ATHLET-CD analysis of water injection into a partly-damaged BWR core for accident management improvement

The present work is aimed at investigating the influence of water injection into a partly-damage core as a result of hypothetical multiple failures of the safety systems during a small break (SB) LOCA in a boiling water reactor (BWR). The work is carried out in the frame of the research project WASA-BOSS (Weiterentwicklung und Anwendung von Severe Accident Codes – Bewertung und Optimierung von Störfallmaßnahmen) funded by the German Federal Ministry of Economics and Technology which is devoted to the analysis of severe accident scenarios for the assessment of the safety of German nuclear power plants. The reference power plant is a generic 72-type BWR with a thermal power of 3840 MW. According to probabilistic safety analyses, a SBLOCA scenario at one of the steam lines inside the containment was defined for this work. Calculations have been carried out with the thermal-hydraulic system code ATHLET-CD up to 2 hours after reactor SCRAM, corresponding to a configuration where about half of the core material is molten in case of no water injections. The coolability of a partly-damaged core has been investigated by assuming the recovery of the safety systems operation at different core damage states. The results of the present calculations show that injection of water at different points in time has a significant impact on the produced hydrogen and on the accident progression. This study represents the starting point for further analysis, which may include reflooding at different core damage states as well as simulation of the interaction between molten material and lower plenum, and provides useful information for the improvement of the accident management measures. (author)