Basis for Extending Limits in ANSI Standard for Mixed Oxides to Heterogeneous Systems

Subcommittee 8 of the Standards Committee of the American Nuclear Society is revising the Standard for Nuclear Criticality Control and Safety of Homogeneous Plutonium-Uranium Fuel Mixtures Outside Reactors to include limits on heterogeneous systems. In connection with this effort, a number of criticality calculations were completed for mixed-oxide (PuO₂ + UO₂) fuel pins in water. The concentration of PuO₂ in the UO₂ (natural uranium) covered the range from 3.0 to 34 wt%. The isotopic makeup of the plutonium was also varied, up to 25 wt% ²⁴⁰Pu and 15 wt% ²⁴¹Pu. A search was made on fuel pin diameters and water-to-fuel volume ratios to obtain minimum critical dimensions and masses for a given fuel composition. Calculations made independently by several different members of the Work Group are compiled and compared, together with the proposed subcritical control limits for the Standard. Some difficulties were encountered with calculations pertaining to 30% PuO₂ at ²⁴⁰Pu concentrations at water-to-fuel volume ratios and fuel pin diameters outside the area covered by any critical experiment. For this reason, dimensional limits on heterogeneous systems are not being proposed at this time for the Standard with 30% PuO₂ at a ²⁴⁰Pu content of 25%. In general, for a given fuel composition of mixed oxides, a heterogeneous arrangement of fuel pins of optimum diameter in water results in substantially smaller minimum critical dimensions than are obtainable for an aqueous homogeneous plutonium-uranium fuel mixture.