Editing Fast-neutron reactor (section)

==Disadvantages==

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 neutron]]s, 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 rod]]s, 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 (cameras, 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, although 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 Nuclear Power Plant|Monju]]).
: Some sodium-cooled fast reactors have operated safely for long periods (notably the [[Phénix]] and [[Experimental Breeder Reactor II|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 release 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 ({{chem|24|Na}}) is created in the reactor loop of the sodium cooled fast reactor, from natural sodium-23 by [[neutron irradiation|neutron bombardment]]. With a 15-hour half-life, {{chem|24|Na}} decays to {{chem|link=Isotopes of magnesium|24|Mg}} by emission of an [[electron]] and two [[gamma ray]]s. As the half life of this isotope is very short, after e.g. two weeks, almost no {{chem|24|Na}} 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 [[Lead-bismuth eutectic]] 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 exchanger 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. 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 fast reactor|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 [[enriched uranium|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.