Nuclear criticality of heterogeneous mixed-oxide systems

An existing American National Standard, ANSI/ANS- .12-1978 provides guidance for nuclear criticality control and safety of homogeneous plutonium-uranium fuel mixtures outside reactors. Critical dimensions for heterogeneous systems, however, will differ significantly from those of homogeneous systems, and the effect of higher isotopes of Pu also will differ due to self-shielding effects in the lattice (heterogeneous) systems. Therefore, data are required for criticality control of heterogeneous systems of mixed oxide fuels and Work Group ANS 8.12.1 of Subcommittee 8 of the American Nuclear Society Standards Committee was formed to provide this data. In connection with these activities, some data were previously reported on critical dimensions for arrays of (U,Pu)O2 rods in water. Since that information was presented, calculations have been extended to include the effects of higher isotopes of Pu, i.e., 240Pu and 241Pu, on the criticality of heterogeneous mixed oxide systems and a number of additional lattice benchmarks have been calculated as validation of the methods used. The results of these calculations are presented and discussed and compliment the data already available on homogeneous systems. Calculations were made with three different compositions of mixed oxides, 3 wt% Pu, 15 wt% Pu and 30 wt% Pu in (U,Pu)O2 for three different isotopic compositions of Pu in the mixed oxide: 100 wt% 239Pu; 79 wt% 239Pu, 15 wt% 240Pu, 6 wt% 241Pu; and 60 wt% 239Pu, 25 wt% 240Pu, 15 wt% 241Pu