Tritium absorption in Type 304L stainless steel

Tritium adsorption was determined in Type 304L austenitic stainless steel by analyzing concentration gradients obtained during prolonged exposures to high-pressure gaseous tritium. The calculated tritium diffusivities at temperatures greater than 3730K were shown to be in excellent agreement with the equation D = (1/√ m) (4.7 x 10-3) exp(-12,900/RT) cm2sec-2, where m is the isotopic mass. This equation was previously developed for deuterium in several types of austenitic stainless steels. There was strong evidence for ''short-circuit'' diffusion paths and a grain size effect on tritium absorption. Such effects are assumed to cause the tritium diffusivities measured for exposures at less than 3730K to be higher than expected from the above equation. Cold work, either prior to or during exposure, significantly increased the effective tritium diffusivity. The increase in tritium diffusivity observed in the samples cold-worked prior to exposure is believed to be caused by preferential (short-circuit) diffusion through strain-induced martensite. The increase in diffusivity in specimens deformed during exposure is believed to be caused by enhanced tritium transport with moving dislocations. This analysis of concentration radients also shows that tritium permeation rates through austenitic stainless steels will often be significantly less than rates expected from analysis of diffusion-controlled transport properties. This is because of surface barriers that limit tritium absorption, even at pressures to 69 MPa. Solubilities derived from analyses of the concentration gradients were consistently lower than expected and were significantly influenced by specimen surface conditions. (U.S.)