Pressurized water reactor

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Pressurized_water_reactor

 

Nuclear Regulatory Commission image of pressurized water reactor vessel heads

A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.

PWRs were originally designed to serve as nuclear marine propulsion for nuclear submarines and were used in the original design of the second commercial power plant at Shippingport Atomic Power Station.

PWRs currently operating in the United States are considered Generation II reactors. Russia's VVER reactors are similar to US PWRs, but the VVER-1200 is not considered Generation II (see below). France operates many PWRs to generate the bulk of its electricity.

History

Rancho Seco PWR reactor hall and cooling tower (being decommissioned, 2004)

Several hundred PWRs are used for marine propulsion in aircraft carriers, nuclear submarines and ice breakers. In the US, they were originally designed at the Oak Ridge National Laboratory for use as a nuclear submarine power plant with a fully operational submarine power plant located at the Idaho National Laboratory. Follow-on work was conducted by Westinghouse Bettis Atomic Power Laboratory. The first purely commercial nuclear power plant at Shippingport Atomic Power Station was originally designed as a pressurized water reactor (although the first power plant connected to the grid was at Obninsk, USSR), on insistence from Admiral Hyman G. Rickover that a viable commercial plant would include none of the "crazy thermodynamic cycles that everyone else wants to build".

The United States Army Nuclear Power Program operated pressurized water reactors from 1954 to 1974. Three Mile Island Nuclear Generating Station initially operated two pressurized water reactor plants, TMI-1 and TMI-2. The partial meltdown of TMI-2 in 1979 essentially ended the growth in new construction of nuclear power plants in the United States for two decades. Watts Bar unit 2 (a Westinghouse 4-loop PWR) came online in 2016, becoming the first new nuclear reactor in the United States since 1996.

The pressurized water reactor has several new Generation III reactor evolutionary designs: the AP1000, VVER-1200, ACPR1000+, APR1400, Hualong One, IPWR-900 and EPR. The first AP1000 and EPR reactors were connected to the power grid in China in 2018. In 2020, NuScale Power became the first U.S. company to receive regulatory approval from the Nuclear Regulatory Commission for a small modular reactor with a modified PWR design. Also in 2020, the Energy Impact Center introduced the OPEN100 project, which published open-source blueprints for the construction of a 100 MW_electric nuclear power plant with a PWR design.

Design

Pictorial explanation of power transfer in a pressurized water reactor. Primary coolant is in orange and the secondary coolant (steam and later feedwater) is in blue.

Primary coolant system showing reactor pressure vessel (red), steam generators (purple), Pressurizer (blue), and pumps (green) in the three coolant loop Hualong One design

Nuclear fuel in the reactor pressure vessel is engaged in a controlled fission chain reaction, which produces heat, heating the water in the primary coolant loop by thermal conduction through the fuel cladding. The hot primary coolant is pumped into a heat exchanger called the steam generator, where it flows through several thousand small tubes. Heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the shell side of the exchanger where the secondary coolant evaporates to pressurized steam. This transfer of heat is accomplished without mixing the two fluids to prevent the secondary coolant from becoming radioactive. Some common steam generator arrangements are u-tubes or single pass heat exchangers.

In a nuclear power station, the pressurized steam is fed through a steam turbine which drives an electrical generator connected to the electric grid for transmission. After passing through the turbine the secondary coolant (water-steam mixture) is cooled down and condensed in a condenser. The condenser converts the steam to a liquid so that it can be pumped back into the steam generator, and maintains a vacuum at the turbine outlet so that the pressure drop across the turbine, and hence the energy extracted from the steam, is maximized. Before being fed into the steam generator, the condensed steam (referred to as feedwater) is sometimes preheated in order to minimize thermal shock.

The steam generated has other uses besides power generation. In nuclear ships and submarines, the steam is fed through a steam turbine connected to a set of speed reduction gears to a shaft used for propulsion. Direct mechanical action by expansion of the steam can be used for a steam-powered aircraft catapult or similar applications. District heating by the steam is used in some countries and direct heating is applied to internal plant applications.

Two things are characteristic for the pressurized water reactor (PWR) when compared with other reactor types: coolant loop separation from the steam system and pressure inside the primary coolant loop. In a PWR, there are two separate coolant loops (primary and secondary), which are both filled with demineralized/deionized water. A boiling water reactor, by contrast, has only one coolant loop, while more exotic designs such as breeder reactors use substances other than water for coolant and moderator (e.g. sodium in its liquid state as coolant or graphite as a moderator). The pressure in the primary coolant loop is typically 15–16 megapascals (150–160 bar), which is notably higher than in other nuclear reactors, and nearly twice that of a boiling water reactor (BWR). As an effect of this, only localized boiling occurs and steam will recondense promptly in the bulk fluid. By contrast, in a boiling water reactor the primary coolant is designed to boil.

Reactor

PWR reactor pressure vessel

Coolant

Light water is used as the primary coolant in a PWR. Water enters through the bottom of the reactor's core at about 548 K (275 °C; 527 °F) and is heated as it flows upwards through the reactor core to a temperature of about 588 K (315 °C; 599 °F). The water remains liquid despite the high temperature due to the high pressure in the primary coolant loop, usually around 155 bar (15.5 MPa 153 atm, 2,250 psi). The water in a PWR cannot exceed a temperature of 647 K (374 °C; 705 °F) or a pressure of 22.064 MPa (3200 psi or 218 atm), because those are the critical point of water. Supercritical water reactors are (as of 2022) only a proposed concept in which the coolant would never leave the supercritical state. However, as this requires even higher pressures than a PWR and can cause issues of corrosion, so far no such reactor has been built.

Pressurizer

Main article: Pressurizer

Pressure in the primary circuit is maintained by a pressurizer, a separate vessel that is connected to the primary circuit and partially filled with water which is heated to the saturation temperature (boiling point) for the desired pressure by submerged electrical heaters. To achieve a pressure of 155 bars (15.5 MPa), the pressurizer temperature is maintained at 345 °C (653 °F), which gives a subcooling margin (the difference between the pressurizer temperature and the highest temperature in the reactor core) of 30 °C (54 °F). As 345 °C is the boiling point of water at 155 bar, the liquid water is at the edge of a phase change. Thermal transients in the reactor coolant system result in large swings in pressurizer liquid/steam volume, and total pressurizer volume is designed around absorbing these transients without uncovering the heaters or emptying the pressurizer. Pressure transients in the primary coolant system manifest as temperature transients in the pressurizer and are controlled through the use of automatic heaters and water spray, which raise and lower pressurizer temperature, respectively.

Pumps

The coolant is pumped around the primary circuit by powerful pumps. These pumps have a rate of ~100,000 gallons of coolant per minute. After picking up heat as it passes through the reactor core, the primary coolant transfers heat in a steam generator to water in a lower pressure secondary circuit, evaporating the secondary coolant to saturated steam — in most designs 6.2 MPa (60 atm, 900 psia), 275 °C (530 °F) — for use in the steam turbine. The cooled primary coolant is then returned to the reactor vessel to be heated again.

Moderator

Main article: Passive nuclear safety

Pressurized water reactors, like all thermal reactor designs, require the fast fission neutrons to be slowed (a process called moderation or thermalizing) in order to interact with the nuclear fuel and sustain the chain reaction. In PWRs the coolant water is used as a moderator by letting the neutrons undergo multiple collisions with light hydrogen atoms in the water, losing speed in the process. This "moderating" of neutrons will happen more often when the water is more dense (more collisions will occur). The use of water as a moderator is an important safety feature of PWRs, as an increase in temperature may cause the water to expand, giving greater 'gaps' between the water molecules and reducing the probability of thermalization — thereby reducing the extent to which neutrons are slowed and hence reducing the reactivity in the reactor. Therefore, if reactivity increases beyond normal, the reduced moderation of neutrons will cause the chain reaction to slow down, producing less heat. This property, known as the negative temperature coefficient of reactivity, makes PWR reactors very stable. This process is referred to as 'Self-Regulating', i.e. the hotter the coolant becomes, the less reactive the plant becomes, shutting itself down slightly to compensate and vice versa. Thus the plant controls itself around a given temperature set by the position of the control rods.

In contrast, the Soviet RBMK reactor design used at Chernobyl, which uses graphite instead of water as the moderator and uses boiling water as the coolant, has a large positive thermal coefficient of reactivity. This means reactivity and heat generation increases when coolant and fuel temperatures increase, which makes the RBMK design less stable than pressurized water reactors at high operating temperature. In addition to its property of slowing down neutrons when serving as a moderator, water also has a property of absorbing neutrons, albeit to a lesser degree. When the coolant water temperature increases, the boiling increases, which creates voids. Thus there is less water to absorb thermal neutrons that have already been slowed by the graphite moderator, causing an increase in reactivity. This property is called the void coefficient of reactivity, and in an RBMK reactor like Chernobyl, the void coefficient is positive, and fairly large, making it very hard to regulate when the reaction begins to run away. The RBMK reactors also have a flawed control rods design in which during rapid scrams, the graphite reaction enhancement tips of the rods would displace water at the bottom of the reactor and locally increase reactivity there. This is called the "positive scram effect" that is unique to the flawed RBMK control rods design. These design flaws, in addition to operator errors that pushed the reactor to its limits, are generally seen as the causes of the Chernobyl disaster. 

The Canadian CANDU heavy water reactor design have a slight positive void coefficient, these reactors mitigate this issues with a number of built-in advanced passive safety systems not found in the Soviet RBMK design. No criticality could occur in a CANDU reactor or any other heavy water reactor when ordinary light water is supplied to the reactor as an emergency coolant. Depending on burnup, boric acid or another neutron poison will have to be added to emergency coolant to avoid a criticality accident.

PWRs are designed to be maintained in an undermoderated state, meaning that there is room for increased water volume or density to further increase moderation, because if moderation were near saturation, then a reduction in density of the moderator/coolant could reduce neutron absorption significantly while reducing moderation only slightly, making the void coefficient positive. Also, light water is actually a somewhat stronger moderator of neutrons than heavy water, though heavy water's neutron absorption is much lower. Because of these two facts, light water reactors have a relatively small moderator volume and therefore have compact cores. One next generation design, the supercritical water reactor, is even less moderated. A less moderated neutron energy spectrum does worsen the capture/fission ratio for ²³⁵U and especially ²³⁹Pu, meaning that more fissile nuclei fail to fission on neutron absorption and instead capture the neutron to become a heavier nonfissile isotope, wasting one or more neutrons and increasing accumulation of heavy transuranic actinides, some of which have long half-lives.

Fuel

Main article: Nuclear fuel

PWR fuel bundle This fuel bundle is from a pressurized water reactor of the nuclear passenger and cargo ship NS Savannah. Designed and built by Babcock & Wilcox.

After enrichment, the uranium dioxide (UO
₂) powder is fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enriched uranium dioxide. The cylindrical pellets are then clad in a corrosion-resistant zirconium metal alloy Zircaloy which are backfilled with helium to aid heat conduction and detect leakages. Zircaloy is chosen because of its mechanical properties and its low absorption cross section. The finished fuel rods are grouped in fuel assemblies, called fuel bundles, that are then used to build the core of the reactor. A typical PWR has fuel assemblies of 200 to 300 rods each, and a large reactor would have about 150–250 such assemblies with 80–100 tons of uranium in all. Generally, the fuel bundles consist of fuel rods bundled 14 × 14 to 17 × 17. A PWR produces on the order of 900 to 1,600 MW_e. PWR fuel bundles are about 4 meters in length.

Refuelings for most commercial PWRs is on an 18–24 month cycle. Approximately one third of the core is replaced each refueling, though some more modern refueling schemes may reduce refuel time to a few days and allow refueling to occur on a shorter periodicity.

Control

In PWRs reactor power can be viewed as following steam (turbine) demand due to the reactivity feedback of the temperature change caused by increased or decreased steam flow. (See: Negative temperature coefficient.) Boron and cadmium control rods are used to maintain primary system temperature at the desired point. In order to decrease power, the operator throttles shut turbine inlet valves. This would result in less steam being drawn from the steam generators. This results in the primary loop increasing in temperature. The higher temperature causes the density of the primary reactor coolant water to decrease, allowing higher neutron speeds, thus less fission and decreased power output. This decrease of power will eventually result in primary system temperature returning to its previous steady-state value. The operator can control the steady state operating temperature by addition of boric acid and/or movement of control rods.

Reactivity adjustment to maintain 100% power as the fuel is burned up in most commercial PWRs is normally achieved by varying the concentration of boric acid dissolved in the primary reactor coolant. Boron readily absorbs neutrons and increasing or decreasing its concentration in the reactor coolant will therefore affect the neutron activity correspondingly. An entire control system involving high pressure pumps (usually called the charging and letdown system) is required to remove water from the high pressure primary loop and re-inject the water back in with differing concentrations of boric acid. The reactor control rods, inserted through the reactor vessel head directly into the fuel bundles, are moved for the following reasons: to start up the reactor, to shut down the primary nuclear reactions in the reactor, to accommodate short term transients, such as changes to load on the turbine,

The control rods can also be used to compensate for nuclear poison inventory and to compensate for nuclear fuel depletion. However, these effects are more usually accommodated by altering the primary coolant boric acid concentration.

In contrast, BWRs have no boron in the reactor coolant and control the reactor power by adjusting the reactor coolant flow rate.

Advantages

PWR reactors are very stable due to their tendency to produce less power as temperatures increase; this makes the reactor easier to operate from a stability standpoint.

PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials.

PWRs can passively scram the reactor in case offsite power is lost to immediately stop the primary nuclear reaction. The control rods are held by electromagnets and fall by gravity when current is lost; full insertion safely shuts down the primary nuclear reaction.

PWR technology is favoured by nations seeking to develop a nuclear navy; the compact reactors fit well in nuclear submarines and other nuclear ships.

PWRs are the most deployed type of reactor globally, allowing for a wide range of suppliers of new plants and parts for existing plants. Due to long experience with their operation they are the closest thing to mature technology that exists in nuclear energy.

PWRs - depending on type - can be fueled with MOX-fuel and/or the Russian Remix Fuel (which has a lower ²³⁹
Pu and a higher ²³⁵
U content than "regular" U/Pu MOX-fuel) allowing for a (partially) closed nuclear fuel cycle.

Water is a nontoxic, transparent, chemically unreactive (by comparison with e.g. NaK) coolant that is liquid at room temperature which makes visual inspection and maintenance easier. It is also easy and cheap to obtain unlike heavy water or even nuclear graphite.

Compared to reactors operating on natural uranium, PWRs can achieve a relatively high burnup. A typical PWR will exchange a quarter to a third of its fuel load every 18-24 months and have maintenance and inspection, that requires the reactor to be shut down, scheduled for this window. While more uranium ore is consumed per unit of electricity produced than in a natural uranium fueled reactor, the amount of spent fuel is less with the balance being depleted uranium whose radiological danger is lower than that of natural uranium.

Disadvantages

The coolant water must be highly pressurized to remain liquid at high temperatures. This requires high strength piping and a heavy pressure vessel and hence increases construction costs. The higher pressure can increase the consequences of a loss-of-coolant accident. The reactor pressure vessel is manufactured from ductile steel but, as the plant is operated, neutron flux from the reactor causes this steel to become less ductile. Eventually the ductility of the steel will reach limits determined by the applicable boiler and pressure vessel standards, and the pressure vessel must be repaired or replaced. This might not be practical or economic, and so determines the life of the plant.

Additional high pressure components such as reactor coolant pumps, pressurizer, and steam generators are also needed. This also increases the capital cost and complexity of a PWR power plant.

The high temperature water coolant with boric acid dissolved in it is corrosive to carbon steel (but not stainless steel); this can cause radioactive corrosion products to circulate in the primary coolant loop. This not only limits the lifetime of the reactor, but the systems that filter out the corrosion products and adjust the boric acid concentration add significantly to the overall cost of the reactor and to radiation exposure. In one instance, this has resulted in severe corrosion to control rod drive mechanisms when the boric acid solution leaked through the seal between the mechanism itself and the primary system.

Due to the requirement to load a pressurized water reactor's primary coolant loop with boron, undesirable radioactive secondary tritium production in the water is over 25 times greater than in boiling water reactors of similar power, owing to the latter's absence of the neutron moderating element in its coolant loop. The tritium is created by the absorption of a fast neutron in the nucleus of a boron-10 atom which subsequently splits into a lithium-7 and tritium atom. Pressurized water reactors annually emit several hundred curies of tritium to the environment as part of normal operation.

Natural uranium is only 0.7% uranium-235, the isotope necessary for thermal reactors. This makes it necessary to enrich the uranium fuel, which significantly increases the costs of fuel production. Compared to reactors operating on natural uranium, less energy is generated per unit of uranium ore even though a higher burnup can be achieved. Nuclear reprocessing can "stretch" the fuel supply of both natural uranium and enriched uranium reactors but is virtually only practiced for light water reactors operating with lightly enriched fuel as spent fuel from e.g. CANDU reactors is very low in fissile material.

Because water acts as a neutron moderator, it is not possible to build a fast-neutron reactor with a PWR design. A reduced moderation water reactor may however achieve a breeding ratio greater than unity, though this reactor design has disadvantages of its own.

Spent fuel from a PWR usually has a higher content of fissile material than natural uranium. Without nuclear reprocessing, this fissile material cannot be used as fuel in a PWR. It can, however, be used in a CANDU with only minimal reprocessing in a process called "DUPIC" - Direct Use of spent PWR fuel in CANDU.

Thermal efficiency, while better than for boiling water reactors, cannot achieve the values of reactors with higher operating temperatures such as those cooled with high temperature gases, liquid metals or molten salts. Similarly process heat drawn from a PWR is not suitable for most industrial applications as those require temperatures in excess of 400 °C (752 °F).

Radiolysis and certain accident scenarios which involve interactions between hot steam and zircalloy cladding can produce hydrogen from the cooling water leading to hydrogen explosions as a potential accident scenario. During the Fukushima nuclear accident a hydrogen explosion damaging the containment building was a major concern. Some reactors contain catalytic recombiners which let the hydrogen react with ambient oxygen in a non-explosive fashion.

Neurotoxicity

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Neurotoxicity

Neurotoxicity is a form of toxicity in which a biological, chemical, or physical agent produces an adverse effect on the structure or function of the central and/or peripheral nervous system. It occurs when exposure to a substance – specifically, a neurotoxin or neurotoxicant– alters the normal activity of the nervous system in such a way as to cause permanent or reversible damage to nervous tissue. This can eventually disrupt or even kill neurons, which are cells that transmit and process signals in the brain and other parts of the nervous system. Neurotoxicity can result from organ transplants, radiation treatment, certain drug therapies, recreational drug use, exposure to heavy metals, bites from certain species of venomous snakes, pesticides, certain industrial cleaning solvents, fuels and certain naturally occurring substances. Symptoms may appear immediately after exposure or be delayed. They may include limb weakness or numbness, loss of memory, vision, and/or intellect, uncontrollable obsessive and/or compulsive behaviors, delusions, headache, cognitive and behavioral problems and sexual dysfunction. Chronic mold exposure in homes can lead to neurotoxicity which may not appear for months to years of exposure. All symptoms listed above are consistent with mold mycotoxin accumulation.

The term neurotoxicity implies the involvement of a neurotoxin; however, the term neurotoxic may be used more loosely to describe states that are known to cause physical brain damage, but where no specific neurotoxin has been identified.

The presence of neurocognitive deficits alone is not usually considered sufficient evidence of neurotoxicity, as many substances may impair neurocognitive performance without resulting in the death of neurons. This may be due to the direct action of the substance, with the impairment and neurocognitive deficits being temporary, and resolving when the substance is eliminated from the body. In some cases the level or exposure-time may be critical, with some substances only becoming neurotoxic in certain doses or time periods. Some of the most common naturally occurring brain toxins that lead to neurotoxicity as a result of long term drug use are amyloid beta (Aβ), glutamate, dopamine, and oxygen radicals. When present in high concentrations, they can lead to neurotoxicity and death (apoptosis). Some of the symptoms that result from cell death include loss of motor control, cognitive deterioration and autonomic nervous system dysfunction. Additionally, neurotoxicity has been found to be a major cause of neurodegenerative diseases such as Alzheimer's disease (AD).

Neurotoxic agents

Amyloid beta

Amyloid beta (Aβ) was found to cause neurotoxicity and cell death in the brain when present in high concentrations. Aβ results from a mutation that occurs when protein chains are cut at the wrong locations, resulting in chains of different lengths that are unusable. Thus they are left in the brain until they are broken down, but if enough accumulate, they form plaques which are toxic to neurons. Aβ uses several routes in the central nervous system to cause cell death. An example is through the nicotinic acetylcholine receptor (nAchRs), which is a receptor commonly found along the surfaces of the cells that respond to nicotine stimulation, turning them on or off. Aβ was found manipulating the level of nicotine in the brain along with the MAP kinase, another signaling receptor, to cause cell death. Another chemical in the brain that Aβ regulates is JNK; this chemical halts the extracellular signal-regulated kinases (ERK) pathway, which normally functions as memory control in the brain. As a result, this memory favoring pathway is stopped, and the brain loses essential memory function. The loss of memory is a symptom of neurodegenerative disease, including AD. Another way Aβ causes cell death is through the phosphorylation of AKT; this occurs as the phosphate group is bound to several sites on the protein. This phosphorylation allows AKT to interact with BAD, a protein known to cause cell death. Thus an increase in Aβ results in an increase of the AKT/BAD complex, in turn stopping the action of the anti-apoptotic protein Bcl-2, which normally functions to stop cell death, causing accelerated neuron breakdown and the progression of AD.

Glutamate

Glutamate is a chemical found in the brain that poses a toxic threat to neurons when found in high concentrations. This concentration equilibrium is extremely delicate and is usually found in millimolar amounts extracellularly. When disturbed, an accumulation of glutamate occurs as a result of a mutation in the glutamate transporters, which act like pumps to clear glutamate from the synapse. This causes glutamate concentration to be several times higher in the blood than in the brain; in turn, the body must act to maintain equilibrium between the two concentrations by pumping the glutamate out of the bloodstream and into the neurons of the brain. In the event of a mutation, the glutamate transporters are unable to pump the glutamate back into the cells; thus a higher concentration accumulates at the glutamate receptors. This opens the ion channels, allowing calcium to enter the cell causing excitotoxicity. Glutamate results in cell death by turning on the N-methyl-D-aspartic acid receptors (NMDA); these receptors cause an increased release of calcium ions (Ca²⁺) into the cells. As a result, the increased concentration of Ca²⁺ directly increases the stress on mitochondria, resulting in excessive oxidative phosphorylation and production of reactive oxygen species (ROS) via the activation of nitric oxide synthase, ultimately leading to cell death. Aβ was also found aiding this route to neurotoxicity by enhancing neuron vulnerability to glutamate.

Oxygen radicals

The formation of oxygen radicals in the brain is achieved through the nitric oxide synthase (NOS) pathway. This reaction occurs as a response to an increase in the Ca²⁺ concentration inside a brain cell. This interaction between the Ca²⁺ and NOS results in the formation of the cofactor tetrahydrobiopterin (BH4), which then moves from the plasma membrane into the cytoplasm. As a final step, NOS is dephosphorylated yielding nitric oxide (NO), which accumulates in the brain, increasing its oxidative stress. There are several ROS, including superoxide, hydrogen peroxide and hydroxyl, all of which lead to neurotoxicity. Naturally, the body utilizes a defensive mechanism to diminish the fatal effects of the reactive species by employing certain enzymes to break down the ROS into small, benign molecules of simple oxygen and water. However, this breakdown of the ROS is not completely efficient; some reactive residues are left in the brain to accumulate, contributing to neurotoxicity and cell death. The brain is more vulnerable to oxidative stress than other organs, due to its low oxidative capacity. Because neurons are characterized as postmitotic cells, meaning that they live with accumulated damage over the years, accumulation of ROS is fatal. Thus, increased levels of ROS age neurons, which leads to accelerated neurodegenerative processes and ultimately the advancement of AD.

Dopaminergic Neurotoxicity

Endogenous

The endogenously produced autotoxin metabolite of dopamine, 3,4-Dihydroxyphenylacetaldehyde (DOPAL), is a potent inducer of programmed cell death (apoptosis) in dopaminergic neurons. DOPAL may play an important role in the pathology of Parkinson's disease.

Drug induced

Certain drugs, most famously the pesticide and metabolite MPP+ (1-methyl-4-phenylpyridin-1-ium) can induce Parkinson's disease by destroying dopaminergic neurons in the substantia nigra. MPP+ interacts with the electron transport chain in the mitochondria to generate reactive oxygen species which cause generalized oxidative damage and ultimately cell death. MPP+ is produced by monoamine oxidase B as a metabolite of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), and its toxicity is particularly significant to dopaminergic neurons because of an active transporter on those cells that bring it into the cytoplasm. The neurotoxicity of MPP+ was first investigated after MPTP was produced as a contaminant in the pethidine synthesized by a chemistry graduate student, who injected the contaminated drug and developed overt Parkinson's within weeks. Discovery of the mechanism of toxicity was an important advance in the study of Parkinson's disease, and the compound is now used to induce the disease in research animals.

Prognosis

The prognosis depends upon the length and degree of exposure and the severity of neurological injury. In some instances, exposure to neurotoxins or neurotoxicants can be fatal. In others, patients may survive but not fully recover. In other situations, many individuals recover completely after treatment.

The word neurotoxicity (/ˌnʊəroʊtɒkˈsɪsɪti/) uses combining forms of neuro- + tox- + -icity, yielding "nervous tissue poisoning".

Muon-catalyzed fusion

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Muon-catalyzed_fusion

Muon-catalyzed fusion (abbreviated as μCF or MCF) is a process allowing nuclear fusion to take place at temperatures significantly lower than the temperatures required for thermonuclear fusion, even at room temperature or lower. It is one of the few known ways of catalyzing nuclear fusion reactions.

Muons are unstable subatomic particles which are similar to electrons but 207 times more massive. If a muon replaces one of the electrons in a hydrogen molecule, the nuclei are consequently drawn 186 times closer than in a normal molecule, due to the reduced mass being 186 times the mass of an electron. When the nuclei move closer together, the fusion probability increases, to the point where a significant number of fusion events can happen at room temperature.

Methods for obtaining muons, however, require far more energy than can be produced by the resulting fusion reactions. Muons have a mean lifetime of 2.2 μs, much longer than many other subatomic particles but nevertheless far too brief to allow their useful storage.

To create useful room-temperature muon-catalyzed fusion, reactors would need a cheap, efficient muon source and/or a way for each individual muon to catalyze many more fusion reactions.

History

Andrei Sakharov and F.C. Frank predicted the phenomenon of muon-catalyzed fusion on theoretical grounds before 1950. Yakov Borisovich Zel'dovich also wrote about the phenomenon of muon-catalyzed fusion in 1954. Luis W. Alvarez et al., when analyzing the outcome of some experiments with muons incident on a hydrogen bubble chamber at Berkeley in 1956, observed muon-catalysis of exothermic p–d, proton and deuteron, nuclear fusion, which results in a helion, a gamma ray, and a release of about 5.5 MeV of energy. The Alvarez experimental results, in particular, spurred John David Jackson to publish one of the first comprehensive theoretical studies of muon-catalyzed fusion in his ground-breaking 1957 paper. This paper contained the first serious speculations on useful energy release from muon-catalyzed fusion. Jackson concluded that it would be impractical as an energy source, unless the "alpha-sticking problem" (see below) could be solved, leading potentially to an energetically cheaper and more efficient way of utilizing the catalyzing muons.

Viability as a power source

Potential benefits

If muon-catalyzed d–t nuclear fusion is realized practically, it will be a much more attractive way of generating power than conventional nuclear fission reactors because muon-catalyzed d–t nuclear fusion (like most other types of nuclear fusion), produces far fewer harmful (and far less long-lived) radioactive wastes.

The large number of neutrons produced in muon-catalyzed d–t nuclear fusions may be used to breed fissile fuels from fertile material – for example, thorium-232 could breed uranium-233 in this way. The fissile fuels that have been bred can then be "burned," either in a conventional critical nuclear fission reactor or in an unconventional subcritical fission reactor, for example, a reactor using nuclear transmutation to process nuclear waste, or a reactor using the energy amplifier concept devised by Carlo Rubbia and others.

Another benefit of muon-catalyzed fusion is that the fusion process can start with pure deuterium gas without tritium. Plasma fusion reactors like ITER or Wendelstein X7 need tritium to initiate and also need a tritium factory. Muon-catalyzed fusion generates tritium under operation and increases operating efficiency up to an optimum point when the deuterium:tritium ratio reaches about 1:1. Muon-catalyzed fusion can operate as a tritium factory and deliver tritium for material and plasma fusion research.

Problems facing practical exploitation

Except for some refinements, little has changed since Jackson's 1957 assessment of the feasibility of muon-catalyzed fusion other than Vesman's 1967 prediction of the hyperfine resonant formation of the muonic (d–μ–t)⁺ molecular ion which was subsequently experimentally observed. This helped spark renewed interest in the whole field of muon-catalyzed fusion, which remains an active area of research worldwide. However, as Jackson observed in his paper, muon-catalyzed fusion is "unlikely" to provide "useful power production ... unless an energetically cheaper way of producing μ⁻-mesons can be found."

One practical problem with the muon-catalyzed fusion process is that muons are unstable, decaying in 2.2 μs (in their rest frame). Hence, there needs to be some cheap means of producing muons, and the muons must be arranged to catalyze as many nuclear fusion reactions as possible before decaying.

Another, and in many ways more serious, problem is the "alpha-sticking" problem, which was recognized by Jackson in his 1957 paper. The α-sticking problem is the approximately 1% probability of the muon "sticking" to the alpha particle that results from deuteron-triton nuclear fusion, thereby effectively removing the muon from the muon-catalysis process altogether. Even if muons were absolutely stable, each muon could catalyze, on average, only about 100 d-t fusions before sticking to an alpha particle, which is only about one-fifth the number of muon catalyzed d–t fusions needed for break-even, where as much thermal energy is generated as electrical energy is consumed to produce the muons in the first place, according to Jackson's rough estimate.

More recent measurements seem to point to more encouraging values for the α-sticking probability, finding the α-sticking probability to be around 0.3% to 0.5%, which could mean as many as about 200 (even up to 350) muon-catalyzed d–t fusions per muon. Indeed, the team led by Steven E. Jones achieved 150 d–t fusions per muon (average) at the Los Alamos Meson Physics Facility. The results were promising and almost enough to reach theoretical break-even. Unfortunately, these measurements for the number of muon-catalyzed d–t fusions per muon are still not enough to reach industrial break-even. Even with break-even, the conversion efficiency from thermal energy to electrical energy is only about 40% or so, further limiting viability. The best recent estimates of the electrical "energy cost" per muon is about 6 GeV with accelerators that are (coincidentally) about 40% efficient at transforming electrical energy from the power grid into acceleration of the deuterons.

As of 2012, no practical method of producing energy through this means has been published, although some discoveries using the Hall effect show promise.

Alternative estimation of breakeven

According to Gordon Pusch, a physicist at Argonne National Laboratory, various breakeven calculations on muon-catalyzed fusion omit the heat energy the muon beam itself deposits in the target. By taking this factor into account, muon-catalyzed fusion can already exceed breakeven; however, the recirculated power is usually very large compared to power out to the electrical grid (about 3–5 times as large, according to estimates). Despite this rather high recirculated power, the overall cycle efficiency is comparable to conventional fission reactors; however the need for 4–6 MW electrical generating capacity for each megawatt out to the grid probably represents an unacceptably large capital investment. Pusch suggested using Bogdan Maglich's "migma" self-colliding beam concept to significantly increase the muon production efficiency, by eliminating target losses, and using tritium nuclei as the driver beam, to optimize the number of negative muons.

In 2021, Kelly, Hart and Rose produced a μCF model whereby the ratio, Q, of thermal energy produced to the kinetic energy of the accelerated deuterons used to create negative pions (and thus negative muons through pion decay) was optimized. In this model, the heat energy of the incoming deuterons as well as that of the particles produced due to the deuteron beam impacting a tungsten target was recaptured to the extent possible, as suggested by Gordon Pusch in the previous paragraph. Additionally, heat energy due to tritium breeding in a lithium-lead shell was recaptured, as suggested by Jändel, Danos and Rafelski in 1988. The best Q value was found to be about 130% assuming that 50% of the muons produced were actually utilized for fusion catalysis. Furthermore, assuming that the accelerator was 18% efficient at transforming electrical energy into deuteron kinetic energy and conversion efficiency of heat energy into electrical energy of 60%, they estimate that, currently, the amount of electrical energy that could be produced by a μCF reactor would be 14% of the electrical energy consumed. In order for this to improve, they suggest that some combination of a) increasing accelerator efficiency and b) increasing the number of fusion reactions per negative muon above the assumed level of 150 would be needed.

Process

To create this effect, a stream of negative muons, most often created by decaying pions, is sent to a block that may be made up of all three hydrogen isotopes (protium, deuterium, and/or tritium), where the block is usually frozen, and the block may be at temperatures of about 3 kelvin (−270 degrees Celsius) or so. The muon may bump the electron from one of the hydrogen isotopes. The muon, 207 times more massive than the electron, effectively shields and reduces the electromagnetic repulsion between two nuclei and draws them much closer into a covalent bond than an electron can. Because the nuclei are so close, the strong nuclear force is able to kick in and bind both nuclei together. They fuse, release the catalytic muon (most of the time), and part of the original mass of both nuclei is released as energetic particles, as with any other type of nuclear fusion. The release of the catalytic muon is critical to continue the reactions. The majority of the muons continue to bond with other hydrogen isotopes and continue fusing nuclei together. However, not all of the muons are recycled: some bond with other debris emitted following the fusion of the nuclei (such as alpha particles and helions), removing the muons from the catalytic process. This gradually chokes off the reactions, as there are fewer and fewer muons with which the nuclei may bond. The number of reactions achieved in the lab can be as high as 150 d–t fusions per muon (average).

Deuterium–tritium (d–t or dt)

In the muon-catalyzed fusion of most interest, a positively charged deuteron (d), a positively charged triton (t), and a muon essentially form a positively charged muonic molecular heavy hydrogen ion (d–μ–t)⁺. The muon, with a rest mass 207 times greater than the rest mass of an electron, is able to drag the more massive triton and deuteron 207 times closer together to each other in the muonic (d–μ–t)⁺ molecular ion than can an electron in the corresponding electronic (d–e–t)⁺ molecular ion. The average separation between the triton and the deuteron in the electronic molecular ion is about one angstrom (100 pm), so the average separation between the triton and the deuteron in the muonic molecular ion is 207 times smaller than that. Due to the strong nuclear force, whenever the triton and the deuteron in the muonic molecular ion happen to get even closer to each other during their periodic vibrational motions, the probability is very greatly enhanced that the positively charged triton and the positively charged deuteron would undergo quantum tunnelling through the repulsive Coulomb barrier that acts to keep them apart. Indeed, the quantum mechanical tunnelling probability depends roughly exponentially on the average separation between the triton and the deuteron, allowing a single muon to catalyze the d–t nuclear fusion in less than about half a picosecond, once the muonic molecular ion is formed.

The formation time of the muonic molecular ion is one of the "rate-limiting steps" in muon-catalyzed fusion that can easily take up to ten thousand or more picoseconds in a liquid molecular deuterium and tritium mixture (D₂, DT, T₂), for example. Each catalyzing muon thus spends most of its ephemeral existence of 2.2 microseconds, as measured in its rest frame, wandering around looking for suitable deuterons and tritons with which to bind.

Another way of looking at muon-catalyzed fusion is to try to visualize the ground state orbit of a muon around either a deuteron or a triton. Suppose the muon happens to have fallen into an orbit around a deuteron initially, which it has about a 50% chance of doing if there are approximately equal numbers of deuterons and tritons present, forming an electrically neutral muonic deuterium atom (d–μ)⁰ that acts somewhat like a "fat, heavy neutron" due both to its relatively small size (again, 207 times smaller than an electrically neutral electronic deuterium atom (d–e)⁰) and to the very effective "shielding" by the muon of the positive charge of the proton in the deuteron. Even so, the muon still has a much greater chance of being transferred to any triton that comes near enough to the muonic deuterium than it does of forming a muonic molecular ion. The electrically neutral muonic tritium atom (t–μ)⁰ thus formed will act somewhat like an even "fatter, heavier neutron," but it will most likely hang on to its muon, eventually forming a muonic molecular ion, most likely due to the resonant formation of a hyperfine molecular state within an entire deuterium molecule D₂ (d=e²=d), with the muonic molecular ion acting as a "fatter, heavier nucleus" of the "fatter, heavier" neutral "muonic/electronic" deuterium molecule ([d–μ–t]=e²=d), as predicted by Vesman, an Estonian graduate student, in 1967.

Once the muonic molecular ion state is formed, the shielding by the muon of the positive charges of the proton of the triton and the proton of the deuteron from each other allows the triton and the deuteron to tunnel through the Coulomb barrier in time span of order of a nanosecond The muon survives the d–t muon-catalyzed nuclear fusion reaction and remains available (usually) to catalyze further d–t muon-catalyzed nuclear fusions. Each exothermic d–t nuclear fusion releases about 17.6 MeV of energy in the form of a "very fast" neutron having a kinetic energy of about 14.1 MeV and an alpha particle α (a helium-4 nucleus) with a kinetic energy of about 3.5 MeV. An additional 4.8 MeV can be gleaned by having the fast neutrons moderated in a suitable "blanket" surrounding the reaction chamber, with the blanket containing lithium-6, whose nuclei, known by some as "lithions," readily and exothermically absorb thermal neutrons, the lithium-6 being transmuted thereby into an alpha particle and a triton.

Deuterium–deuterium and other types

The first kind of muon–catalyzed fusion to be observed experimentally, by L.W. Alvarez et al., was protium (H or ₁H¹) and deuterium (D or ₁H²) muon-catalyzed fusion. The fusion rate for p–d (or pd) muon-catalyzed fusion has been estimated to be about a million times slower than the fusion rate for d–t muon-catalyzed fusion.

Of more practical interest, deuterium–deuterium muon-catalyzed fusion has been frequently observed and extensively studied experimentally, in large part because deuterium already exists in relative abundance and, like protium, deuterium is not at all radioactive. (Tritium rarely occurs naturally, and is radioactive with a half-life of about 12.5 years.)

The fusion rate for d–d muon-catalyzed fusion has been estimated to be only about 1% of the fusion rate for d–t muon-catalyzed fusion, but this still gives about one d–d nuclear fusion every 10 to 100 picoseconds or so. However, the energy released with every d–d muon-catalyzed fusion reaction is only about 20% or so of the energy released with every d–t muon-catalyzed fusion reaction. Moreover, the catalyzing muon has a probability of sticking to at least one of the d–d muon-catalyzed fusion reaction products that Jackson in this 1957 paper estimated to be at least 10 times greater than the corresponding probability of the catalyzing muon sticking to at least one of the d–t muon-catalyzed fusion reaction products, thereby preventing the muon from catalyzing any more nuclear fusions. Effectively, this means that each muon catalyzing d–d muon-catalyzed fusion reactions in pure deuterium is only able to catalyze about one-tenth of the number of d–t muon-catalyzed fusion reactions that each muon is able to catalyze in a mixture of equal amounts of deuterium and tritium, and each d–d fusion only yields about one-fifth of the yield of each d–t fusion, thereby making the prospects for useful energy release from d–d muon-catalyzed fusion at least 50 times worse than the already dim prospects for useful energy release from d–t muon-catalyzed fusion.

Potential "aneutronic" (or substantially aneutronic) nuclear fusion possibilities, which result in essentially no neutrons among the nuclear fusion products, are almost certainly not very amenable to muon-catalyzed fusion. One such essentially aneutronic nuclear fusion reaction involves a deuteron from deuterium fusing with a helion (He⁺²) from helium-3, which yields an energetic alpha particle and a much more energetic proton, both positively charged (with a few neutrons coming from inevitable d–d nuclear fusion side reactions). However, one muon with only one negative electric charge is incapable of shielding both positive charges of a helion from the one positive charge of a deuteron. The chances of the requisite two muons being present simultaneously are exceptionally remote.

In culture

The term "cold fusion" was coined to refer to muon-catalyzed fusion in a 1956 New York Times article about Luis W. Alvarez's paper.

In 1957 Theodore Sturgeon wrote a novelette, "The Pod in the Barrier", in which humanity has ubiquitous cold fusion reactors that work with muons. The reaction is "When hydrogen one and hydrogen two are in the presence of Mu mesons, they fuse into helium three, with an energy yield in electron volts of 5.4 times ten to the fifth power". Unlike the thermonuclear bomb contained in the Pod (which is used to destroy the Barrier) they can become temporarily disabled by "concentrated disbelief" that muon fusion works.

In Sir Arthur C. Clarke's third novel in the Space Odyssey series, 2061: Odyssey Three, muon-catalyzed fusion is the technology that allows mankind to achieve easy interplanetary travel. The main character, Heywood Floyd, compares Luis Alvarez to Lord Rutherford for underestimating the future potential of their discoveries.

Nuclear fusion–fission hybrid

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Nuclear_fusion%E2%80%93fission_hybrid

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 basic idea is to use high-energy fast neutrons from a fusion reactor to trigger fission in non-fissile fuels like U-238 or Th-232. Each neutron can trigger several fission events, multiplying the energy released by each fusion reaction hundreds of times. As the fission fuel is not fissile, there is no self-sustaining chain reaction from fission. This would not only make fusion designs more economical in power terms, but also be able to burn fuels that were not suitable for use in conventional fission plants, even their nuclear waste.

In general terms, the hybrid is similar in concept to the fast breeder reactor, which uses a compact high-energy fission core in place of the hybrid's fusion core. Another similar concept is the accelerator-driven subcritical reactor, which uses a particle accelerator to provide the neutrons instead of nuclear reactions.

History

The concept dates to the 1950s, and was strongly advocated by Hans Bethe during the 1970s. At that time the first powerful fusion experiments were being built, but it would still be many years before they could be economically competitive. Hybrids were proposed as a way of greatly accelerating their market introduction, producing energy even before the fusion systems reached break-even. However, detailed studies of the economics of the systems suggested they could not compete with existing fission reactors.

The idea was abandoned and lay dormant until the 2000s, when the continued delays in reaching break-even led to a brief revival around 2009. These studies generally concentrated on the nuclear waste disposal aspects of the design, as opposed to the production of energy. The concept has seen cyclical interest since then, based largely on the success or failure of more conventional solutions like the Yucca Mountain nuclear waste repository

Another major design effort for energy production was started at Lawrence Livermore National Laboratory (LLNL) under their LIFE program. Industry input led to the abandonment of the hybrid approach for LIFE, which was then re-designed as a pure-fusion system. LIFE was cancelled when the underlying technology, from the National Ignition Facility, failed to reach its design performance goals.

Apollo Fusion, a company founded by Google executive Mike Cassidy in 2017, was also reported to be focused on using the subcritical nuclear fusion-fission hybrid method. Their web site is now focussed on their hall effect thrusters, and mentions fusion only in passing.

On 2022, September 9, Professor Peng Xianjue of the Chinese Academy of Engineering Physics announced that the Chinese government had approved the construction of the world's largest pulsed-powerplant - the Z-FFR, namely Z(-pinch)-Fission-Fusion Reactor- in Chengdu, Sichuan province. Neutrons produced in a Z-pinch facility (endowed with cylindrical symmetry and fuelled with deuterium and tritium) will strike a coaxial blanket including both uranium and lithium isotopes. Uranium fission will boost the facility's overall heat output by 10 to 20 times. Interaction of lithium and neutrons will provide tritium for further fueling. Innovative, quasi-spherical geometry near the core of Z-FFR leads to high performance of Z-pinch discharge. According to Prof. Xianjue, this will considerably speed up the use of fusion energy and prepare it for commercial power production by 2035.

Fission basics

Conventional fission power systems rely on a chain reaction of nuclear fission events that release a few neutrons that cause further fission events. By careful arrangement and the use of various absorber materials the system can be set in a balance of released and absorbed neutrons, known as criticality.

Natural uranium is a mix of several isotopes, mainly a trace amount of ²³⁵U and over 99% ²³⁸U. When they undergo fission, both of these isotopes release fast neutrons with an energy distribution peaking around 1 to 2 MeV. This energy is too low to cause fission in ²³⁸U, which means it cannot sustain a chain reaction. ²³⁵U will undergo fission when struck by neutrons of this energy, so it is possible for ²³⁵U to sustain a chain reaction. There are too few ²³⁵U atoms in natural uranium to sustain a chain reaction, the atoms are spread out too far and the chance a neutron will hit one is too small. Chain reactions are accomplished by concentrating, or enriching, the fuel, increasing the amount of ²³⁵U to produce enriched uranium, while the leftover, now mostly ²³⁸U, is a waste product known as depleted uranium. ²³⁵U will sustain a chain reaction if enriched to about 20% of the fuel mass.

²³⁵U will undergo fission more easily if the neutrons are of lower energy, the so-called thermal neutrons. Neutrons can be slowed to thermal energies through collisions with a neutron moderator material, the easiest to use are the hydrogen atoms found in water. By placing the fission fuel in water, the probability that the neutrons will cause fission in another ²³⁵U is greatly increased, which means the level of enrichment needed to reach criticality is greatly reduced. This leads to the concept of reactor-grade enriched uranium, with the amount of ²³⁵U increased from just less than 1% in natural ore to between 3 and 5%, depending on the reactor design. This is in contrast to weapons-grade enrichment, which increases to the ²³⁵U to at least 20%, and more commonly, over 90%.

In order to maintain criticality, the fuel has to retain that extra concentration of ²³⁵U. A typical fission reactor burns off enough of the ²³⁵U to cause the reaction to stop over a period on the order of a few months. A combination of burnup of the ²³⁵U along with the creation of neutron absorbers, or poisons, as part of the fission process eventually results in the fuel mass not being able to maintain criticality. This burned up fuel has to be removed and replaced with fresh fuel. The result is nuclear waste that is highly radioactive and filled with long-lived radionuclides that present a safety concern.

The waste contains most of the ²³⁵U it started with, only 1% or so of the energy in the fuel is extracted by the time it reaches the point where it is no longer fissile. One solution to this problem is to reprocess the fuel, which uses chemical processes to separate the ²³⁵U (and other non-poison elements) from the waste, and then mixes the extracted ²³⁵U5 in fresh fuel loads. This reduces the amount of new fuel that needs to be mined and also concentrates the unwanted portions of the waste into a smaller load. Reprocessing is expensive, however, and it has generally been more economical to simply buy fresh fuel from the mine.

Like ²³⁵U, ²³⁹Pu can maintain a chain reaction, so it is a useful reactor fuel. However, ²³⁹Pu is not found in commercially useful amounts in nature. Another possibility is to breed ²³⁹Pu from the ²³⁸U through neutron capture, or various other means. This process only occurs with higher-energy neutrons than would be found in a moderated reactor, so a conventional reactor only produces small amounts of Pu when the neutron is captured within the fuel mass before it is moderated.

More typically, special reactors are used that are designed specifically for the breeding of ²³⁹Pu. The simplest way to achieve this is to further enrich the original ²³⁵U fuel well beyond what is needed for use in a moderated reactor, to the point where the ²³⁵U maintains criticality even with the fast neutrons. The extra fast neutrons escaping the fuel load can then be used to breed fuel in a ²³⁸U assembly surrounding the reactor core, most commonly taken from the stocks of depleted uranium. ²³⁹Pu can also be used for the core, which means once the system is up and running, it can be refuelled using the ²³⁹Pu it creates, with enough left over to feed into other reactors as well.

Extracting the ²³⁹Pu from the ²³⁸U feedstock can be achieved with chemical processing, in the same fashion as normal reprocessing. The difference is that the mass will contain far fewer other elements, particularly some of the highly radioactive fission products found in normal nuclear waste. Unfortunately it is a tendency that breeder reactors in the "free world" (like the SNR-300, the Integral fast reactor) that have been built were demolished before operation, as a "symbol" (as Bill Clinton has stated). The Prototype Fast Breeder Reactor passed tests in 2017 and apparently is about to face the same fate, leaving some military reactors and the Russian BN-800 reactor operating, mostly consuming spent nuclear fuel.

Fusion basics

Fusion reactors typically burn a mixture of deuterium (D) and tritium (T). When heated to millions of degrees, the kinetic energy in the fuel begins to overcome the natural electrostatic repulsion between nuclei, the so-called coulomb barrier, and the fuel begins to undergo fusion. This reaction gives off an alpha particle and a high energy neutron of 14 MeV. A key requirement to the economic operation of a fusion reactor is that the alphas deposit their energy back into the fuel mix, heating it so that additional fusion reactions take place. This leads to a condition not unlike the chain reaction in the fission case, known as ignition.

Deuterium can be obtained by the separation of hydrogen isotopes in sea water (see heavy water production). Tritium has a short half life of just over a decade, so only trace amounts are found in nature. To fuel the reactor, the neutrons from the reaction are used to breed more tritium through a reaction in a blanket of lithium surrounding the reaction chamber. Tritium breeding is key to the success of a D-T fusion cycle, and to date this technique has not been demonstrated. Predictions based on computer modeling suggests that the breeding ratios are quite small and a fusion plant would barely be able to cover its own use. Many years would be needed to breed enough surplus to start another reactor.

Hybrid concepts

Fusion–fission designs essentially replace the lithium blanket with a blanket of fission fuel, either natural uranium ore or even nuclear waste. The fusion neutrons have more than enough energy to cause fission in the ²³⁸U, as well as many of the other elements in the fuel, including some of the transuranic waste elements. The reaction can continue even when all of the ²³⁵U is burned off; the rate is controlled not by the neutrons from the fission events, but the neutrons being supplied by the fusion reactor.

Fission occurs naturally because each event gives off more than one neutron capable of producing additional fission events. Fusion, at least in D-T fuel, gives off only a single neutron, and that neutron is not capable of producing more fusion events. When that neutron strikes fissile material in the blanket, one of two reactions may occur. In many cases, the kinetic energy of the neutron will cause one or two neutrons to be struck out of the nucleus without causing fission. These neutrons still have enough energy to cause other fission events. In other cases the neutron will be captured and cause fission, which will release two or three neutrons. This means that every fusion neutron in the fusion–fission design can result in anywhere between two and four neutrons in the fission fuel.

This is a key concept in the hybrid concept, known as fission multiplication. For every fusion event, several fission events may occur, each of which gives off much more energy than the original fusion, about 11 times. This greatly increases the total power output of the reactor. This has been suggested as a way to produce practical fusion reactors in spite of the fact that no fusion reactor has yet reached break-even, by multiplying the power output using cheap fuel or waste. However, a number of studies have repeatedly demonstrated that this only becomes practical when the overall reactor is very large, 2 to 3 GWt, which makes it expensive to build.

These processes also have the side-effect of breeding ²³⁹Pu or ²³³U, which can be removed and used as fuel in conventional fission reactors. This leads to an alternate design where the primary purpose of the fusion–fission reactor is to reprocess waste into new fuel. Although far less economical than chemical reprocessing, this process also burns off some of the nastier elements instead of simply physically separating them out. This also has advantages for non-proliferation, as enrichment and reprocessing technologies are also associated with nuclear weapons production. However, the cost of the nuclear fuel produced is very high, and is unlikely to be able to compete with conventional sources.

Neutron economy

A key issue for the fusion–fission concept is the number and lifetime of the neutrons in the various processes, the so-called neutron economy.

In a pure fusion design, the neutrons are used for breeding tritium in a lithium blanket. Natural lithium consists of about 92% ⁷Li and the rest is mostly ⁶Li. ⁷Li breeding requires neutron energies even higher than those released by fission, around 5 MeV, well within the range of energies provided by fusion. This reaction produces tritium and helium-4, and another slow neutron. ⁶Li can react with high or low energy neutrons, including those released by the ⁷Li reaction. This means that a single fusion reaction can produce several tritiums, which is a requirement if the reactor is going to make up for natural decay and losses in the fusion processes.

When the lithium blanket is replaced, or supplanted, by fission fuel in the hybrid design, neutrons that do react with the fissile material are no longer available for tritium breeding. The new neutrons released from the fission reactions can be used for this purpose, but only in ⁶Li. One could process the lithium to increase the amount of ⁶Li in the blanket, making up for these losses, but the downside to this process is that the ⁶Li reaction only produces one tritium atom. Only the high-energy reaction between the fusion neutron and ⁷Li can create more than one tritium, and this is essential for keeping the reactor running.

To address this issue, at least some of the fission neutrons must also be used for tritium breeding in ⁶Li. Every one that does is no longer available for fission, reducing the reactor output. This requires a very careful balance if one wants the reactor to be able to produce enough tritium to keep itself running, while also producing enough fission events to keep the fission side energy positive. If these cannot be accomplished simultaneously, there is no reason to build a hybrid. Even if this balance can be maintained, it might only occur at a level that is economically infeasible.

Overall economy

Through the early development of the hybrid concept the question of overall economics appeared difficult to handle. A series of studies starting in the late 1970s provided a much clearer picture of the hybrid in a complete fuel cycle, and allowed the economics to be better understood. These studies appeared to indicate there was no reason to build a hybrid.

One of the most detailed of these studies was published in 1980 by Los Alamos National Laboratory (LANL). Their study noted that the hybrid would produce most of its energy indirectly, both through the fission events in its own reactor, and much more by providing Pu-239 to fuel conventional fission reactors. In this overall picture, the hybrid is essentially identical to the breeder reactor, which uses fast neutrons from plutonium fission to breed more fuel in a fission blanket in largely the same fashion as the hybrid. Both require chemical processing to remove the bred Pu-239, both presented the same proliferation and safety risks as a result, and both produced about the same amount of fuel. Since that fuel is the primary source of energy in the overall cycle, the two systems were almost identical in the end.

What was not identical, however, was the technical maturity of the two designs. The hybrid would require considerable additional research and development before it would be known if it could even work, and even if that were demonstrated, the result would be a system essentially identical to breeders which were already being built at that time. The report concluded:

The investment of time and money required to commercialize the hybrid cycle could only be justified by a real or perceived advantage of the hybrid over the classical FBR. Our analysis leads us to conclude that no such advantage exists. Therefore, there is not sufficient incentive to demonstrate and commercialize the fusion–fission hybrid.

Rationale

The fusion process alone currently does not achieve sufficient gain (power output over power input) to be viable as a power source. By using the excess neutrons from the fusion reaction to in turn cause a high-yield fission reaction (close to 100%) in the surrounding subcritical fissionable blanket, the net yield from the hybrid fusion–fission process can provide a targeted gain of 100 to 300 times the input energy (an increase by a factor of three or four over fusion alone). Even allowing for high inefficiencies on the input side (i.e. low laser efficiency in ICF and Bremsstrahlung losses in Tokamak designs), this can still yield sufficient heat output for economical electric power generation. This can be seen as a shortcut to viable fusion power until more efficient pure fusion technologies can be developed, or as an end in itself to generate power, and also consume existing stockpiles of nuclear fissionables and waste products.

In the LIFE project at the Lawrence Livermore National Laboratory LLNL, using technology developed at the National Ignition Facility, the goal is to use fuel pellets of deuterium and tritium surrounded by a fissionable blanket to produce energy sufficiently greater than the input (laser) energy for electrical power generation. The principle involved is to induce inertial confinement fusion (ICF) in the fuel pellet which acts as a highly concentrated point source of neutrons which in turn converts and fissions the outer fissionable blanket. In parallel with the ICF approach, the University of Texas at Austin is developing a system based on the tokamak fusion reactor, optimising for nuclear waste disposal versus power generation. The principles behind using either ICF or tokamak reactors as a neutron source are essentially the same (the primary difference being that ICF is essentially a point-source of neutrons while Tokamaks are more diffuse toroidal sources).

Use to dispose of nuclear waste

The surrounding blanket can be a fissile material (enriched uranium or plutonium) or a fertile material (capable of conversion to a fissionable material by neutron bombardment) such as thorium, depleted uranium or spent nuclear fuel. Such subcritical reactors (which also include particle accelerator-driven neutron spallation systems) offer the only currently-known means of active disposal (versus storage) of spent nuclear fuel without reprocessing. Fission by-products produced by the operation of commercial light water nuclear reactors (LWRs) are long-lived and highly radioactive, but they can be consumed using the excess neutrons in the fusion reaction along with the fissionable components in the blanket, essentially destroying them by nuclear transmutation and producing a waste product which is far safer and less of a risk for nuclear proliferation. The waste would contain significantly reduced concentrations of long-lived, weapons-usable actinides per gigawatt-year of electric energy produced compared to the waste from a LWR. In addition, there would be about 20 times less waste per unit of electricity produced. This offers the potential to efficiently use the very large stockpiles of enriched fissile materials, depleted uranium, and spent nuclear fuel.

Safety

In contrast to current commercial fission reactors, hybrid reactors potentially demonstrate what is considered inherently safe behavior because they remain deeply subcritical under all conditions and decay heat removal is possible via passive mechanisms. The fission is driven by neutrons provided by fusion ignition events, and is consequently not self-sustaining. If the fusion process is deliberately shut off or the process is disrupted by a mechanical failure, the fission damps out and stops nearly instantly. This is in contrast to the forced damping in a conventional reactor by means of control rods which absorb neutrons to reduce the neutron flux below the critical, self-sustaining, level. The inherent danger of a conventional fission reactor is any situation leading to a positive feedback, runaway, chain reaction such as occurred during the Chernobyl disaster. In a hybrid configuration the fission and fusion reactions are decoupled, i.e. while the fusion neutron output drives the fission, the fission output has no effect whatsoever on the fusion reaction, eliminating any chance of a positive feedback loop.

Fuel cycle

There are three main components to the hybrid fusion fuel cycle: deuterium, tritium, and fissionable elements. Deuterium can be derived by the separation of hydrogen isotopes in seawater (see heavy water production). Tritium may be generated in the hybrid process itself by absorption of neutrons in lithium bearing compounds. This would entail an additional lithium-bearing blanket and a means of collection. Small amounts of tritium are also produced by neutron activation in nuclear fission reactors, particularly when heavy water is used as a neutron moderator or coolant. The third component is externally derived fissionable materials from demilitarized supplies of fissionables, or commercial nuclear fuel and waste streams. Fusion driven fission also offers the possibility of using thorium as a fuel, which would greatly increase the potential amount of fissionables available. The extremely energetic nature of the fast neutrons emitted during the fusion events (up to 0.17 the speed of light) can allow normally non-fissioning ²³⁸U to undergo fission directly (without conversion first to ²³⁹Pu), enabling refined natural Uranium to be used with very low enrichment, while still maintaining a deeply subcritical regime.

Engineering considerations

Practical engineering designs must first take into account safety as the primary goal. All designs should incorporate passive cooling in combination with refractory materials to prevent melting and reconfiguration of fissionables into geometries capable of un-intentional criticality. Blanket layers of Lithium bearing compounds will generally be included as part of the design to generate Tritium to allow the system to be self-supporting for one of the key fuel element components. Tritium, because of its relatively short half-life and extremely high radioactivity, is best generated on-site to obviate the necessity of transportation from a remote location. D-T fuel can be manufactured on-site using Deuterium derived from heavy water production and Tritium generated in the hybrid reactor itself. Nuclear spallation to generate additional neutrons can be used to enhance the fission output, with the caveat that this is a tradeoff between the number of neutrons (typically 20-30 neutrons per spallation event) against a reduction of the individual energy of each neutron. This is a consideration if the reactor is to use natural Thorium as a fuel. While high energy (0.17c) neutrons produced from fusion events are capable of directly causing fission in both Thorium and ²³⁸U, the lower energy neutrons produced by spallation generally cannot. This is a tradeoff that affects the mixture of fuels against the degree of spallation used in the design.