55Fe effect on enhancing ferritic steel He/dpa ratio in fission reactor irradiations to simulate fusion conditions

This study evaluated methods for increasing the helium production rate in ferritic steel irradiation in a fission reactor neutron spectrum in order to increase the helium to atomic displacement ratio to values typical of fusion reactor first wall conditions. An early experiment showed that the accelerated He(appm)/dpa ratio of about 2.3 was achieved for 96% enriched 54Fe in iron in the High Flux Isotope Reactor (HFIR), ORNL. In the current work, the ferritic steel He(appm)/dpa ratio was studied in the neutron spectrum of HFIR with the 55Fe thermal neutron helium production taken into account. A benchmark calculation for the same sample, as used in the aforementioned experiment, was then used to adjust and evaluate the 55Fe (n, a) cross section values in TALYS-based Evaluated Nuclear Data Library (TENDL). The analysis showed that a decrease of a factor of 6700 for the TENDL 55Fe (n, a) cross section in the intermediate and low energy regions was required in order to fit the experimental results. The best fit to the cross section value at thermal neutron energy was about 27 mb. With the adjusted 55Fe (n, a) cross sections, calculation showed that the 54Fe and 55Fe isotopes could be enriched by the isotopic tailoring technique in a ferritic steel sample irradiated in HFIR to significantly enhance the helium production rate. This new calculation can be used to guide future isotopic tailoring experiments designed to increase the He(appm)/dpa ratio in fission reactors. A benchmark experiment is suggested to be performed to evaluate the 55Fe (n, a) cross section at thermal energy.