A reevaluation of helium/dpa and hydrogen/dpa ratios for fast reactor and thermal reactor data used in fission-fusion correlations

The Ni-59 isotope produced by the Ni-58 (n,gamma) reaction has three highly exothermic reactions, (n,alpha), (n,p), and (n,gamma). These reactions contribute to the displacement damage and two of them can generate significant amounts of helium and hydrogen. The production of helium in the Experimental Breeder Reactor-II (EBR-II) at high neutron exposure has been measured and found not to include a significant contribution from Ni-59 in those core volumes where most material's data were generated. This is in contrast to a significant Ni-59 contribution that occurs in the softer below-core areas of the Fast Flux Test Facility (FFTF). The often overlooked Ni-59 contribution to hydrogen generation is of particular importance when considering neutron spectra with a substantial thermal neutron component, however. Inclusion of the (n,p) and (n,gamma) recoil events in the dpa damage calculations requires only a 3% correction to the previously published value, indicating that most of the damage arises from the (n,alpha) reaction.