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The BN-350 fast-neutron reactor at Aktau, Kazakhstan.It operated between 1973 and 1994. 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, on average), as opposed to slow thermal neutrons used in thermal-neutron reactors.
A thermal-neutron reactor is a nuclear reactor that uses slow or thermal neutrons.. ("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and structure, which is much lower energy than the fast neutrons initially produced by fission.)
Plutonium is fissionable with both fast and thermal neutrons, which make it ideal for either nuclear reactors or nuclear bombs. Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant.
A fast neutron reactor uses fast neutrons, so it does not use a moderator. Moderators may absorb a lot of neutrons in a thermal reactor, and fast fission produces a higher average number of neutrons per fission, so fast reactors have better neutron economy making a plutonium breeder reactor possible.
This is especially the case for companies studying fast neutron reactors of 4th generation (molten salts reactors, metal-cooled reactors (sodium-cooled fast reactor, or lead-cooled fast reactor). Fast breeder reactors "burn" 235 U (0.7% of natural uranium), but also convert fertile materials such as 238 U (99.3% of natural uranium) into fissile 239
The projected increase in uranium price did not materialize, but if uranium demand increases in the future, then there may be renewed interest in fast reactors. The GFR base design is a fast reactor, but in other ways similar to a high temperature gas-cooled reactor. It differs from the HTGR design in that the core has a higher fissile fuel ...
A fast reactor is therefore more efficient than a thermal reactor for using plutonium and higher actinides as fuel. These fast reactors are better suited for the transmutation of other actinides than thermal reactors. Because thermal reactors use slow or moderated neutrons, the actinides that are not fissionable with thermal neutrons tend to ...
The CNRS project EVOL (Evaluation and viability of liquid fuel fast reactor system) project, with the objective of proposing a design of the molten salt fast reactor (MSFR), [72] released its final report in 2014. [73] Various MSR projects like FHR, MOSART, MSFR, and TMSR have common research and development themes. [74]