Rates of adsorption of moisture onto plutonium oxide powders exposed to air are modeled. The moisture contents of these powders must be limited to minimize the radiolytic generation of flammable hydrogen gas when the plutonium oxide subsequently is stored in containment vessels. The pressure in the vessels is related to the amount of moisture adsorbed. Moisture adsorption rates are modeled for powders in two different containers used by the Savannah River Site (SRS) HB-Line facility, a B vial and a product can. The adsorption models examine the effects of the powder layer fill height, gas mixing conditions above the powder layer, and ambient relative humidity. Moisture distribution profiles are calculated to enable the evaluation of the effect of sampling location on the measured moisture content. The adsorption models are applied using the COMSOL Multiphysics® finite element code. The COMSOL® models couple moisture diffusion with thermal conduction and radiation. The models incorporate an equilibrium adsorption isotherm and a detailed model for combined radiation and conduction heat transfer in the powder, both developed at Los Alamos National Laboratory. The COMSOL® adsorption rate calculations are successfully benchmarked using an analytical, one-dimensional ash and pore diffusion model.

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