INVESTIGATIONS OF SLOSHING FLUID MOTIONS IN POOLS RELATED TO RECRITICALITIES IN LIQUID-METAL FAST BREEDER REACTOR CORE MELTDOWN ACCIDENTS

Analyses of unprotected loss-of-flow accidents for medium-size cores of current liquid-metal fast breeder reactors have shown that the accident proceeds into a transition phase were further meltdown is accompanied by recriticalities and secondary excursions. Assuming very pessimistic conditions concerning fuel discharge and blockage formation, a neutronically active whole-core pool of molten material can form. Neutronic or thermohydraulic disturbances may initiate a special motion pattern in these pools, called centralized sloshing, which can lead to energetic power excursions. If such a whole-core pool is formed, its energetic potential must be adequately assessed. This requires sufficiently correct theoretical tools (codes) and proper consideration of the fluid-dynamic and thermohydraulic conditions of these pools. A series of experiments has been performed that serves as a benchmark for the SIMMER-II and the AFDM codes in assessing their adequacy in modeling such sloshing motions. Additional phenomenologically oriented experiments provide deeper insight into general motion patterns of sloshing fluids while taking special notice of asymmetries and obstacles that exist in such pools.