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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 or greater, 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 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 reactor years of operation globally. Fast reactors have been intensely studied since the s, as they provide certain decisive advantages over the existing fleet of water cooled and water moderated reactors. These are:.
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. 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. Natural uranium consists mostly of two isotopes :. Of these two, U undergoes fission only by fast 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 times greater of causing a fission in U 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 U.
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 U , Th or U.
In this case, only the U has a high probability of fission. Although U rarely undergoes fission by the fast neutrons released in fission, thermal neutrons i. When Plutonium in turn absorbs a thermal neutron to become a heavier isotope Pu which is also fissionable with thermal neutrons very close in probability to Plutonium These effects combined have the result of creating, in a light water 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 create new neutrons , but instead just absorb the neutrons.
Most moderated reactors use low enriched fuel. As power production continues, around 12—18 months of stable operation in light water moderated reactors, the thermal nuclear reactor both consumes more fissionable material than it breeds and accumulated neutron absorbing fission products which make it difficult to sustain the fission process, and the reactor has to be refueled.
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, water can absorb a neutron and remove it from the reaction. It does this just enough that the concentration of U in natural uranium is too low to sustain the chain reaction; the neutrons lost through absorption in the water and U , 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 U in the fuel to produce enriched uranium , with the leftover U 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.
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. In Russia and the UK, reactors are operational that use graphite as moderator, and resp. 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. 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. 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. 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 is more likely to fission after absorbing a fast neutron, than a slow one.
Although U and Pu have a lower capture cross section with higher-energy neutrons, they still remain reactive well into the MeV range. If the density of U or Pu 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 U captures a fast neutron it will also undergo fission at a low rate with the remainder of captures being "radiative" and entering the decay chain to Plutonium 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.
By removing the moderator, the size of the reactor core volume can be greatly reduced, and to some extent the complexity. Test runs at various facilities have also been done using U and Th.
The natural uranium mostly U will be turned into Pu , while in the case of Th , U is the result. As new fuel is created during the operation, this process is called breeding. All fast reactors can be used for breeding, or by carefully selecting the materials in the core and eliminating the blanket they can be operated to maintain the same level of fissionable material without creating any excess material. This is a process called Conversion because it transmutes fertile materials into fissile fuels on a basis.
By surrounding the reactor core with a blanket of U or Th which captures excess neutrons, the extra neutrons breed more Pu or U respectively. The blanket material can then be processed to extract the new fissile material, which can then be mixed with depleted uranium to produce MOX fuel , mixed with lightly enriched Uranium fuel to form REMIX fuel both for conventional slow-neutron reactors. A single fast reactor can thereby supply its own fuel indefinitely as well as feed several thermal ones, greatly increasing the amount of energy extracted from the natural uranium.
Given the current inventory of spent nuclear fuel which contains reactor grade plutonium , it is possible to process this spent fuel material and reuse the Actinide isotopes as fuel in a large number of fast reactors. This effectively consumes the Np , reactor grade plutonium , Am , Cu. 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 that energy can be extracted for useful purposes.
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 years. 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. 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 , which has a half life of 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. 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. 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.
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.
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.
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.
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