Helium bubble characterization in tritium aged austenitic stainless steels

Tritium, as an isotope of hydrogen, permeates throughout most known materials and decays over time into 3 He causing both hydrogen and helium embrittlement. Tritium embrittlement is fundamentally different to He exposure under irradiation since it is purely the transmutation of tritium into 3 He without an accompanying atomic displacement (dpa = 0). Typically, austenitic stainless steel is used for tritium handling due to its high fracture resistance and favorable hydrogen embrittlement resistance. In this study, 304 L stainless steel and 21-6-9 steel samples were pre-charged with tritium gas to 3700 appm and 6100 appm, respectively. Samples were then stored at −80°C to allow 3 He to accumulate while minimizing the loss of tritium to off-gassing. When helium content was estimated to be 300 and 600 appm for 304 L and 1200 appm for 21-6-9, the samples were imaged with transmission electron microscopy (TEM). He bubble densities, sizes, and regions of agglomeration or absence were measured and no apparent agglomeration or void denuded zones were observed, suggesting that there are no or minimal grain boundary effects under these conditions.