Criticality parameters for tank waste evaluation

Nuclear criticality parameters were developed as a basis for evaluating criticality safety for waste stored in the high-level waste tank farms on the Hanford Site in Washington State. The plutonium critical concentration and critical mass were calculated using a conservative waste model (CWM). The primary requirement of a CWM is that it have a lower neutron absorption than any actual waste. Graphs are provided of the critical mass as a function of plutonium concentration for spheres and for uniform slab layers in a 22.9-m-diameter tank. Minimum subcritical absorber-to-plutonium mass rates were calculated for waste components selected for their relative abundance and neutron absorption capacity. Comparison of measured absorber-to-plutonium mass ratios in their corresponding subcritical limit mass ratios provides a means of assessing whether criticality is possible for waste of the measured composition. A comparison is made between the plutonium critical concentrations in CWM solids and in a postulated real waste. This comparison shows that the actual critical parameters are likely to be significantly larger than those obtained using the CWM, thus providing confidence that the margin of safety obtained to the criticality safety evaluation is conservative