The buildup of uranium isotopes with time in a U 'a3 breeder core was calculated for five different cases, the difference depending on the starting fuel and the isotopic composition of the continuously added make-up fuel. The total uranium concentration was found to approach slowly an equilibrium value of 2.6 to 3.5 times the starting value, depending on the composition of the make-up fuel. In all cases the net neutron losses per net neutron reproduced in the core go through a maximum of less than 1% at a flux time of about 3 x 10zl, go through a minimum of about -0.6 % (i.e. a net gain) at about a flux time of 4 x 10z2, and approach equilibrium values of about 0.3 % at flux times above 6 x 10za.
THE principal heavy isotopes in the core of a U 233 breeder reactor and their modes of formation may be depicted by the following diagram. * This document is based on work performed for the Atomic Energy Commission at the Oak Ridge National Laboratory.
t Actually this starting material will contain a small amount of UzS4 and Uzas. The Uza4 and UzB8 will increase the losses over those calculated in this paper for Cases IV and V at the shorter times; however, the magnitude of this additional loss will be small even at zero time and it will steadily decrease with increasing time.