A new technique for enhancing helium production in ferritic materials

Analyses of iron samples irradiated up to 1027 n/m2 in HFIR found more helium than was expected from fast neutron reactions at high neutron fluences. The helium excess increased systematically with neutron exposure, suggesting a transmutation-driven process. The extra helium may be produced in two different ways, either by fast neutron reactions on the transmuted isotopes of iron or by a thermal neutron reaction with the radioactive isotope 55Fe. Radiometric and mass spectrometric measurements of the iron isotopes composing the irradiated samples have been used to determine limits on the cross sections for each process. Either of these processes can be used to enhance helium production in ferritic materials during irradiations in mixed-spectrum reactors by isotopically enriching the samples. Further work is needed to clarify the reaction mechanisms and helium-production cross sections. Our measurements determined the thermal neutron total absorption cross section of 55Fe to be 13.2±2.1 barn. (orig.)