A fast-neutron reactor (FNR) or 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, on average), as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is comparatively rich in fissile material.
The fast spectrum is key to breeder reactors, which convert highly abundant uranium-238 into fissile plutonium-239, without requiring enrichment. It also leads to high burnup: many transuranic isotopes, such as of americium and curium, accumulate in thermal reactor spent fuel; in fast reactors they undergo fast fission, reducing total nuclear waste. As a strong fast-spectrum neutron source, they can also be used to transmute existing nuclear waste into manageable or non-radioactive isotopes.
These characteristics also cause fast reactors to be judged a higher nuclear proliferation risk, especially as breeder reactors require nuclear reprocessing, which can be redirected to produce weapons-grade plutonium.
, every fast reactor has used a liquid metal coolant, typically sodium-cooled or lead-cooled. This allows high thermal efficiency, without pressurization systems, however it also contributes to historical high costs and operational difficulties.
In total, 13 fast breeder reactors have been constructed for commercial nuclear power, alongside 65 fast-spectrum research reactors of various configurations. The first fast reactor was Los Alamos National Laboratory's Clementine, operated from 1946. The largest was Superphénix, in France, designed to deliver 1,242 MWe. In the GEN IV initiative, about two thirds of the proposed reactors for the future use a fast spectrum.
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:
99.3% 0.7%
Of these two, undergoes fission only by fast neutrons. About 0.7% of natural uranium is , which will undergo fission by both fast and slow (thermal) neutrons. When the uranium undergoes fission, it releases neutrons with a high energy ("fast"). However, these fast neutrons have a much lower probability of causing another fission than neutrons which are slowed down after they have been generated by the fission process. Slower neutrons have a much higher chance (about 585 times greater) of causing a fission in 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. The most common moderator is ordinary water, which acts by elastic scattering until the neutrons reach thermal equilibrium with the water (hence the term "thermal neutron"), at which point the neutrons become highly reactive with the . Other moderators include heavy water, beryllium and 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 is stationary or moving slowly, 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 more likely to be absorbed by another heavy element, such as , or . In this case, only the has a high probability of fission.
Although undergoes fission by the fast neutrons released in fission about 11% of the time, this can not sustain the chain reaction alone. Neutrons produced by fission of have lower energies than the original neutron, usually below 1 MeV, the fission threshold to cause subsequent fission of , so fission of does not sustain a nuclear chain reaction. When hit by thermal neutrons (i.e. neutrons that have been slowed down by a moderator) the neutron can be captured by the nucleus to transmute the uranium into which rapidly decays into which in turn decays into . has a thermal neutron cross section larger than that of .
About 73% of the created this way will undergo fission from capturing a thermal neutron while the remaining 27% absorbs a thermal neutron without undergoing fission, creating , which rarely fissions with thermal neutrons. When plutonium-240 in turn absorbs a thermal neutron to become a heavier isotope which is also fissionable with thermal neutrons very close in probability to plutonium-239. In a fast spectrum reactor all three isotopes have a high probability of fission when absorbing a high energy neutron which limits their accumulation in the fuel.
These effects combined have the result of creating, in a moderated reactor, the presence of the transuranic elements. Such isotopes are themselves unstable, 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 release new fast neutrons), but instead just absorb the thermal neutrons. Most moderated reactors use natural uranium or low enriched fuel. As power production continues, around 12–18 months of stable operation in all moderated reactors, the reactor both consumes more fissionable material than it breeds and accumulates neutron absorbing fission products which make it difficult to sustain the fission process. When too much fuel has been consumed the reactor has to be refueled.
Drawbacks of light water as the moderator in conventional nuclear reactors
The following disadvantages of the use of a moderator have instigated the research and development of fast reactors.
Although cheap, readily available and easily purified, light water can absorb a neutron and remove it from the reaction. It does this enough that the concentration of in natural uranium is too low to sustain the chain reaction; the neutrons lost through absorption in the water and , along with those lost to the environment, results in too few left in the fuel. The most common solution to this problem is to concentrate the amount of in the fuel to produce enriched uranium, with the leftover known as depleted uranium.
Other thermal neutron designs use different moderators, like heavy water or graphite that are much less likely to absorb neutrons, allowing them to run on natural uranium fuel. See CANDU, X-10 Graphite Reactor. 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 30 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, the coolant being respectively water in Russian and gas in British reactors.
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 , with pressures of around 85 bar (1233 psi). Compared to for example modern coal fired steam circuits, where main steam temperatures in excess of 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 occurs 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 . 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 and have a lower capture cross section with higher-energy neutrons, they still remain reactive well into the MeV range. If the density of or is sufficient, a threshold will be reached where there are enough fissile atoms in the fuel to maintain a chain reaction with fast neutrons. In fact, in the fast spectrum, when captures a fast neutron it will also undergo fission around 11% of the time with the remainder of captures being "radiative" and entering the decay chain to plutonium-239.
Crucially, when a reactor runs on fast neutrons, the isotope is likely to fission 74% of the time instead of the 62% of fissions when it captures a thermal neutron. In addition, the probability of a atom fissioning upon absorbing a fast neutron is 70% while for a thermal neutron it is less than 20%. Fast neutrons have a smaller chance of being captured by the uranium and plutonium, but when they are captured, have a significantly higher probability of causing a fission. The inventory of spent fast reactor fuel therefore contains virtually no actinides except for uranium and plutonium, which can be effectively recycled. Even when the core is initially loaded with 20% mass reactor-grade plutonium (containing on average 2% , 53% , 25% , 15% , 5% and traces of ), the fast spectrum neutrons are capable of causing each of these to fission at significant rates. By the end of a fuel cycle of some 24 months, these ratios will have shifted with an increase of to over 80% while all the other plutonium isotopes will have decreased in proportion.