Nuclear Criticality Safety Evaluation of a Mixture of MOX, UO2 and Additive in the Most Conservative Concentration Distribution

Abstract

The nuclear criticality safety evaluation of blenders that are used at a mixed uranium-plutonium oxide (MOX) fuel plant must take into account the nonuniform distribution of powders in three principal components, i.e., MOX, uranium dioxide (UO2) and zinc stearate, which is a fuel additive. The model blender considered in this article contained a mixture of 33 wt% PUO2-enriched MOX, depleted UO2 and zinc stearate in the form of an upside-down truncated cone that was surrounded by 30-cm-thick polyethylene. To limit the number of calculation cases, the fissile plutonium mass of the mixture was fixed at 98 kg, and the total concentration of MOX and UO2 was fixed at 4.0 g/cm3. The most conservative fuel distribution with respect to nuclear criticality safety under these constraints was calculated with a two-dimensional optimum fuel distribution code OPT-TWO, so that the importance distribution of MOX and that of zinc stearate could be individually flattened by conserving the mass of each component. The OPT-TWO calculation was followed by a criticality calculation performed with the MCNP code to obtain the neutron multiplication factor of the fuel system in the optimum fuel distribution. The most conservative fuel distribution obtained in this research was typically depicted as a layer of zinc stearate embedded within the central MOX region that was surrounded by the peripheral UO2 region. An increase of up to 25% in the neutron multiplication factor was found; two factors with comparable but independent contributions were the nonuniform concentrations of plutonium enrichment and zinc stearate.