Fast neutron reactor

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The BN-350 fast-neutron reactor at Aktau, Kazakhstan. It operated between 1973 and 1994.

A fast-neutron reactor (FNR), a.k.a. fast-spectrum reactor or simply a fast reactor, is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as opposed to thermal neutrons used in thermal-neutron reactors. Such a reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain decisive advantages over the existing fleet of water cooled and water moderated reactors. These are:

  • Atoms heavier than uranium have a much greater chance of fission with a fast neutron, than with a thermal one. This means that the inventory of heavier atoms in the nuclear waste stream, for example Curium, is greatly reduced, leading to a substantial lower waste management requirement.[1]
  • FNRs are capable of burning spent nuclear fuel, old bomb cores, depleted uranium, and thorium.[2]
  • More neutrons are produced when a fission occurs, resulting from the absorption of a fast neutron, than the comparable process with slow (thermal, or moderated) neutrons. Thus, criticality is easier to attain than with slower neutrons.
  • All fast reactor design built to this date use liquid metals as coolant, such as the sodium fast reactor and the Lead-cooled fast reactor. As the boiling points of these metals is very high, the pressure in the reactor can be maintained at a low level, which improves safety considerably.
  • As temperatures in the core can also be substantially higher than in a water cooled design, such reactors have a greater thermodynamic efficiency; a larger percentage of the heat generated is turned into usable electricity.

In the GEN IV initiative, about two thirds of the proposed reactors for the future use a fast spectrum for these reasons.

In order to describe the properties of a fast reactor design, an overview of neutron moderated reactor properties is first needed.

Fission processes

Fast reactors operate by the fission of uranium and other heavy atoms, similar to thermal reactors. However, there are crucial differences, arising from the fact that by far most commercial nuclear reactors use a moderator, and fast reactors do not.

Moderators in conventional nuclear reactors

Natural uranium consists mostly of two isotopes:

Of these two, uranium-238 undergoes fission only by fast neutrons.[3] About 0.7% of natural uranium is uranium-235, which will fission by both fast and thermal neutrons. When the uranium undergoes fission, it releases neutrons with a high energy ("fast"). However, such fast neutrons which are liberated by fission processes, have a much lower probability to cause another fission than neutron which are slowed down after they have been generated by the fission process. Slower neutrons have a much bigger chance (about a thousand times) of causing a fission in uranium-235 than the fast neutrons.

The common solution to this problem is to slow the neutrons down using a neutron moderator, which interacts with the neutrons to slow them ("thermal" neutrons). The most common moderator is ordinary water, which acts by elastic scattering until the neutrons reach thermal equilibrium with the water, at which point the neutrons become highly reactive with the uranium-235. Other moderators include graphite. The elastic scattering of the neutrons can be likened to the collision of two ping pong balls; when a fast ping pong ball hits one that hardly moves, they will both end up having about half of the original kinetic energy of the fast ball. This is in contrast to a fast ping pong ball hitting a bowling ball, where the ping pong ball keeps virtually all of its energy.

Such thermal neutrons are about a thousand times more likely to be absorbed by another heavy element, such as uranium-238 or uranium-235. In this case, only the uranium-235 undergoes fission. Although uranium-238 does not undergo fission by the neutrons released in fission, thermal neutrons (i.e. neutrons that have been slowed down by a moderator) can be captured by the nucleus to transmute the uranium into plutonium-239. plutonium-239 has a neutron cross section similar to that of uranium-235, which means that in turn, it can absorb yet another thermal neutron.

Unfortunately, only about 40% of the plutonium-239 created this way will undergo fission from the thermal neutrons. When plutonium-239 absorbs another neutron without undergoing fission, plutonium-240 is created, which virtually never fissions with the slower neutrons, when it in turn absorbs one, but just absorbs the neutron to become a heavier isotope. These effects combined have the result of creating, in a (water) moderated reactor, the presence of the transuranic elements. Such isotopes are themselves often not stable, and undergo Beta decay to create ever heavier elements, such as Americium and Curium. Thus, in moderated reactors, plutonium isotopes in many instances do not fission (and so do not create new neutrons), but instead just absorb the neutrons. Therefore, after a certain amount of time (around 12–18 months of stable operation in water moderated reactors), the nuclear reactor can no longer sustain the fission process, and the reactor has to be refueled.

Drawbacks of (water) moderators in conventional nuclear reactors

The following disadvantages of the use of a moderator have instigated the research and development of fast reactors.[4]

Although cheap, readily available and easily purified, water can absorb a neutron and remove it from the reaction. It does this just enough that the concentration of uranium-235 in natural uranium is too low to sustain the chain reaction; the neutrons lost through absorption in the water and uranium-238, along with those lost to the environment, results in too few left in the fuel. The most common solution to this problem is to slightly concentrate the amount of uranium-235 in the fuel to produce enriched uranium, with the leftover uranium-238 known as depleted uranium. Other designs use different moderators, like heavy water, that are much less likely to absorb neutrons, allowing them to run on unenriched fuel. See CANDU. In either case, the reactor's neutron economy is based on thermal neutrons.

A second drawback of using water for cooling is that it has a relatively low boiling point. The vast majority of electricity production uses steam turbines. These become more efficient as the pressure (and thus the temperature) of the steam is higher. A water cooled and moderated nuclear reactor therefore needs to operate at high pressures to enable the efficient production of electricity. Thus, such reactors are constructed using very heavy steel vessels, for example 25 cm (12 inch) thick. This high pressure operation adds complexity to reactor design and requires extensive physical safety measures. The vast majority of nuclear reactors in the world are water cooled and moderated with water. Examples include the PWR, the BWR and the CANDU reactors. In Russia and the UK, reactors are operational that use graphite as moderator, and resp. water and gas as coolant.

As the operational temperature and pressure of these reactors is dictated by engineering and safety constraints, both are limited. Thus, the temperatures and pressures that can be delivered to the steam turbine are also limited. Typical water temperatures of a modern Pressurized water reactor are around 350 Celsius, with pressures of around 85 bar. Compared to for example modern coal-fired steam circuits, where main steam temperatures in excess of 500 Celsius are obtained, this is low, leading to a relatively low thermal efficiency. In a modern PWR, around 30-33 % of the nuclear heat is converted into electricity.

A third drawback is that when a (any) nuclear reactor is shut down after operation, the fuel in the reactor no longer undergoes fission processes. However, there is an inventory present of highly radioactive elements, some of which generate substantial amounts of heat. If the fuel elements were to be exposed (i.e. there is no water to cool the elements), this heat is no longer removed. The fuel will then start to heat up, and temperatures can then exceed the melting temperature of the zircaloy cladding. When this occur the fuel elements melt, and a meltdown occurs, such as the multiple meltdowns that occurred in the Fukushima disaster. When the reactor is in shutdown mode, the temperature and pressure are slowly reduced to atmospheric, and thus water will boil at 100 Celsius. This relatively low temperature, combined with the thickness of the steel vessels used, could lead to problems in keeping the fuel cool, as was shown by the Fukushima accident.

Lastly, the fission of uranium and plutonium in a thermal spectrum yields a smaller number of neutrons than in the fast spectrum, so in a fast reactor, more losses are acceptable.

The proposed fast reactors solve all of these problems (next to the fundamental fission properties, where for example plutonium 239 is more likely to fission after absorbing a fast neutron, than a slow one.)

Fast fission and breeding

Although uranium-235 and plutonium-239 are less sensitive to higher-energy neutrons, they still remain somewhat reactive well into the MeV range. If the density of uranium-235 or plutonium-239 is sufficient, eventually a threshold will be reached where there are enough fissile atoms in the fuel to maintain a chain reaction even with fast neutrons. In fact, in the fast spectrum, uranium-238 will also undergo fission. Crucially, when a reactor runs on fast neutrons, the plutonium isotopes are far more likely to fission upon absorbing a neutron. Fast neutrons have a smaller chance of being captured by the uranium and plutonium, but when they are captured, have a much bigger chance of causing a fission. The inventory of spent fuel therefore contains virtually no actinides except for non fissioned uranium and plutonium, which can be effectively recycled.

By removing the moderator, the size of the reactor can be greatly reduced, and to some extent the complexity. As plutonium-239 and particularly plutonium-240 are far more likely to fission when they capture a fast neutron, it is possible to fuel such reactors with a mixture of plutonium and natural uranium, or with enriched material, containing around 20% Template:Chem/link . During the operation, the natural uranium (mostly uranium-238) will be turned into plutonium, or fission in the first place, and thus the reactor never runs out of neutrons, as new fuel is created during the operation, a process called breeding. Fast reactors can be used for breeding. By surrounding the reactor core with a blanket of uranium-238 which captures the neutrons, the extra neutrons breed more plutonium-239.

The blanket material can then be processed to extract the plutonium-239, which is then mixed with uranium to produce MOX fuel that can be fed into both conventional slow-neutron reactors, as well as fast reactors. A single fast reactor can thereby feed several thermal ones, greatly increasing the amount of energy extracted from the natural uranium: from less than 1% in a normal once-through cycle, to as much as 60% in the best existing fast reactor cycles. Given the current inventory of spent nuclear fuel (which contains the plutonium), it is possible to treat this waste material and reuse the fuel in fast reactors, effectively destroying the plutonium. Enormous amounts of energy are still present in the spent reactor fuel inventories, if fast reactor types were to be employed to use this material.

Waste recycling

Fast-neutron reactors can potentially reduce the radiotoxicity of nuclear waste. Each commercial scale reactor would have an annual waste output of a little more than a ton of fission products, plus trace amounts of transuranics if the most highly radioactive components could be recycled. The remaining waste should be stored for about 500 years.[5]

With fast neutrons, the ratio between splitting and the capture of neutrons by plutonium and the minor actinides is often larger than when the neutrons are slower, at thermal or near-thermal "epithermal" speeds. Simply put, fast neutrons have a smaller chance of being absorbed by plutonium or Uranium, but when they are, they almost always cause a fission. The transmuted even-numbered actinides (e.g. Template:Chem/link , Template:Chem/link ) split nearly as easily as odd-numbered actinides in fast reactors. After they split, the actinides become a pair of "fission products". These elements have less total radiotoxicity. Since disposal of the fission products is dominated by the most radiotoxic fission products, strontium-90, which has a half life of 28.8 years, and caesium-137, which has a half life of 30.1 years,[5] the result is to reduce nuclear waste lifetimes from tens of millennia (from transuranic isotopes) to a few centuries. The processes are not perfect, but the remaining transuranics are reduced from a significant problem to a tiny percentage of the total waste, because most transuranics can be used as fuel.

Fast reactors technically solve the "fuel shortage" argument against uranium-fueled reactors without assuming undiscovered reserves, or extraction from dilute sources such as granite or seawater. They permit nuclear fuels to be bred from almost all the actinides, including known, abundant sources of depleted uranium and thorium, and light-water reactor wastes. On average, more neutrons per fission are produced by fast neutrons than from thermal neutrons. This results in a larger surplus of neutrons beyond those required to sustain the chain reaction. These neutrons can be used to produce extra fuel, or to transmute long half-life waste to less troublesome isotopes, as was done at the Phénix reactor in Marcoule, France, or some can be used for each purpose. Though conventional thermal reactors also produce excess neutrons, fast reactors can produce enough of them to breed more fuel than they consume. Such designs are known as fast breeder reactors.[4]

In the spent fuel from water moderated reactors, several plutonium isotopes are present, along with the heavier, transuranic elements. Nuclear reprocessing, a complex series of chemical extraction processes, mostly based on the PUREX process, can be used to extract the unchanged uranium, the fission products, the plutonium, and the heavier elements.[6] Such waste streams can be divided in categories; 1) Unchanged uranium 238, which is the vast bulk of the material and has a very low radioactivity, 2) a collection of fission products and 3) the transuranic elements.


All nuclear reactors produce heat which must be removed from the reactor core. Water, the most common coolant in thermal reactors, is generally not feasible for a fast reactor, because it acts as a neutron moderator.

All operating fast reactors are liquid metal cooled reactors. The early Clementine reactor used mercury coolant and plutonium metal fuel. In addition to its toxicity to humans, mercury has a high cross section (thus, it readily absorbs the radiation, which causes nuclear reactions) for the (n,gamma) reaction, causing activation in the coolant and losing neutrons that could otherwise be absorbed in the fuel, which is why it is no longer considered as a coolant.

Russia has developed reactors that use Molten lead and lead-bismuth eutectic alloys, which have been used on a larger scale in naval propulsion units, particularly the Soviet Alfa-class submarine, as well as some prototype reactors. Sodium-potassium alloy (NaK) is popular in test reactors due to its low melting point.

Another proposed fast reactor is a molten salt reactor, in which the salt's moderating properties are insignificant.[7]

Gas-cooled fast reactors have been the subject of research commonly using helium, which has small absorption and scattering cross sections, thus preserving the fast neutron spectrum without significant neutron absorption in the coolant.Template:Citation needed

However, all large-scale fast reactors have used molten metal coolant. Advantages of molten metals are low cost, the small activation potential and the large liquid ranges. The latter means that the material has a low melting point, and a high boiling point. Examples of these reactors include Sodium cooled fast reactor, which are still being pursued worldwide. Russia currently operates two such reactors on a commercial scale. Additionally, Russia has around eighty reactor years of experience with the Lead-cooled fast reactor which is rapidly gaining interest.


In practice, sustaining a fission chain reaction with fast neutrons means using relatively enriched uranium or plutonium. The reason for this is that fissile reactions are favored at thermal energies, since the ratio between the Template:Chem/link

fission cross section and 


absorption cross section is ~100 in a thermal spectrum and 8 in a fast spectrum. Fission and absorption cross sections are low for both 




at high (fast) energies, which means that fast neutrons are likelier to pass through fuel without interacting than thermal neutrons; thus, more fissile material is needed. Therefore a fast reactor cannot run on natural uranium fuel. However, it is possible to build a fast reactor that breeds fuel by producing more than it consumes. After the initial fuel charge such a reactor can be refueled by reprocessing. Fission products can be replaced by adding natural or even depleted uranium without further enrichment. This is the concept of the fast breeder reactor or FBR.

So far, most fast-neutron reactors have used either MOX (mixed oxide) or metal alloy fuel. Soviet fast-neutron reactors use (high Template:Chem/link

enriched) uranium fuel. The Indian prototype reactor uses uranium-carbide fuel.

While criticality at fast energies may be achieved with uranium enriched to 5.5 (weight) percent uranium-235, fast reactor designs have been proposed with enrichments in the range of 20 percent for reasons including core lifetime: if a fast reactor were loaded with the minimal critical mass, then the reactor would become subcritical after the first fission. Rather, an excess of fuel is inserted with reactivity control mechanisms, such that the reactivity control is inserted fully at the beginning of life to bring the reactor from supercritical to critical; as the fuel is depleted, the reactivity control is withdrawn to support continuing fission. In a fast breeder reactor, the above applies, though the reactivity from fuel depletion is also compensated by breeding either Template:Chem/link





from thorium-232 or 

Template:Chem/link , respectively.


Like thermal reactors, fast-neutron reactors are controlled by keeping the criticality of the reactor reliant on delayed neutrons, with gross control from neutron-absorbing control rods or blades.

They cannot, however, rely on changes to their moderators because there is no moderator. So Doppler broadening in the moderator, which affects thermal neutrons, does not work, nor does a negative void coefficient of the moderator. Both techniques are common in ordinary light-water reactors.

Doppler broadening from the molecular motion of the fuel, from its heat, can provide rapid negative feedback. The molecular movement of the fissionables themselves can tune the fuel's relative speed away from the optimal neutron speed. Thermal expansion of the fuel can provide negative feedback. Small reactors as in submarines may use Doppler broadening or thermal expansion of neutron reflectors.


As the perception of the reserves of uranium ore in the 1960s was rather low, and the rate that nuclear power was expected to take over baseload generation, through the 1960s and 1970s fast breeder reactors were considered to be the solution to the world's energy needs. Using twice-through processing, a fast breeder increases the energy capacity of known ore deposits, meaning that existing ore sources would last hundreds of years. The disadvantage to this approach is that the breeder reactor has to be fed fuel that must be treated in a spent fuel treatment plant. It was widely expected that this would still be below the price of enriched uranium as demand increased and known resources dwindled.

Through the 1970s, experimental breeder designs were examined, especially in the US, France and the USSR. However, this coincided with a crash in uranium prices. The expected increased demand led mining companies to expand supply channels, which came online just as the rate of reactor construction stalled in the mid-1970s. The resulting oversupply caused fuel prices to decline from about US$40 per pound in 1980 to less than $20 by 1984. Breeders produced fuel that was much more expensive, on the order of $100 to $160, and the few units that reached commercial operation proved to be economically unfeasible.


Fast reactors are widely seen as an essential development because of several advantages over moderated designs.[8] The most studied and built Fast reactor type is the Sodium-cooled fast reactor. Some of the advantages of this design are discussed below; other designs such as the Lead-cooled fast reactor have similar advantages.

  • A fission event creates more neutrons than in the thermal reactor. This gives flexibility and allows breeding of uranium.
  • As


becomes slightly reactive to fast neutrons, a significant percentage of the fission events in the reactor occur with this isotope.
  • There is a fine balance between the production of neutrons from fission on the one hand, and the many processes that remove them from the equation on the other. If the temperature increases in a fast reactor , this will have two effects;

1) Doppler broadening of the neutron spectrum, and 2) a very small increase in the physical size of the reactor core. These two effects serve to reduce the reactivity because it allows more neutrons to escape the core, as was shown in a demonstration at EBR-II in 1986.[9] In this test, the additional heat was readily absorbed by the large volume of liquid sodium, and the reactor shut itself down, without operator interference.

  • Because sodium has a boiling point of 883  Celsius (Expression error: Missing operand for round. {{{3}}}), and lead has a boiling point of 1749  Celsius (Expression error: Missing operand for round. {{{3}}}) but reactors operates typically around 500  Celsius (Expression error: Missing operand for round. {{{3}}}) to 550  Celsius (Expression error: Missing operand for round. {{{3}}}), there is a large margin where the metals will stay liquid, and thermal increases can be easily absorbed, without any pressure increase.
  • As no water is present in the core at high temperatures, the reactor is essentially at atmospheric pressure. Most often, an inert gas blanket at a modest pressure (e.g. 0.5 atmospheres) is present to ensure that any leak results in mass transport to the outside of the reactor. This means that there is no pressure vessel with associated problems (high pressure systems are complex), nor will a leak from the reactor emit high pressure jets.
  • The entire vessel being at atmospheric pressure, and the sodium is very hot, and can be allowed to remain at these temperatures even in shutdown, passive cooling (i.e. no pumping requirements) with air is possible. Accidents such as the Fukushima Daiichi nuclear accident [10] are impossible with such a design.
  • The higher temperature of the liquid metal, and therefore the higher temperature of the steam generated by this liquid metal, allows a considerable increase in the electric generating efficiency (around 40% thermal efficiency, as opposed to 30% ).[11]
  • Such reactors have the potential to significantly reduce the waste streams from nuclear power, while at the same time increasing vastly the fuel utilization.


As most fast reactors to date have been either sodium, lead or lead-bismuth cooled, the disadvantages of such systems are described here.

  • As a result of running the reactors on fast neutrons, the reactivity of the core is determined by these neutrons, as opposed to moderated reactors. In the moderated reactors, a significant amount of control of the reactivity is obtained from delayed neutrons, which allow time for operators or computers to adjust reactivity. As delayed neutrons play virtually no role in fast reactors, other mechanisms are required for the very short term reactivity control (e.g within one second) in fast reactors, which are thermal expansion and Doppler broadening. Longer term reactivity is obtained from control rods, which are filled with a neutron absorption material.
  • As the entire reactor is filled with large volumes of molten metal, refuelling is not trivial, as optical tools (camera's etc) are of no use. Costly, carefully calibrated and positioned robotic tools are needed for the operation of refueling. Also, completely removing fuel elements from the reactor is not easy.
  • The fact that the entire reactor is filled with a metal that has a melting point much higher than room temperature, all the tubing, heat exchangers, and the entire reactor volume must be heated electrically, before any nuclear operation can take place. However, once the reactor produces heat, this is no longer of any concern.
  • To date most fast reactor types have proven costly to build and operate, and are not very competitive with thermal-neutron reactors unless the price of uranium increased dramatically, or building costs decreased. It is thought that given the perception of problematic nuclear waste disposal, such reactors will be necessary. As moderated reactor construction costs are rising (among other) due to ever more stringent safety mechanisms, this could mean a better economic viability of fast reactors.
  • Sodium is often used as a coolant in fast reactors, because it does not moderate neutron speeds much and has a high heat capacity. However, it burns and foams in air. However, the combustion reaction of sodium in air should not be confused with the extremely violent reaction of sodium and water. Sodium leaks can ignite with air, causing difficulties in reactors such as (e.g. USS Seawolf (SSN-575) and Monju).

However, some sodium-cooled fast reactors have operated safely for long periods (notably the Phénix and EBR-II for 30 years, or the BN-600 and BN-800 in operation since resp. 1980 and 2016, despite several minor leaks and fires. It is important to note that sodium leaks (and possibly fires) do not releases radioactive elements, as the sodium fast reactors are always designed with a two loop system.

  • Since liquid metals other than lithium and beryllium have low moderating ability, the primary interaction of neutrons with fast reactor coolant is the (n,gamma) reaction, which induces radioactivity in the coolant. Sodium-24 (

Template:Chem/link ) is created in the reactor loop of the sodium cooled fast reactor, from natural sodium-23 by neutron bombardment. With a 15-hour half-life, Template:Chem/link

decays to 


by emission of an electron and two gamma rays. As the half life of this isotope is very short, after e.g. two weeks, almost no 


is left. Fast spectrum reactors that use sodium, must remove this magnesium from the sodium, which is achieved with a 'cold' trap.
  • From the liquid lead or liquid lead-bismuth fast reactor designs, only the liquid eutectic lead-bismuth will have activation. As pure lead will have virtually no activation, a pure lead reactor design could operate in a single loop, saving significant costs on heat exchangers and separate systems.
  • A defective fast reactor design could have positive void coefficient: boiling of the coolant in an accident would reduce coolant density and thus the absorption rate. However, no such designs are proposed for commercial service, as they are potentially dangerous and undesirable from a safety and accident standpoint. This can be avoided with a gas-cooled reactor, since voids do not form in such a reactor during an accident; however, reactivity control in a gas cooled fast reactor is difficult.
  • Due to the low cross sections of most materials at high neutron energies, critical mass in a fast reactor is much higher than in a thermal reactor. In practice, this means significantly higher enrichment: >20% enrichment in a fast reactor compared to <5% enrichment in typical thermal reactors. Alternatively, a mixture of plutonium from nuclear waste, combined with natural or depleted uranium could be used.


The BN-350 reactor was also used for desalination of sea water.

US interest in breeder reactors were muted by Jimmy Carter's April 1977 decision to defer construction of breeders in the US due to proliferation concerns, and the suboptimal operating record of France's Superphénix reactor.[12] The French reactors also met with serious opposition of environmentalist groups, who regarded these as very dangerous.[13] Despite such setbacks, a number of countries still invest in the fast reactor technology. Around 25 reactors have been built since the 1970-ies, accumulating over 400 reactor years of experience.

A 2008 IAEA proposal for a Fast Reactor Knowledge Preservation System[14] noted that:

during the past 15 years there has been stagnation in the development of fast reactors in the industrialized countries that were involved, earlier, in intensive development of this area. All studies on fast reactors have been stopped in countries such as Germany, Italy, the United Kingdom and the United States of America and the only work being carried out is related to the decommissioning of fast reactors. Many specialists who were involved in the studies and development work in this area in these countries have already retired or are close to retirement. In countries such as France, Japan and the Russian Federation that are still actively pursuing the evolution of fast reactor technology, the situation is aggravated by the lack of young scientists and engineers moving into this branch of nuclear power.

As of 2021, Russia operates two fast reactors on commercial scale.[15] The GEN IV initiative, an international working group on new reactor designs has proposed six new reactor types, three of which would operate with a fast spectrum.[16]


Some content on this page may previously have appeared on Wikipedia.
This page is a shortened and simplified version of the Wikipedia article, focused on just the issues raised in Nuclear power reconsidered.
See the original for more details and additional information on the history and list of reactors in many countries.