Hydrogen generation arising from the Ni-59(n,p) reaction and its impact on fission-fusion correlations

While the influence of transmutant helium on radiation-induced microstructural evolution has often been studied, there is a tendency to overlook the influence of concurrently-generated hydrogen. Recent studies suggest that the influence of hydrogen may be enhanced in the presence of large amounts of helium, especially at lower irradiation temperatures typical of projected ITER operation. This interaction may have an impact on potentially hydrogen-sensitive processes such as irradiation-assisted stress corrosion cracking (IASCC) of stainless steel. In nickel-bearing alloys, one of the major sources of helium in neutron spectra with a significant thermal neutron component is the two-step Ni-58(n, gamma)Ni-59(n, alpha)Fe-56 reaction. It has generally been overlooked, however, that the competing (n, p) reaction on Ni-59 can dominate the hydrogen production process since about one hydrogen atom is generated for every six helium atoms. Inclusion of the damage energy deposited by the (n, p) reaction in the alloy matrix also requires a slight modification in the Ni-59(n, alpha) damage enhancement formula published earlier. The impact of the (n, p) reaction on both hydrogen generation and displacement rates is evaluated in this paper for a variety of neutron spectra often employed in fission-fusion correlations.