Capture-to-Fission Ratios of U²³³, U²³⁵, U²³⁸ and Pu²³⁹ in EBR-I, Mark III

In order to compare the breeding capabilities of the major nuclear fuels in the spectrum of a fast-breeder reactor, integral measurements have been made for the ratio of their capture and fission cross sections in the third loading of the First Experimental Breeder Reactor (EBR-I, Mark III). The capture-to-fission ratio has been determined as a function of position in the reactor for U²³³, U²³⁵ and Pu²³⁹. In addition, for U²³³ the ratio of (n, 2n) and fission cross sections has been determined. Further, for U²³⁸ the following cross-section ratios have been determined: σ_nγ(U²³⁸) / σƒ(U²³⁸)(X), where σ_f(X) refers to the fission cross sections of U²³³, U²³⁵ and Pu²³⁹. The capture-to-fission ratio results for the three primary fissile species have been compared with calculations based upon 16-group neutron diffusion theory using two different sets of monoenergetic neutron cross sections, and the agreement is good. The present data show that of the three major fissile species, Pu²³⁹ has the highest value of η-1, the maximum number of neutrons available for breeding, for each fissile nucleus consumed.