Importance and status of (n,α)-cross sections for a reliable prediction of radiation damage in stainless steel

It is well established that helium formed in stainless steel by various (n,α) processes has a pronounced effect on its mechanical and dimensional properties. There has been strong discrepancy between the helium contents measured in irradiated stainless steel and that calculated on the basis of known (n,α) cross sections on the stable constituents of stainless steel. This discrepancy seems to be resolved by attributing the excessive helium to the two step process: 58Ni(n,γ)59Ni(n,α)56Fe. However, a large discrepancy exists between the value of the thermal cross section for the 59Ni(n,α)56Fe process used to calculate the helium production data and that obtained out of the direct measurements of this cross section. In this paper the role of the (n,α) cross section is investigated on the irradiation induced swelling as an example. It is shown that insoluble gases like helium do influence swelling by way of nucleating and stabilising voids. It is further shown that helium produced via 59Ni constitutes a substantial amount of helium produced in stainless steel under thermal as well as under fast reactor conditions. In fast reactors helium produced via 59Ni gains its importance at high neutron fluences. A linear extrapolation of helium production data measured at low fluences to the fluences of the order of l.5.1023 n/cm2 is not justified. It is shown that the usual practise to relate helium production data to thermal and fast neutron fluence is inadequate. The details of neutron spectrum are necessary to reliably predict helium production rate. A measurement of 59Ni(n,α)56Fe cross section in keV and MeV range is recommended. (orig.)