Evaluation of reflooding effects on an overheated BWR core in a small steam line break accident using MAAP, MELCOR and SCDAP/RELAP5 computer codes

Selected core reflooding situations were investigated in the case of a Finnish boiling water reactor with three severe accident analysis computer codes (MAAP, MELCOR, and SCDAP/RELAP5). The unmitigated base case accident scenario was a 10% steam-line break without water makeup to the reactor pressure vessel initially. The pumping of water to the core was started with the auxiliary feed water system when the maximum fuel cladding temperature reached 1500 K. The auxiliary feedwater system pumps water (temperature 303 K) through the core spray spargers (core spray) on the top of the core and through feedwater nozzles to the downcomer (downcomer injection). The scope of the study was restricted to cases where the overheated core was still geometrically intact at the start of the reflooding. The following different core reflooding situations were investigated: 1. auxiliary feedwater injection to core spray (45 kg/s); 2. auxiliary feedwater injection to downcomer (45 kg/s); 3. auxiliary feedwater injection to downcomer (45 kg/s) and to core spray (45 kg/s); 4. no reflooding of the core.
All the three codes predicted debris formation after the water addition, and in all MAAP and MELCOR reflooding results the core was quenched. The major difference between the code predictions was in the amount of H₂ produced, though the trends in H2 production were similar. Additional steam production during the quenching process accelerated the oxidation in the unquenched parts of the core. This result is in accordance with several experimental observations.