The most important feature of the fission process is of course the enormous energy release from each reaction. Another significant fact is that for each neutron absorbed in a fuel such as U-235, more than two neutrons are released. To maintain the chain reaction, only one in needed.
If losses of neutrons can be reduced enough the possibility exists for new fuel to be generated in quantities as large or even larger than the amount consumed a condition called "breeding".
In the ultimate and ideal breeder cycle all materials are recycled. There is a revival of interest in some level of recycling to help reduce radioactive waste and to use all fuel energy values.
All reactors produce new fissile material. However, most of them do this at the cost of consuming an even greater quantity of fissile fuel. A true breeder reactor produces more new fuel than it consumes.
Its effectiveness is measured by the breeding ratio B which is defined as the number of new fissile atoms formed per atom of existing fuel consumed. If B = 1 fuel is replaced, if B > 1 the amount of fuel is increased and if B < 1 there is a net decrease.
These reactors were considered to "breed"
more fissile material by neutron capture than was present in the original core loading, by surrounding the core with a blanket of "fertile"
material. The term "fast"
comes from the fact that the majority of the fission events are caused by fast neutrons, rather than slow or thermal neutrons. In fact no moderator is present at all to slow down the fast neutrons.
Almost all fast breeder reactors are cooled by liquid sodium rather than water. Examples of this type of reactor are the Enrico Fermi 1 station near Monroe, Michigan, now decommissioned; the prototupe fast reactor at Dounreay scotland; and the Phenix and super-phenix reactors in France.
Breeder reactor are advanced type of nuclear reactor for generating electrical power and converting non-fissile uranium U-238 into fissile Plutonium-239 by neutron bombardment. The later is a highly desired artificial radioactive isotope that is very fissile and can be used as a fuel in NPPs or as the principal explosive material in fission nuclear weapons.
A nuclear breeder reactor produces fuel in the process of consuming it. India has expressed massive interest in developing effective thorium breeder nuclear reactors. In 2008, it stated that by 2050 it planned to meet 30% of its energy demands via thorium reactors.
Thermal breeder reactors primarily operate on the basis of neutron absorption by fertile isotopes in a thermal spectrum, producing more fissile fuel than they consume. Earlier studies on breeders have shown that the absorption cross-section is an important factor in choosing fertile material for the core.
The fact that Th-232 breeds U-233 through neutron absorption and successive beta decays with higher neutron absorption cross-section than U-238 was an overriding factor favoring thorium in thermal breeders.
The incorporation of some low enriched uranium provides another safety advantage relative to all thorium fuel because the lower absorption cross-section for epithermal neutrons in pure Th-232, reduces the negative power co-efficient in case of a power transient. But too much uranium in the fuel will result in a higher concentration of plutonium produced by the fertile isotope U-238.
In a thermal breeder reactor of the suggested design, the low enriched fuel reaches high burn-up and achieves higher Pu-240/Pu-239 ratio for the EOC fuel. The blanket with half the size of the driver also breeds only reactor-grade plutonium in lower quantity.