Calculated minimum critical masses of (sup 239)Pu homogeneously mixed with polyethylene moderator

摘要

The minimum critical masses of Plutonium 239 in a polyethylene moderator were calculated as a function of plutonium density for several polyethylene densities (various void fractions). This study has applications for solid transuranic (TRU) waste and for plutonium scrap dissolving operations where polyethylene bags may be present within the cans to be dissolved. Polyethylene is usually present in TRU waste as a result of glovebox bagout operations and as liners in 55 gallon drums. The methodology utilized the SCALE driver CSAS1X. MCNP4A with ENDF/B-V was also used as an independent check due to the lack of critical experiments for polyethylene moderator. For TRU solid waste, tests with 55 gallon drums indicate that polyethylene bagging cannot be tightly stuffed to a volume fraction greater than 15.5%. To allow for settling, calculations were conducted for 20% and 50% polyethylene volume fractions as well as for full density for comparison. The effects of 10% concrete or steel mixed with 20% polyethylene were also evaluated. Since water egress into underground solid waste containers would be possible, additional calculations evaluated critical masses of water in polyethylene moderator. Calculated critical masses for the various moderators were determined for a range of plutonium concentrations. For full density polyethylene, the minimum critical plutonium mass is about 345 grams (at 30 grams per liter) versus 510 for water. Added concrete substantially decreases the critical mass, and added steel substantially increases the critical mass. This study indicates that in some situations the minimum critical plutonium mass in polyethylene can be less than that of metal in water (about 510 grams). TRU waste fissile limits are usually based on safe masses determined from plutonium in water so that this result has obvious implications on criticality safety. 1 fig.