Reactor, breeder poisons

Many of the fission products formed in a nuclear reactor are themselves strong neutron absorbers (i.e. poisons ) and so will stop the chain reaction before all the (and Pu which has also been formed) has been consumed. If this wastage is to be avoided the irradiated fuel elements must be removed periodically and the fission products separated from the remaining uranium and the plutonijjm. Such reprocessing is of course inherent in the operation of fast-breeder reactors, but whether or not it is used for thermal reactors depends on economic and political factors. Reprocessing is currently undertaken in the UK, France and Russia but is not considered to be economic in the USA. 

The effect of poisoning can be compensated to a certain extent by an excess reactivity or by installation of a breeder blanket (an outer layer of Th) in which new fissile material is produced. In fast reactors the effect of poisoning is less important. 

Plutonium-239 is the fissionable isotope produced in breeder reactors it is also produced in ordinary nuclear plants and in weapons tests. It is an extremely poisonous substance with a half-life of 24,100 years. 

Ill steady-state operation, the concentration of the various nuclides within the reactor system does not change with time. During the initial period of reactor operation this situation is not true, but is approached after some time interval if neutron poisons arc removed by fuel processing. Under steady-state operation it is necessary to consider the equilibrium isotope relationships. In thorium breeder reactors this involves rate material lialanccs on Th, Pa - fi.ssion-product poisons,... 

Computations have been performed for several spherical reactors using an Oracle eode [31] for two-region, time-dependent, thorium breeder systems. The variation with time of the breeding ratio and the concentrations of 11- , Pa, and fission-product poisons were... 

Pa233 ijj tije blanket, breeding ratio, and core and blanket poison fractions as functions of time for a two-region 60-Mw thorium breeder reactor. Core thorium concentration = 150 g Th/liter, blanket thorium concentration = 1000 g Th/liter, temperature = 280°C. 

Studies have been carried out at BNL on control requirements for an LMFR experiment. The control requirements depend not only on the choice of operating temperatures, the possible xenon and fission-product poisoning, etc., but also on conceivable emergency situations. such as errors in fuel concentration control. In a reactor with a full breeding blanket, the control requirements may have to include the effect of complete loss of the breeder fluid. 

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