Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

In the framework of the joint research project WASA-BOSS (Weiterentwicklung und Anwendung von Severe Accident Codes – Bewertung und Optimierung von Störfallmaßnahmen) of the Federal Ministry of Education and Research, an ATHLET-CD model for a generic German PWR of type KONVOI was developed. The model is applied to analyze the nuclear power plant response in case of a hypothetical SBLOCA severe accident scenario. The SBLOCA 50 cm² leak is modelled in cold leg No 2 (loop with pressurizer) close to the reactor pressure vessel. The scenario is initiated at nominal power conditions with additional assumptions for multiple systems failures, leading to a core degradation scenario. The model covers the in-vessel phase of the accident including core degradation, cladding oxidation, core quenching, hydrogen production, fission products release, material relocation, and RPV failure. The progress of the accident and timing of the main events is presented. Despite continuous improvement of the severe accidents codes, the model parameters for late accident phenomena are still subject to significant uncertainties. A parameter study investigates the influence of one of those uncertain parameters in ATHLET-CD, the onset of core melt relocation to the RPV lower head, to the late in-vessel phase of accident progression. In the ATHLET-CD model, the relocation to the lower head is initiated by a user defined criterion based on the amount of molten mass. The variation of this parameter within the range of 20 % – 60 % of total core mass leads to a time-shift of the lower head pool-formation of 45 min and a time-shift of all subsequent processes till RPV failure. This analysis contributes to the application and assessment of the ATHLET-CD code for simulation of severe accident scenarios.