Large-scale multi-dimensional phenomena found in CCTF and SCTF experiment

Thermal-hydraulic behaviors in pressure vessel during reflood phase of a PWR-LOCA are discussed based on the results from tests with Cylindrical Core and Slab Core Test Facilities, which model a 1100-MWe-class PWR with scaling ratio of about 1/20. Major findings on core thermal-hydraulics are, (i) much water accumulation in the upper part of the core, (ii) radially uniform water accumulations, and (iii) fluid circulation and/or concentration to high power bundles. These multi-dimensional phenomena cause better core cooling in high power bundle than expected from an evaluation model for licensing based on one-dimensional reflood experiments. The much water accumulation is considered to be caused by recirculation flow by comparing with data of the FLECHT low-flooding tests. Based on small scale reflood experiment, effect of local mass flow on heat transfer enhancement in a film boiling was correlated and all phenomena except the flow concentration effect were numerically well-simulated by using an analytical model with this correlation. The flow concentration effect on core heat transfer was empirically correlated with local power ratio. It was observed that non-uniform fluid mixing between core flow and subcooled water from hot legs for PWRs with combined-injection ECCS, which enhances the core coolability. This is multi-dimensional phenomena. The heat transfer enhancement was also well-simulated with the above-mentioned model. (authors). 15 figs., 20 refs