A comparison of measured and calculated helium production in nickel using newly evaluated neutron cross sections for 59Ni

Abstract

Fusion reactors will produce high levels of helium in surrounding materials with a helium (appm)-to-displacement ratio of about 10-to-1 in stainless steel. This high ratio can be obtained in mixed-spectrum reactors, which are used for fusion materials testing, due to unusually high thermal neutron cross sections for the sequential reactions 58Ni(n,γ) 59Ni(n,α) 56Fe. The highenergy (∼340 keV) 56Fe recoils also add significantly to the displacement damage at the rate of one DPA per 567 appm helium. Until now, the calculation of helium production in nickel has been done in a semi-empirical manner due to a lack of evaluated cross sections for 59Ni. However, this approach cannot be readily transferred between different reactors since we do not know the contributions from epithermal neutrons in different neutron spectra. A new evaluation of the 59Ni cross sections has recently been completed, permitting us to calculate all of the required reaction rates for any given neutron spectrum. Radiometric dosimetry and helium measurements have recently been completed for several different mixed-spectrum reactors. Precise comparisons of the helium production cross sections and measurements can thus be made in well-characterized neutron spectra. Data are presented for several recent fusion materials irradiations in the Oak Ridge Research Reactor and High Flux Isotope Reactor at Oak Ridge National Laboratory and for the Experimental Breeder Reactor II at Argonne National Laboratory. Procedures are recommended for calculating helium production for nickel-bearing materials in any neutron spectrum.