Experimental studies of helium release from aged PdT» show that the helium-to-metal-atom ratio saturates at a value of [He]/[Pd]=0. 5 under conditions of ambient-temperature storage. Below this value, very little helium release occurs. Thermal desorption experiments show that release from a sample with [He]/[Pd]=0. 3 requires temperatures in excess of -600 K, while release from a sample with [He]/[Pd] =0.02 requires temperatures in excess of at least 1300 K.These results are related to the question of the disposition of helium that would be produced by hypothetical fusion reactions in a Pdo"electrode.
Hydrogen and helium measurements in pure nickel irradiated to 100 dpa in HFIR at temperatures between 300 and 600°C show higher gas concentrations than predicted from fast-neutron reactions and the two-step 58Ni(n,γ)59Ni (n,p and n,α) reactions. This additional gas production suggests previously unidentified nuclear sources of helium and possibly hydrogen that assert themselves at very high neutron exposure. The elevated hydrogen measurements are especially surprising since it is generally accepted that hydrogen is very mobile in nickel at elevated temperatures and therefore is easily lost, never reaching large concentrations. However, it appears that relatively large hydrogen concentrations can be reached and retained for many years after irradiation at reactor-relevant temperatures. These new effects may have a significant impact on the performance of nickel-bearing alloys at high neutron fluences in both fission and fusion reactor irradiations.
The 59Ni isotope produced by the 58Ni (n,γ) reaction has three highly exothermic reactions, (n,α), (n,p), and (n,γ). These reactions contribute to the displacement damage and two of them can generate significant amounts of helium and hydrogen. The production of helium in the Experimental Breeder Reactor-II (EBR-II) at high neutron exposure has been measured and found not to include a significant contribution from 59Ni in those core volumes where most material's data were generated. This is in contrast to a significant 59Ni contribution that occurs in the softer below-core areas of the Fast Flux Test Facility (FFTF).
The often overlooked 59Ni contribution to hydrogen generation is of particular importance when considering neutron spectra with a substantial thermal neutron component, however. Inclusion of the (n,p) and (n,γ) recoil events in the dpa damage calculations requires only a 3% correction to the previously published value, indicating that most of the damage arises from the (n,α) reaction.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.