This paper provides an analysis of the values of additional reactivity occurs in fast reactor after shutdown. Modern designs of advanced nuclear reactors are based on the realization of the internal safety of the reactor, with the key requirement is to keep the maximum reactivity margin below the delayed-neutron fraction and the exclusion of the possibility of increasing power on prompt neutrons. The additional reactivity arising after a shutdown of the fast reactor approximately is proportional to the relation of burnout in a year to enrichment of the fuel. The accuracy of this approximation is about 5 % compared with the detailed calculation of the entire system of kinetic equations. The limit in case of which the increase in reactivity reaches a delayed-neutron fraction is specified and, therefore, reactor system becomes incompatible with the requirements of internal safety.
Keywords: reactivity margin, neptunium effect of reactivity, internal safety.
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