Enrichment of 57 Fe isotope in neutron flux of nuclear reactors observed by M?ssbauer spectroscopy

摘要

Abstract The abundance of 57 Fe isotope in nuclear reactor core materials can be considerably enriched by neutron-capture 56 Fe(n,γ) reactions. This is demonstrated using the sections of Zr-2.5 wt.%Nb pressure tubes removed from two CANDU* reactors. The tubes contained 0.11 and 0.04wt% Fe and were irradiated for about 10 effective full power years (EFPY) up to ~10 26 n/m 2 fast neutron (E > 1MeV) fluencies. The M?ssbauer spectra of 57 Fe in irradiated samples indicated up to 10 times larger areas than unirradiated off-cuts from the same pressure tubes. The observed effect is in accord with the values calculated for known thermal neutron-capture cross-sections and resonance capture integrals, neutron flux profiles and spectra, and times of irradiation. The build-up of 57 Fe facilitated recording M?ssbauer absorption spectra of alloys with minor amount of Fe down to ~ 400ppm, despite intense background radiation emitted by samples. These findings can open new possibilities in post-irradiation studies of alloys used in nuclear reactors and in other objects subjected to large neutron fluencies. Highlights ? The abundance of 57 Fe isotope in nuclear reactor core is considerably enriched by neutron-capture 56 Fe(n,γ) reactions. ? M?ssbauer spectroscopy of 57 Fe is used to observe this effect. ? In Zr-2.5 wt.%Nb pressure tubes removed from CANDU reactors after 10 years of service 57 Fe abundance increased about 10 times. ? Calculations of 56 Fe(n,γ) 57 Fe transmutation are performed to predict the degree of enrichment in various nuclear reactors.