Hydrogen concentrations near cracks in target materials for high-power spallation neutron sources

Materials in the target region of spallation neutron sources which are exposed, or are close to the proton beam suffer from high rates of displacement damage and the production of foreign elements, particularly hydrogen and helium isotopes. Whilst some results on the effect of helium are now available, there is little information on the potential influence of hydrogen in the environment of a spallation source. We therefore investigate theoretically the establishment of hydrogen background concentrations and their enhancement near microcracks in proton-irradiated plates of ferritic/martensitic steel and tantalum, respectively, for the conditions of the envisaged 5 MW European Spallation Source (ESS). A comparison of the predicted hydrogen concentrations with empirical threshold concentrations for hydrogen embrittlement and hydride formation indicates that the use of ferritic/martensitic steels for water-cooled windows may be problematic, although these materials remain good candidates for the mercury container. Tantalum as a prospective material for the solid target option of ESS is not expected to undergo hydrogen degradation.