Overview of the ESS target and moderator R and D

The major concern in current R and D effort is radiation damage and foreign atom production (mainly hydrogen and helium) in structural materials of the ESS mercury target. Mechanical tests on specimens from spent targets at LAMPF (Los Alamos Meson Physics Facility) and ISIS are being performed. The results show a remarkable strengthening and embrittlement with all three investigated materials classes (austenitic and martensitic steels as well as nickel-based alloys). The residual ductility observed with specimens subject to the highest available dose of 10 dpa (corresponding to about 2 months of operation of ESS) are, however, sufficient for being employed as structural materials of ESS targets. Furthermore, an additional extended irradiation program at PSI (Paul Sherrer Institute) (STIP) is just about to be started. Pressure waves in mercury as well as the resulting stress waves in the container walls of the ASTE (AGS Spallation Target Experiment) mercury target are being investigated at the AGS (Alternating Gradient Synchrotron) in Brookhaven. A comparison with results from finite elements calculations is under way. Heat transfer experiments in mercury loops in connection with computational fluid dynamics studies are being conducted. This is to investigate the active cooling of the proton beam window of the target container. Within the present R and D phase several paths for developing radiation resistant or at least better manageable cold moderators such as small pebbles of solid methane (2 to 3 mm diameter) or methane clathrates (e.g. from water ice) are being pursued. Irradiation behavior of solid methane is being performed at the IBR-2 reactor in Russia. The neutronic properties (intensities and pulse shapes) of the different variants will be studied employing the ESS target-moderator-reflector module JESSICA (Juelich Experimental Spallation target Set-up In Cosy Area), which became operational recently at the Juelich Cooler Synchrotron. (author)