Helium release from type 304 stainless steel

Abstract

Helium in very low concentration (less than 1 atomic ppB) has been introduced into type 304 stainless steel by radioactive decay of dissolved tritium. Release of this helium during subsequent annealing was monitored with a high sensitivity mass spectrometric gas analyzer. With isochronal annealing, helium is released in two temperature ranges, one near 300$sup 0$C and the other between 800$sup 0$C and the melting point. The latter release is interpreted as attributable to helium gas bubbles. The release near 300$sup 0$C was studied isothermally between 150 and 300$sup 0$C and is analyzed in terms of two stages of exponential decay. The fast and slow release stages have relaxation times near 10$sup 2$ and 10$sup 3$ s, respectively, and the fast release accounts for roughly 85 percent of the total release at low temperature. From an analysis of the temperature dependence of the release rate, it is concluded that volume diffusion is the controlling mechanism for the outgassing. (auth