Radiochemical aspects of liquid mercury spallation targets

Liquid metal spallation targets using mercury as target material are used in state-of-the-art high power pulsed neutron sources that have been constructed in the USA and Japan within the last decade. Similar target concepts were also proposed for next generation ISOL, beta-beam and neutrino facilities. A large amount of radioactivity will be induced in the liquid metal during operation caused by the interaction of the target material with the intense proton beam. This radioactivity - carried by a wide range of radioisotopes of all the elements of the periodic table from hydrogen up to thallium - must be considered for the assessment of safe operation and maintenance procedures as well as for a final disposal of the used target material and components. This report presents an overview on chemical investigations performed in our laboratory that deal with the behavior of radionuclides in proton irradiated mercury samples. The solubility of elements in mercury was calculated using thermodynamical data obtained by a semi-empirical method. It is found that several important nuclear reaction products will reach their solubility limit in a high power mercury spallation target. It is also experimentally proven that the radionuclides produced by proton irradiation are generally not homogeneously distributed in the liquid metal but tend to accumulate on vessel walls and free surfaces. The consequences resulting from this behavior for the operation of mercury spallation targets are discussed. (C) 2011 Elsevier B.V. All rights reserved.