Fast-neutron reactor
The largest was the Superphénix sodium cooled fast reactor in France that was designed to deliver 1,242 MWe.
These are: In the GEN IV initiative, about two thirds of the proposed reactors for the future use a fast spectrum for these reasons.
A water cooled and moderated nuclear reactor therefore needs to operate at high pressures to enable the efficient production of electricity.
This high pressure operation adds complexity to reactor design and requires extensive physical safety measures.
When the reactor is in shutdown mode, the temperature and pressure are slowly reduced to atmospheric, and thus water will boil at 100 °C (210 °F).
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.
If the density of 235U or 239Pu is sufficient, a threshold will be reached where there are enough fissile atoms in the fuel to maintain a chain reaction with fast neutrons.
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% 238Pu, 53% 239Pu, 25% 240Pu, 15% 241Pu, 5% 242Pu and traces of 244Pu), 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 239Pu 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 239Pu and particularly 240Pu 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% 235U.
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.
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.
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.
Since disposal of the fission products is dominated by the most radiotoxic fission products, strontium-90, which has a half life of 28.8 years, and caesium-137, which has a half life of 30.1 years,[6] the result is to reduce nuclear waste lifetimes from tens of millennia (from transuranic isotopes) to a few centuries.
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.
These neutrons can be used to produce extra fuel, or to transmute long half-life waste to less troublesome isotopes, as was done at the Phénix reactor in Marcoule, France, or some can be used for each purpose.
[3] In the spent fuel from water moderated reactors, several plutonium isotopes are present, along with the heavier, transuranic elements.
[7] Such waste streams can be divided in categories; 1) unchanged uranium-238, which is the vast bulk of the material and has a very low radioactivity, 2) a collection of fission products and 3) the transuranic elements.
In addition to its toxicity to humans, mercury has a high capture cross section (thus, it readily absorbs the neutrons, 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 useful as a coolant.
Sodium-potassium alloy (NaK) is popular in test reactors due to its low melting point.
Purified nitrogen-15 has also been proposed as a coolant gas because it is more common than helium and also has a very low neutron absorption cross section.
Advantages of molten metals are low cost, the small activation potential and the large liquid ranges.
In practice, sustaining a fission chain reaction with fast neutrons means using relatively enriched uranium or plutonium.
Soviet fast-neutron reactors used (highly 235U enriched) uranium fuel initially, then in 2022 switched to using MOX.
As the perception of the reserves of uranium ore in the 1960s was rather low, and the rate that nuclear power was expected to take over baseload generation, through the 1960s and 1970s fast breeder reactors were considered to be the solution to the world's energy needs.
The expected increased demand led mining companies to expand supply channels, which came online just as the rate of reactor construction stalled in the mid-1970s.
Breeders produced fuel that was much more expensive, on the order of $100 to $160, and the few units that reached commercial operation proved to be economically unfeasible.
In countries such as France, Japan and the Russian Federation that are still actively pursuing the evolution of fast reactor technology, the situation is aggravated by the lack of young scientists and engineers moving into this branch of nuclear power.
