Safety relevant nuclides in advanced spallation neutron sources

Advanced spallation sources for generation of short neutron pulses (∝ 1 μs) are under construction in USA (SNS) and Japan (J-PARC); a decision about the European project ESS is expected not earlier than 2006. Spallation sources consist mainly of an accelerator (1 - 10 MW beam power, 0.8 - 3 GeV proton energy), a compressor ring and one or more target stations, where neutrons are formed by spallation. A liquid mercury target is favoured by most concepts, but (liquid) lead bismuth eutecticum (LBE) and solid state targets (tungsten) are in discussion, too. An advantage of liquid targets lies in the fact that the heat removal is easier, because no direct target cooling is required; this avoids H-3 formation as in the direct target cooling by water. On the other hand, cavitation problems associated with microsecond pressure pulses in liquid targets are not yet completely solved for power values > 1 MW. Further on, the absence of a crystal lattice as activity release barrier has to be taken into account in the safety design of liquid target systems. This paper describes accumulated activities for an ESS liquid mercury target station as well as dose/emission values for the most radiotoxic nuclides. Comments are made about the final waste disposal, including the necessary solidification process for liquid mercury. More details are found in the ESS Project Volume III Update Report. (orig.)