Measurement of Helium Generation in AISI 304 Reflector and Blanket Assemblies after Long-Term Irradiation in EBR-II

Five hexagonal ducts constructed from AISI 304 stainless steel in the annealed state were removed from rows 8-14 of the EBR-II fast reactor after many years of irradiation to study the effect of atomic displacement rate on void swelling. For this objective it was important to assure that the observed differences in void swelling were not strongly influenced by variations in helium/dpa ratio. The two major contributions to helium production arise from the nickel and boron content of the steel. However, the boron content was unspecified and therefore unknown, but it was determined from an unirradiated archive duct by converting a well-defined fraction to helium and then measuring the helium content using isotopic dilution mass spectrometry. The same technique was used to measure the total helium in the irradiated duct specimens. After separating the boron contribution from the total measured helium it was possible to determine the contribution from various fast and thermal neutron interactions with the other major elements in the steel and compare the helium generation with predicted values. One important conclusion of the study is that the range of He/dpa ratios over the five subassemblies was not very large, allowing the observed changes in swelling to be attributed primarily to variations in displacement rate and temperature.