Thermal-hydraulic analysis of light water reactor severe core damage accident

A computer code SEFDAN has been developed in JAERI for analysis of severe fuel damage process under core uncovery condition such as occurred in TMI-2 accident, and applied to analyses of severe fuel damage test (SFD-ST and SFD 1-1 tests) in the PBF reactor. Further, the degree of the core damage in the TMI-2 accident has been evaluated. The major influential factors to the fuel behavior under the core uncovery are height of the dryout level of fuel rods (core froth level), heat transfer in the uncovered region, and the metal-water reactions of the cladding material. The froth level is calculated in the SEFDAN code by solving conservation equations for mass and energy of the two-phase mixture. Above the froth level, flow rate and the temperature of gas flow are calculated in taking account of steam consumption and hydrogen generation due to metal-water reaction. Heat transfer in the uncovered region consists of convection mode and radiation mode. The convective heat transfer is calculated with experimental correlations, which are chosen in accordance with newly developed convection flow regime map. The calculated results of the PBF.SFD-ST and 1-1 tests are in good agreement with experimental ones. Further the calculated result of the TMI-2 core damage is also in good agreement with the result of core examination by TV camera and ultrasonic core topography system. (author)