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

Materials in the target region of spallation neutron sources which are expo sed, or are close to the proton beam suffer from high rates of displacement damage and the production of foreign elements, particularly hydrogen and h elium isotopes. Whilst some results on the effect of helium are now availab le, there is little information on the potential influence of hydrogen in t he environment of a spallation source. We therefore investigate theoretical ly the establishment of hydrogen background concentrations and their enhanc ement 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 c oncentrations with empirical threshold concentrations for hydrogen embrittl ement and hydride formation indicates that the use of ferritic/martensitic steels for water-cooled windows may be problematic, although these material s remain good candidates for the mercury container. Tantalum as a prospecti ve material for the solid target option of ESS is not expected to undergo h ydrogen degradation. (C) 2001 Elsevier Science B.V. All rights reserved.