Retention of Very High Levels of Helium and Hydrogen Generated in Various Structural Alloys by 800 MeV Protons and Spallation Neutrons

Abstract

A series of irradiations were conducted at relatively low temperatures (<100°C) in the Los Alamos Neutron Science Center (LANSCE) as part of the test program supporting the Accelerator Production of Tritium Program sponsored by USDOE. In this irradiation campaign, a variety of candidate structural alloys were placed in various particle spectra, ranging from 800 MeV protons, to mixed energy distributions of both protons and spallation neutrons, and finally to distributions consisting primarily of high energy neutrons. At proton energies on the order of hundreds of MeV, exceptionally high levels of gas atoms are generated in all elemental constituents of typical iron-based and nickel-based structural alloys, with helium typically on the order of ∼150 appm per dpa and hydrogen at approximately an order of magnitude greater. Since both of these gases are considered to exert a negative influence on structural properties of interest, their retention after both energetic recoil loss and subsequent diffusional loss is of strong programmatic interest. Helium is essentially immobile at all temperatures of nuclear interest, but hydrogen has some limited temperature-dependent mobility, even at <100°C. To assess the degree of generation and retention, each gas was measured in a number of highly irradiated specimens of different alloy compositions and dpa levels. The results show that helium production is relatively insensitive to composition. The retained hydrogen levels, however, are somewhat sensitive to composition, reflecting different levels of diffusional loss, but are still at rather large concentrations. The measured helium concentrations are larger than calculated using a calculational code optimized for prediction of neutron/proton ratios in the target tungsten source rods. The use of these gas measurements to provide benchmarks for determination of gas production cross sections for typical elements that comprise common structural alloys is also examined.