Thermal-hydraulics in uncovered core of light water reactor in severe core damage accident, 4

Analysis of the TMI-2 core damage behavior has been performed with the SEFDAN code. The scope of the analysis is by the time of restarting the reactor coolant pump RCP-2B at 2 h 54 min into the accident. The analysis indicates that fuel temperature would have reached the melting point of UO2 in the upper-most part of the most central region in the core. In a large portion of the upper part of the core, fuel cladding temperature would have reached the melting point of ZrO2 and/or highly oxidized. Therefore it is expected that large amount of fuel rod material would have melted and relocated, or embrittled and have been thermally or mechanically shocked by restarting RCP-2B, resulting in formation of large amount of debris. The calculated maximum temperature of fuel rods and expected extent of the core void are in good agreement with the results of core examinations. Total hydrogen generation, however, is calculated to be about 1/3 of that estimated from the accident record. The discrepancy is probably due to heat-up and melting of the collapsed core after restarting RCP-2B in which zirconium-water reaction should have proceeded. (author)