Generic issues on sump clogging

On July 28, 1992, strainers on the suction side of the ECCS pumps in Barsebaeck NPP Unit 2 became partially plugged with mineral wool after a safety valve opened because steam impinged on the thermally-insulated equipment and released mineral wool. This event pointed out that the plugging of strainers after loss-of-coolant accidents was underestimated. The findings from this event led first to modifications in other BWRs but also to research activities for PWR sump designs. Results of experiments at Karlstein and GKSS have shown that under realistic boundary conditions during LOCA released insulation material leads to a higher transportation rate in the sump and a larger deposition rate at the sump strainer and in the core than assumed before. Based on the new knowledge a new statement on the 'Requirements for the demonstration of effective emergency core cooling during loss-of-coolant accidents with release of insulation material and other substances' was issued by the German Reactor Safety Commission in July 2004. Modifications of the insulation, the strainer size and the mesh size at the strainers were carried out in most of the German NPPs to improve the design for emergency core cooling during sump recirculation mode. (authors)