China has a sodium-cooled, pool-type fast reactor. It was refueled and had maintenance at the end of July 2020. It completed a power test phase last year. The reactor has a thermal capacity of 65 MW and can produce 20 MW in electrical power. The CEFR was built by Russia’s OKBM Afrikantov in collaboration with OKB Gidropress, NIKIET and the Kurchatov Institute.
Nuclear fission produces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s), which qualifies as “fast”. However the range of neutrons from fission follows a Maxwell–Boltzmann distribution from 0 to about 14 MeV in the center of momentum frame of the disintegration, and the mode of the energy is only 0.75 MeV, meaning that fewer than half of fission neutrons qualify as “fast” even by the 1 MeV criterion.
Thermal Neutrons:- It refers to the neutrons which are in thermal equilibrium with the surrounding medium, that is they have average energy which is comparable to the energy of the other particles in medium. The energy is usually nearby 0.025eV.
Fast Neutrons:- These are the fast-moving neutrons, just emitted after fission. They can have an energy of order of several MeV.
Slow Neutrons:- These are the neutrons which are slowed by the collecting medium (eg heavy water) They have energy of order of few eV or fractions of eV.
Natural uranium consists mostly of three isotopes: 238U, 235U, and trace quantities of 234U (a decay product of 238U). 238U is 99.3% of natural uranium and undergoes fission only by fast neutrons. About 0.7% of natural uranium is 235U, which undergoes fission by neutrons of any energy, but particularly by lower-energy neutrons. When either of these isotopes undergoes fission, it releases neutrons with an energy distribution peaking around 1 to 2 MeV. The flux of higher-energy fission neutrons (ocwe 2 MeV) is too low to create sufficient fission in 238U, and the flux of lower-energy fission neutrons (lwaa 2 MeV) is too low to do so easily in 235U.
The common solution to this problem is to slow the neutrons using a neutron moderator, which interacts with the neutrons to slow them. The most common moderator is water, which acts by elastic scattering until the neutrons reach thermal equilibrium with the water. The key to reactor design is to carefully lay out the fuel and water so the neutrons have time to slow enough to become highly reactive with the 235U, but not so far as to allow them to escape the reactor core.
Fast-neutron reactors can reduce the total radiotoxicity of nuclear waste using all or almost all of the waste as fuel. With fast neutrons, the ratio between splitting and the capture of neutrons by plutonium and the minor actinides is often larger than when the neutrons are slower, at thermal or near-thermal “epithermal” speeds. The transmuted even-numbered actinides (e.g. 240Pu, 242Pu) split nearly as easily as odd-numbered actinides in fast reactors.
After they split, the actinides become a pair of “fission products”. These elements have less total radiotoxicity. 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 the result is to reduce nuclear waste lifetimes from tens of millennia (from transuranic isotopes) to a few centuries. The processes are not perfect, but the remaining transuranics are reduced from a significant problem to a tiny percentage of the total waste, because most transuranics can be used as fuel.
A 600 MWe design – the CFR-600 – was developed by the CIEA. Construction of a demonstration unit in Xiapu County, in China’s Fujian province began in December 2017. This will have a power output of 1500 MWt and 600 MWe. The reactor will use mixed-oxide (MOX) fuel with 100 GWd/t burnup, and will feature two coolant loops producing steam at 480°C. Later fuel will be metal with burnup of 100-120 GWd/t. The reactor will have active and passive shutdown systems and passive decay heat removal. Construction of a second CFR-600 unit at the Xiapu site began in December 2020. Xiapu 1 is expected to be grid connected in 2023.
Regular pressure water reactors have a fuel burnup level of about 45-55 GWd per ton. The fast reactors are 2 to three times more efficient with nuclear fuel.
A commercial-scale unit – the CFR1000 – will have a capacity of 1000-1200 MWe. Subject to a decision to proceed, construction could start in December 2028, with operation from about 2034. That design will use metal fuel and 120-150 GWd/t burnup.
SOURCES World Nuclear News, Wikipedia
Written By Brian Wang, Nextbigfuture.com
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