Untersuchungen zur Getterung metallischer Spaltprodukte im Primärkühlkreislauf eines Hochtemperaturreaktors

Abstract

Supplementary to the recommended method to retain fission products by ceramic coatings (e.g. pyrocarbon, silicon carbide) in small spherical fuel particles, a new concept of gettering silver 110m and cesium 134/137 in the primary circuit of a High Temperature Reactor (HTR) is presented. It is based upon the known fact that the vapor pressure of metallic fission products in solid or liquid solutions is lower compared with that of the pure fission products. Although metallic additives were found not to influence the silver release from oxide fuel kernels, the effective diffusion coefficient of silver 110m in graphite matrix is reduced by about two orders of magnitude by small amounts of the metallic Cu, Ge, Sn or Au additions. However, these reduced silver diffusion coefficients are not sufficiently low in order to retain silver 110m in the fuel-free zone of spherical HTR fuel elements. On the other hand, metallic additives were found to be very efficient in gettering silver 110m from the gaseous phase: During a contact time of only 0,15 seconds at 950°C more than 80 %,at 850°C even more than 99 % of the Ag 110m could be absorbed from the streaming gas by using a metal-containing graphite filter. The best results were obtained by using tin or gold additives. By optimizing the filter geometry further increase of the efficiency should be possible