cooled reactor. Theoretical models of breeders with liquid sodium coolant flowing through tubes inside fuel elements ("tube-in-shell" construction) suggest breeding ratios of at least 1.8 are possible on an industrial scale. The Soviet BR-1 test reactor achieved a breeding ratio of 2.5 under non-commercial conditions.

Types of breeder reactor

Production of heavy transuranic actinides in current thermal-neutron fission reactors through neutron capture and decays. Starting at uranium-238, isotopes of plutonium, americium, and curium are all produced. In a fast neutron-breeder reactor, all these isotopes may be burned as fuel.

Many types of breeder reactor are possible:

A 'breeder' is simply a reactor designed for very high neutron economy with an associated conversion rate higher than 1.0. In principle, almost any reactor design could be tweaked to become a breeder. An example of this process is the evolution of the Light Water Reactor, a very heavily moderated thermal design, into the Super Fast Reactor concept, using light water in an extremely low-density supercritical form to increase the neutron economy high enough to allow breeding.

Aside from water cooled, there are many other types of breeder reactor currently envisioned as possible. These include molten-salt cooled, gas cooled, and liquid-metal cooled designs in many variations. Almost any of these basic design types may be fueled by uranium, plutonium, many minor actinides, or thorium, and they may be designed for many different goals, such as creating more fissile fuel, long-term steady-state operation, or active burning of nuclear wastes.

Extant reactor designs are sometimes divided into two broad categories based upon their neutron spectrum, which generally separates those designed to use primarily uranium and transuranics from those designed to use thorium and avoid transuranics. These designs are:

• Fast breeder reactor (FBR) which use fast (i.e.: unmoderated) neutrons to breed fissile plutonium and possibly higher transuranics from fertile uranium-238. The fast spectrum is flexible enough that it can also breed fissile uranium-233 from thorium, if desired.
• Thermal breeder reactor which use thermal-spectrum (i.e.: moderated) neutrons to breed fissile uranium-233 from thorium (thorium fuel cycle). Due to the behavior of the various nuclear fuels, a thermal breeder is thought commercially feasible only with thorium fuel, which avoids the buildup of the heavier transuranics.

Reprocessing

Fission of the nuclear fuel in any reactor produces neutron-absorbing fission products. Because of this unavoidable physical process, it is necessary to reprocess the fertile material from a breeder reactor to remove those neutron poisons. This step is required to fully utilize the ability to breed as much or more fuel than is consumed. All reprocessing can present a proliferation concern, since it extracts weapons-usable material from spent fuel. The most-common reprocessing technique, PUREX, presents a particular concern, since it was expressly designed to separate pure plutonium. Early proposals for the breeder-reactor fuel cycle posed an even greater proliferation concern because they would use PUREX to separate plutonium in a highly attractive isotopic form for use in nuclear weapons.

Several countries are developing reprocessing methods that do not separate the plutonium from the other actinides. For instance, the non-water-based pyrometallurgical electrowinning process, when used to reprocess fuel from an integral fast reactor, leaves large amounts of radioactive actinides in the reactor fuel. More-conventional water-based reprocessing systems include SANEX, UNEX, DIAMEX, COEX, and TRUEX, and proposals to combine PUREX with co-processes.

All these systems have modestly better proliferation resistance than PUREX, though their adoption rate is low.

In the thorium cycle, thorium-232 breeds by converting first to protactinium-233, which then decays to uranium-233. If the protactinium remains in the reactor, small amounts of uranium-232 are also produced, which has the strong gamma emitter thallium-208 in its decay chain. Similar to uranium-fueled designs, the longer the fuel and fertile material remain in the reactor, the more of these undesirable elements build up. In the envisioned commercial thorium reactors, high levels of uranium-232 would be allowed to accumulate, leading to extremely high gamma-radiation doses from any uranium derived from thorium. These gamma rays complicate the safe handling of a weapon and the design of its electronics; this explains why uranium-233 has never been pursued for weapons beyond proof-of-concept demonstrations.

While the thorium cycle may be proliferation-resistant with regard to uranium-233 extraction from fuel (because of the presence of uranium-232), it poses a proliferation risk from an alternate route of uranium-233 extraction, which involves chemically extracting protactinium-233 and allowing it to decay to pure uranium-233 outside of the reactor. This process could happen beyond the oversight of organizations such as the International Atomic Energy Agency (IAEA).

Waste reduction

Actinides and fission products by half-life
Actinides by decay chain				Half-life range (a)	Fission products of 235U by yield
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6 a	†		155Euþ
244Cmƒ	241Puƒ	250Cf	227Ac№	10–29 a		90Sr	85Kr	113mCdþ
232Uƒ		238Puƒ	243Cmƒ	29–97 a		137Cs	151Smþ	121mSn
248Bk	249Cfƒ	242mAmƒ		141–351 a	No fission products have a half-life in the range of 100–210 ka ...
	241Amƒ		251Cfƒ	430–900 a
		226Ra№	247Bk	1.3–1.6 ka
240Pu	229Th	246Cmƒ	243Amƒ	4.7–7.4 ka
	245Cmƒ	250Cm		8.3–8.5 ka
			239Puƒ	24.1 ka
		230Th№	231Pa№	32–76 ka
236Npƒ	233Uƒ	234U№		150–250 ka	‡	99Tc₡	126Sn
248Cm		242Pu		327–375 ka			79Se₡
				1.53 Ma		93Zr
	237Npƒ			2.1–6.5 Ma		135Cs₡	107Pd
236U			247Cmƒ	15–24 Ma			129I₡
244Pu				80 Ma	... nor beyond 15.7 Ma
232Th№		238U№	235Uƒ№	0.7–14.1 Ga
Legend for superscript symbols ₡  has thermal neutron capture cross section in the range of 8–50 barns ƒ  fissile m  metastable isomer №  primarily a naturally occurring radioactive material (NORM) þ  neutron poison (thermal neutron capture cross section greater than 3k barns) †  range 4–97 a: Medium-lived fission product ‡  over 200 ka: Long-lived fission product

Nuclear waste became a greater concern by the 1990s. Breeding fuel cycles attracted renewed interest because of their potential to reduce actinide wastes, particularly plutonium and minor actinides. Since breeder reactors on a closed fuel cycle would use nearly all of the actinides fed into them as fuel, their fuel requirements would be reduced by a factor of about 100. The volume of waste they generate would be reduced by a factor of about 100 as well. While there is a huge reduction in the volume of waste from a breeder reactor, the activity of the waste is about the same as that produced by a light-water reactor.

In addition, the waste from a breeder reactor has a different decay behavior, because it is made up of different materials. Breeder reactor waste is mostly fission products, while light-water reactor waste has a large quantity of transuranics. After spent nuclear fuel has been removed from a light-water reactor for longer than 100,000 years, these transuranics would be the main source of radioactivity. Eliminating them would eliminate much of the long-term radioactivity from the spent fuel.

In principle, breeder fuel cycles can recycle and consume all actinides, leaving only fission products. As the graphic in this section indicates, fission products have a peculiar 'gap' in their aggregate half-lives, such that no fission products have a half-life between 91 years and two hundred thousand years. As a result of this physical oddity, after several hundred years in storage, the activity of the radioactive waste from a Fast Breeder Reactor would quickly drop to the low level of the long-lived fission products. However, to obtain this benefit requires the highly efficient separation of transuranics from spent fuel. If the fuel reprocessing methods used leave a large fraction of the transuranics in the final waste stream, this advantage would be greatly reduced.

Both types of breeding cycles can reduce actinide wastes:

• The fast breeder reactor's fast neutrons can fission actinide nuclei with even numbers of both protons and neutrons. Such nuclei usually lack the low-speed "thermal neutron" resonances of fissile fuels used in LWRs.
• The thorium fuel cycle inherently produces lower levels of heavy actinides. The fertile material in the thorium fuel cycle has an atomic weight of 232, while the fertile material in the uranium fuel cycle has an atomic weight of 238. That mass difference means that thorium-232 requires six more neutron capture events per nucleus before the transuranic elements can be produced. In addition to this simple mass difference, the reactor gets two chances to fission the nuclei as the mass increases: First as the effective fuel nuclei U233, and as it absorbs two more neutrons, again as the fuel nuclei U235.

A reactor whose main purpose is to destroy actinides, rather than increasing fissile fuel-stocks, is sometimes known as a burner reactor. Both breeding and burning depend on good neutron economy, and many designs can do either. Breeding designs surround the core by a breeding blanket of fertile material. Waste burners surround the core with non-fertile wastes to be destroyed. Some designs add neutron reflectors or absorbers.

Breeder reactor concepts

There are several concepts for breeder reactors; the two main ones are:

• Reactors with a fast neutron spectrum are called fast breeder reactors (FBR) – these typically utilize uranium-238 as fuel.
• Reactors with a thermal neutron spectrum are called thermal breeder reactors – these typically utilize thorium-232 as fuel.

Fast breeder reactor

Schematic diagram showing the difference between the Loop and Pool types of LMFBR.

In 2006 all large-scale fast breeder reactor (FBR) power stations were liquid metal fast breeder reactors (LMFBR) cooled by liquid sodium. These have been of one of two designs:

• Loop type, in which the primary coolant is circulated through primary heat exchangers outside the reactor tank (but inside the biological shield due to radioactive sodium-24 in the primary coolant)

Experimental Breeder Reactor II, which served as the prototype for the Integral Fast Reactor

• Pool type, in which the primary heat exchangers and pumps are immersed in the reactor tank

All current fast neutron reactor designs use liquid metal as the primary coolant, to transfer heat from the core to steam used to power the electricity generating turbines. FBRs have been built cooled by liquid metals other than sodium—some early FBRs used mercury, other experimental reactors have used a sodium-potassium alloy called NaK. Both have the advantage that they are liquids at room temperature, which is convenient for experimental rigs but less important for pilot or full-scale power stations. Lead and lead-bismuth alloy have also been used.

Three of the proposed generation IV reactor types are FBRs:

• Gas-cooled fast reactor (GFR) cooled by helium.
• Sodium-cooled fast reactor (SFR) based on the existing liquid-metal FBR (LMFBR) and integral fast reactor designs.
• Lead-cooled fast reactor (LFR) based on Soviet naval propulsion units.

FBRs usually use a mixed oxide fuel core of up to 20% plutonium dioxide (PuO₂) and at least 80% uranium dioxide (UO₂). Another fuel option is metal alloys, typically a blend of uranium, plutonium, and zirconium (used because it is "transparent" to neutrons). Enriched uranium can also be used on its own.

Many designs surround the core in a blanket of tubes that contain non-fissile uranium-238, which, by capturing fast neutrons from the reaction in the core, converts to fissile plutonium-239 (as is some of the uranium in the core), which is then reprocessed and used as nuclear fuel. Other FBR designs rely on the geometry of the fuel itself (which also contains uranium-238), arranged to attain sufficient fast neutron capture. The plutonium-239 (or the fissile uranium-235) fission cross-section is much smaller in a fast spectrum than in a thermal spectrum, as is the ratio between the ²³⁹Pu/²³⁵U fission cross-section and the ²³⁸U absorption cross-section. This increases the concentration of ²³⁹Pu/²³⁵U needed to sustain a chain reaction, as well as the ratio of breeding to fission. On the other hand, a fast reactor needs no moderator to slow down the neutrons at all, taking advantage of the fast neutrons producing a greater number of neutrons per fission than slow neutrons. For this reason ordinary liquid water, being a moderator and neutron absorber, is an undesirable primary coolant for fast reactors. Because large amounts of water in the core are required to cool the reactor, the yield of neutrons and therefore breeding of ²³⁹Pu are strongly affected. Theoretical work has been done on reduced moderation water reactors, which may have a sufficiently fast spectrum to provide a breeding ratio slightly over 1. This would likely result in an unacceptable power derating and high costs in a liquid-water-cooled reactor, but the supercritical water coolant of the supercritical water reactor (SCWR) has sufficient heat capacity to allow adequate cooling with less water, making a fast-spectrum water-cooled reactor a practical possibility.

The type of coolants, temperatures and fast neutron spectrum puts the fuel cladding material (normally austenitic stainless or ferritic-martensitic steels) under extreme conditions. The understanding of the radiation damage, coolant interactions, stresses and temperatures are necessary for the safe operation of any reactor core. All materials used to date in sodium-cooled fast reactors have known limits, as explored in ONR-RRR-088 review. Oxide Dispersion Strengthened (ODS) steel is viewed as the long-term radiation resistant fuel-cladding material that overcome the shortcomings of today's material choices.

There are only two commercially operating breeder reactors as of 2017: the BN-600 reactor, at 560 MWe, and the BN-800 reactor, at 880 MWe. Both are Russian sodium-cooled reactors.

Integral fast reactor

One design of fast neutron reactor, specifically conceived to address the waste disposal and plutonium issues, was the integral fast reactor (IFR, also known as an integral fast breeder reactor, although the original reactor was designed to not breed a net surplus of fissile material).

To solve the waste disposal problem, the IFR had an on-site electrowinning fuel-reprocessing unit that recycled the uranium and all the transuranics (not just plutonium) via electroplating, leaving just short half-life fission products in the waste. Some of these fission products could later be separated for industrial or medical uses and the rest sent to a waste repository. The IFR pyroprocessing system uses molten cadmium cathodes and electrorefiners to reprocess metallic fuel directly on-site at the reactor. Such systems not only co-mingle all the minor actinides with both uranium and plutonium, they are compact and self-contained, so that no plutonium-containing material needs to be transported away from the site of the breeder reactor. Breeder reactors incorporating such technology would most likely be designed with breeding ratios very close to 1.00, so that after an initial loading of enriched uranium and/or plutonium fuel, the reactor would then be refueled only with small deliveries of natural uranium metal. A quantity of natural uranium metal equivalent to a block about the size of a milk crate delivered once per month would be all the fuel such a 1 gigawatt reactor would need. Such self-contained breeders are currently envisioned as the final self-contained and self-supporting ultimate goal of nuclear reactor designers. The project was canceled in 1994 by United States Secretary of Energy Hazel O'Leary.

Other fast reactors

The graphite core of the Molten Salt Reactor Experiment

Another proposed fast reactor is a fast molten salt reactor, in which the molten salt's moderating properties are insignificant. This is typically achieved by replacing the light metal fluorides (e.g. LiF, BeF₂) in the salt carrier with heavier metal chlorides (e.g., KCl, RbCl, ZrCl₄).

Several prototype FBRs have been built, ranging in electrical output from a few light bulbs' equivalent (EBR-I, 1951) to over 1,000 MWe. As of 2006, the technology is not economically competitive to thermal reactor technology, but India, Japan, China, South Korea and Russia are all committing substantial research funds to further development of fast breeder reactors, anticipating that rising uranium prices will change this in the long term. Germany, in contrast, abandoned the technology due to safety concerns. The SNR-300 fast breeder reactor was finished after 19 years despite cost overruns summing up to a total of €3.6 billion, only to then be abandoned.

India is also developing FBR technology using both uranium and thorium feedstocks.

Thermal breeder reactor

The Shippingport Reactor, used as a prototype light water breeder for five years beginning in August 1977

The advanced heavy water reactor (AHWR) is one of the few proposed large-scale uses of thorium. India is developing this technology, motivated by substantial thorium reserves; almost a third of the world's thorium reserves are in India, which lacks significant uranium reserves.

The third and final core of the Shippingport Atomic Power Station 60 MWe reactor was a light water thorium breeder, which began operating in 1977. It used pellets made of thorium dioxide and uranium-233 oxide; initially, the U-233 content of the pellets was 5–6% in the seed region, 1.5–3% in the blanket region and none in the reflector region. It operated at 236 MWt, generating 60 MWe and ultimately produced over 2.1 billion kilowatt hours of electricity. After five years, the core was removed and found to contain nearly 1.4% more fissile material than when it was installed, demonstrating that breeding from thorium had occurred.

The liquid fluoride thorium reactor (LFTR) is also planned as a thorium thermal breeder. Liquid-fluoride reactors may have attractive features, such as inherent safety, no need to manufacture fuel rods and possibly simpler reprocessing of the liquid fuel. This concept was first investigated at the Oak Ridge National Laboratory Molten-Salt Reactor Experiment in the 1960s. From 2012 it became the subject of renewed interest worldwide. Japan, India, China, the UK, as well as private US, Czech and Australian companies have expressed intent to develop and commercialize the technology.

Discussion

Like many aspects of nuclear power, fast breeder reactors have been subject to much controversy over the years. In 2010 the International Panel on Fissile Materials said "After six decades and the expenditure of the equivalent of tens of billions of dollars, the promise of breeder reactors remains largely unfulfilled and efforts to commercialize them have been steadily cut back in most countries". In Germany, the United Kingdom, and the United States, breeder reactor development programs have been abandoned. The rationale for pursuing breeder reactors—sometimes explicit and sometimes implicit—was based on the following key assumptions:

• It was expected that uranium would be scarce and high-grade deposits would quickly become depleted if fission power were deployed on a large scale; the reality, however, is that since the end of the cold war, uranium has been much cheaper and more abundant than early designers expected.
• It was expected that breeder reactors would quickly become economically competitive with the light-water reactors that dominate nuclear power today, but the reality is that capital costs are at least 25% more than water-cooled reactors.
• It was thought that breeder reactors could be as safe and reliable as light-water reactors, but safety issues are cited as a concern with fast reactors that use a sodium coolant, where a leak could lead to a sodium fire.
• It was expected that the proliferation risks posed by breeders and their "closed" fuel cycle, in which plutonium would be recycled, could be managed. But since plutonium-breeding reactors produce plutonium from U238, and thorium reactors produce fissile U233 from thorium, all breeding cycles could theoretically pose proliferation risks. However U232, which is always present in U233 produced in breeder reactors, is a strong gamma-emitter via its daughter products, and would make weapon handling extremely hazardous and the weapon easy to detect.

There are some past anti-nuclear advocates that have become pro-nuclear power as a clean source of electricity since breeder reactors effectively recycle most of their waste. This solves one of the most-important negative issues of nuclear power. In the documentary Pandora's Promise, a case is made for breeder reactors because they provide a real high-kW alternative to fossil fuel energy. According to the movie, one pound of uranium provides as much energy as 5,000 barrels of oil.

FBRs have been built and operated in the United States, the United Kingdom, France, the former USSR, India and Japan. The experimental FBR SNR-300 was built in Germany but never operated and eventually shut down amid political controversy following the Chernobyl disaster. As of 2019, two FBRs are being operated for power generation in Russia. Several reactors are planned, many for research related to the Generation IV reactor initiative.

Development and notable breeder reactors

Notable breeder reactors

Reactor	Country when built	Started	Shutdown	Design MWe	Final MWe	Thermal Power MWt	Capacity factor	No of leaks	Neutron temperature	Coolant	Reactor class
DFR	UK	1962	1977	14	11	65	34%	7	Fast	NaK	Test
BN-350	Soviet Union	1973	1999	135	52	750	43%	15	Fast	Sodium	Prototype
Rapsodie	France	1967	1983	0	–	40	–	2	Fast	Sodium	Test
Phénix	France	1975	2010	233	130	563	40.5%	31	Fast	Sodium	Prototype
PFR	UK	1976	1994	234	234	650	26.9%	20	Fast	Sodium	Prototype
KNK II	Germany	1977	1991	18	17	58	17.1%	21	Fast	Sodium	Research/Test
SNR-300	Germany	1985 (partial operation)	1991	327	–	–	–	–	Fast	Sodium	Prototype/Commercial
BN-600	Soviet Union	1981	operating	560	560	1470	74.2%	27	Fast	Sodium	Prototype/Commercial(Gen2)
FFTF	US	1982	1993	0	–	400	–	1	Fast	Sodium	Test
Superphénix	France	1985	1998	1200	1200	3000	7.9%	7	Fast	Sodium	Prototype/Commercial(Gen2)
FBTR	India	1985	operating	13	–	40	–	6	Fast	Sodium	Test
PFBR	India	commissioning	commissioning	500	–	1250	–	–	Fast	Sodium	Prototype/Commercial(Gen3)
Jōyō	Japan	1977	operating	0	–	150	–	–	Fast	Sodium	Test
Monju	Japan	1995	2017	246	246	714	trial only	1	Fast	Sodium	Prototype
BN-800	Russia	2015	operating	789	880	2100	73.4%	–	Fast	Sodium	Prototype/Commercial(Gen3)
MSRE	US	1965	1969	0	–	7.4	–	–	Epithermal	Molten Salt(FLiBe)	Test
Clementine	US	1946	1952	0	–	0.025	–	–	Fast	Mercury	World's First Fast Reactor
EBR-1	US	1951	1964	0.2	0.2	1.4	–	–	Fast	NaK	World's First Power Reactor
Fermi-1	US	1963	1972	66	66	200	–	–	Fast	Sodium	Prototype
EBR-2	US	1964	1994	19	19	62.5	–	–	Fast	Sodium	Experimental/Test
Shippingport	US	1977 as breeder	1982	60	60	236	–	–	Thermal	Light Water	Experimental-Core3

The Soviet Union (comprising Russia and other countries, dissolved in 1991) constructed a series of fast reactors, the first being mercury-cooled and fueled with plutonium metal, and the later plants sodium-cooled and fueled with plutonium oxide.

BR-1 (1955) was 100W (thermal) was followed by BR-2 at 100 kW and then the 5MW BR-5.

BOR-60 (first criticality 1969) was 60 MW, with construction started in 1965.

BN-600 (1981), followed by Russia's BN-800 (2016)

Future plants

The Chinese Experimental Fast Reactor is a 65 MW (thermal), 20 MW (electric), sodium-cooled, pool-type reactor with a 30-year design lifetime and a target burnup of 100 MWd/kg.

India has been an early leader in the FBR segment. In 2012 an FBR called the Prototype Fast Breeder Reactor was due to be completed and commissioned. The program is intended to use fertile thorium-232 to breed fissile uranium-233. India is also pursuing thorium thermal breeder reactor technology. India's focus on thorium is due to the nation's large reserves, though known worldwide reserves of thorium are four times those of uranium. India's Department of Atomic Energy (DAE) said in 2007 that it would simultaneously construct four more breeder reactors of 500 MWe each including two at Kalpakkam.

BHAVINI, an Indian nuclear power company, was established in 2003 to construct, commission and operate all stage II fast breeder reactors outlined in India's three stage nuclear power programme. To advance these plans, the Indian FBR-600 is a pool-type sodium-cooled reactor with a rating of 600 MWe.

The China Experimental Fast Reactor (CEFR) is a 25 MW(e) prototype for the planned China Prototype Fast Reactor (CFRP). It started generating power on 21 July 2011.

China also initiated a research and development project in thorium molten-salt thermal breeder-reactor technology (liquid fluoride thorium reactor), formally announced at the Chinese Academy of Sciences (CAS) annual conference in January 2011. Its ultimate target was to investigate and develop a thorium-based molten salt nuclear system over about 20 years.

Kirk Sorensen, former NASA scientist and chief nuclear technologist at Teledyne Brown Engineering, has long been a promoter of thorium fuel cycle and particularly liquid fluoride thorium reactors. In 2011, Sorensen founded Flibe Energy, a company aimed to develop 20–50 MW LFTR reactor designs to power military bases.

South Korea is developing a design for a standardized modular FBR for export, to complement the standardized PWR (pressurized water reactor) and CANDU designs they have already developed and built, but has not yet committed to building a prototype.

A cutaway model of the BN-600 reactor, superseded by the BN-800 reactor family.

 

Construction of the BN-800 reactor

Russia has a plan for increasing its fleet of fast breeder reactors significantly. A BN-800 reactor (800 MWe) at Beloyarsk was completed in 2012, succeeding a smaller BN-600. In June 2014 the BN-800 was started in the minimum power mode. Working at 35% of nominal efficiency, the reactor contributed to the energy network on 10 December 2015. It reached its full power production in August 2016.

Plans for the construction of a larger BN-1200 reactor (1,200 MWe) was scheduled for completion in 2018, with two additional BN-1200 reactors built by the end of 2030. However, in 2015 Rosenergoatom postponed construction indefinitely to allow fuel design to be improved after more experience of operating the BN-800 reactor, and among cost concerns.

An experimental lead-cooled fast reactor, BREST-300 will be built at the Siberian Chemical Combine (SCC) in Seversk. The BREST (Russian: bystry reaktor so svintsovym teplonositelem, English: fast reactor with lead coolant) design is seen as a successor to the BN series and the 300 MWe unit at the SCC could be the forerunner to a 1,200 MWe version for wide deployment as a commercial power generation unit. The development program is as part of an Advanced Nuclear Technologies Federal Program 2010–2020 that seeks to exploit fast reactors for uranium efficiency while 'burning' radioactive substances that would otherwise be disposed of as waste. Its core would measure about 2.3 metres in diameter by 1.1 metres in height and contain 16 tonnes of fuel. The unit would be refuelled every year, with each fuel element spending five years in total within the core. Lead coolant temperature would be around 540 °C, giving a high efficiency of 43%, primary heat production of 700 MWt yielding electrical power of 300 MWe. The operational lifespan of the unit could be 60 years. The design is expected to be completed by NIKIET in 2014 for construction between 2016 and 2020.

On 16 February 2006, the United States, France and Japan signed an "arrangement" to research and develop sodium-cooled fast reactors in support of the Global Nuclear Energy Partnership. In April 2007 the Japanese government selected Mitsubishi Heavy Industries (MHI) as the "core company in FBR development in Japan". Shortly thereafter, MHI started a new company, Mitsubishi FBR Systems (MFBR) to develop and eventually sell FBR technology.

The Marcoule Nuclear Site in France, location of the Phénix (on the left).

In September 2010 the French government allocated €651.6 million to the Commissariat à l'énergie atomique to finalize the design of ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration), a 600 MW fourth-generation reactor design to be finalized in 2020. As of 2013 the UK had shown interest in the PRISM reactor and was working in concert with France to develop ASTRID. In 2019, CEA announced this design would not be built before mid-century.

In October 2010 GE Hitachi Nuclear Energy signed a memorandum of understanding with the operators of the US Department of Energy's Savannah River Site, which should allow the construction of a demonstration plant based on the company's S-PRISM fast breeder reactor prior to the design receiving full Nuclear Regulatory Commission (NRC) licensing approval. In October 2011 The Independent reported that the UK Nuclear Decommissioning Authority (NDA) and senior advisers within the Department for Energy and Climate Change (DECC) had asked for technical and financial details of PRISM, partly as a means of reducing the country's plutonium stockpile.

The traveling wave reactor (TWR) proposed in a patent by Intellectual Ventures is a fast breeder reactor designed to not need fuel reprocessing during the decades-long lifetime of the reactor. The breed-burn wave in the TWR design does not move from one end of the reactor to the other but gradually from the inside out. Moreover, as the fuel's composition changes through nuclear transmutation, fuel rods are continually reshuffled within the core to optimize the neutron flux and fuel usage at any given point in time. Thus, instead of letting the wave propagate through the fuel, the fuel itself is moved through a largely stationary burn wave. This is contrary to many media reports, which have popularized the concept as a candle-like reactor with a burn region that moves down a stick of fuel. By replacing a static core configuration with an actively managed "standing wave" or "soliton" core, TerraPower's design avoids the problem of cooling a highly variable burn region. Under this scenario, the reconfiguration of fuel rods is accomplished remotely by robotic devices; the containment vessel remains closed during the procedure, and there is no associated downtime.