Editing Fast-neutron reactor (section)

===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.)