Editing Fast-neutron reactor (section)

== Fission processes ==

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

===Moderators in conventional nuclear reactors===

[[Natural uranium]] consists mostly of two [[isotope]]s:

* 99.3% [[uranium-238|{{chem|238|U}}]]
* 0.7% [[uranium-235|{{chem|235|U}}]]

Of these two, {{chem|238|U}} undergoes fission only by fast neutrons.<ref>{{cite web|url=http://www.nuclear-power.net/nuclear-power/reactor-physics/atomic-nuclear-physics/fundamental-particles/neutron/#Neutron_Energy|title=What is Neutron - Neutron Definition|website=www.nuclear-power.net|language=en-US|access-date=2017-09-19}}</ref> 
About 0.7% of natural uranium is {{chem|235|U}}, 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 {{chem|235|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 {{chem|235|U}}. Other moderators include [[heavy water]], [[beryllium]] and [[Nuclear graphite|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 {{chem|238|U}}, {{chem|link=Uranium-232|232|Th}} or {{chem|235|U}}. In this case, only the {{chem|235|U}} has a high probability of fission.

Although {{chem|238|U}} 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 {{chem|238|U}} have lower energies than the original neutron, usually below 1 MeV, the fission threshold to cause subsequent fission of {{chem|238|U}}, so fission of {{chem|238|U}} 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 {{chem|238|U}} nucleus to transmute the uranium into [[uranium-239|{{chem|239|U}}]] which rapidly decays into [[neptunium-239|{{chem|239|Np}}]] which in turn decays into [[plutonium-239|{{chem|239|Pu}}]]. {{chem|239|Pu}} has a thermal [[neutron cross section]] larger than that of {{chem|235|U}}.

About 73% of the {{chem|239|Pu}} created this way will undergo fission from capturing a thermal neutron while the remaining 27% absorbs a thermal neutron without undergoing fission, {{chem|240|Pu}} is created, which rarely fissions with thermal neutrons. When [[plutonium-240]] in turn absorbs a thermal neutron to become a heavier isotope {{chem|241|Pu}} 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.<ref name="difference.minaprem.com">{{cite web|author=Pintu 14/10/2019 Nuclear Power Plant |url=http://www.difference.minaprem.com/npp/difference-between-thermal-reactor-and-fast-reactor/ |title=Difference Between Thermal Reactor and Fast Reactor |publisher=Difference.minaprem.com |date=2019-10-14 |accessdate=2022-04-13}}</ref>

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 {{chem|235|U}} in [[natural uranium]] is too low to sustain the chain reaction; the neutrons lost through absorption in the water and {{chem|238|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 concentrate the amount of {{chem|235|U}} in the fuel to produce [[enriched uranium]], with the leftover {{chem|238|U}} known as [[depleted uranium]].

Other [[thermal neutron]] designs use different moderators, like [[heavy water]] or [[Nuclear graphite|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 neutron]]s.

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 turbine]]s. 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&nbsp;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 [[Pressurized water reactor|PWR]], the [[Boiling Water Reactor|BWR]] and the [[CANDU]] reactors. In Russia and the UK, reactors are operational that use graphite as moderator, and respectively water in Russian and gas in British reactors 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 {{convert|350|Celsius|sigfig=2}}, with pressures of around 85 bar (1233 psi). Compared to for example modern coal fired steam circuits, where main steam temperatures in excess of {{convert|500| Celsius|sigfig=2}} 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 [[Nuclear meltdown|meltdown]] occurs, such as the multiple meltdowns that occurred in the [[Fukushima nuclear disaster|Fukushima disaster]]. When the reactor is in shutdown mode, the temperature and pressure are slowly reduced to atmospheric, and thus water will boil at {{convert|100| Celsius|sigfig=2}}. 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 {{chem|235|U}} and {{chem|239|Pu}} have a lower capture cross section with higher-energy neutrons, they still remain reactive well into the MeV range. If the density of {{chem|235|U}} or {{chem|239|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 {{chem|238|U}} 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 {{chem|239|Pu}} 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 {{chem|240|Pu}} 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% {{chem|238|Pu}}, 53% {{chem|239|Pu}}, 25% {{chem|240|Pu}}, 15% {{chem|241|Pu}}, 5% {{chem|242|Pu}} and traces of {{chem|244|Pu}}), 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 {{chem|239|Pu}} to over 80% while all the other plutonium isotopes will have decreased in proportion.

By removing the moderator, the size of the reactor core volume can be greatly reduced, and to some extent the complexity. As {{chem|239|Pu}} and particularly {{chem|240|Pu}} 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% {{chem|235|U}}. Test runs at various facilities have also been done using {{chem|233|U}} and {{chem|232|Th}}. The natural uranium (mostly {{chem|238|U}}) will be turned into {{chem|239|Pu}}, while in the case of {{chem|232|Th}}, {{chem|233|U}} is the result. As new fuel is created during the operation, this process is called breeding.<ref name=":0" /> 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 1:1 basis. By surrounding the reactor core with a blanket of {{chem|238|U}} or {{chem|232|Th}} which captures excess neutrons, the extra neutrons breed more {{chem|239|Pu}} or {{chem|233|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|REMIX]] fuel, both for conventional slow-neutron reactors. Alternatively it can be mixed as in greater percentage of 17%-19.75% fissile fuel for fast reactor cores. 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. The most effective breeder configuration theoretically is able to produce 14 {{chem|239|Pu}} nuclei for every 10 (14:10) actinide nuclei consumed, however real world fast reactors have so far achieved a ratio of 12:10 ending the fuel cycle with 20% more fissile material than they held at the start of the cycle.<ref>{{Cite web|url=https://www.nuclear-power.com/nuclear-power-plant/nuclear-fuel/conversion-factor-breeding-ratio/|title=Conversion Factor - Breeding Ratio|website=Nuclear Power}}</ref> Less than 1% of the total uranium mined is consumed in a thermal [[Once-through nuclear fuel cycle|once-through cycle]], while up to 60% of the natural uranium is fissioned in the best existing fast reactor cycles.

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 {{chem|237|Np}}, [[reactor-grade plutonium]], {{chem|241|Am}}, and {{chem|244|Cm}}. 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.