The importance of using the mixed neutron flux in activation analysis of D-3He fueled reactors

This paper reports on the D-D and D-T secondary reactions in D-3He reactors which provide the neutron source term for most of the radioactivity produced in the structure of the reactor. radionuclides are produced as a result of neutron interactions with their parent nuclides. The amount of activity produced by any radionuclide depends on the number of its parent atoms present at any given time. One approach to account for the activity induced by both neutron sources in any activation analysis is to add their individual contributions. Performing two separate calculations for the D-D and D-T neutron flux components and adding their contributions yields conservative results due to underestimating the destruction of the parent atoms. The overestimation is more pronounced for short and intermediate lived nuclides, long operation time, large neutron flux and large destruction cross section for the parent atoms. In the steel first wall of a typical d-3He reactor, adding the individual contributions of the tow neutron sources results in overestimating the activities produced by most of the radioactive isotopes of Ag, Lu, Ta, W and Re. After 30 years of reactor operation, the activity of 187W, which is a major source of safety concern in case of an accident, is more than an order of magnitude higher than its value if the mixed neutron flux is used. The activity of 188Re, which is an important source of offsite does in case of accidental release, is overestimated by more than a factor of two