1 Modeling Tritiated Water Desorption from Stainless Steel

A simple mathematical model has been developed to describe the outgassing rate of tritium from solids as a function of time. The model is based on the assumptions of a homogeneous material that can be characterized with a single diffusion coefficient. The model was used to fit the experimental tritium desorption rates from pre-tritiated, type 316, stainless steel over a temperature range of 20 to 400oC. The basic model did not fit the experimental data accurately at longer times, except for data obtained at 20 and 400oC. Including carrier gas flow rate and temperature ramping did not improve the fitting results at the longer times. It is likely that at 20oC, the desorption process is dominated by a single diffusion coefficient that describes tritium diffusion through and desorption from the surface oxide layer. At 400oC, the desorption process is dominated by atom transport through the bulk metal. At intermediate temperatures, atom transport is dependent on diffusivity through the oxide and the bulk, and thus resulted in a poor curve fit since the model assumes a unique diffusion coefficient within a single homogeneous layer. We conclude that the bulk diffusion model alone cannot describe the tritium desorption processes from stainless steel with an oxide top layer. A more accurate model that includes desorption from the surface oxide layer, diffusion through the oxide layer, and diffusion within the bulk has been proposed for future work