Transient parameters analysis of non-scrammed local melting accident scenarios using MCNPx and COBRA/EN

The risk assessment of local nuclear fuel melting in nuclear power plants (NPPs) due to its localized nature and the difficulty in accurate monitoring is of vital importance for safe operation of nuclear reactors. Experiences have shown that flow blockage accident without safety control rod axe man (SCRAM) can lead to local nuclear fuel melting which consequently affect the safety of NPPs. The purpose of this study is to analyze the transient states leading to local fuel melting based on ATWS-related events given in FSAR of the VVER-1000/V446 nuclear reactor including pump failure, local blockage, power level increasing and their combinations. In this work, a coupling framework is first developed based on the MCNPx and the COBRA/EN codes. After validation with available data, the results showed that despite 18% deviation from the mass flow rate reduction limitation, 470 kPa from the channel pressure drop limitation, and 204K from the clad temperature limitation in the most pessimistic situation, the reactor SCRAM does not occur. However, in these conditions (where SCRAM does not occur), 70% void fraction for 12 minitues is observed in some channels. Therefore, there may be dry spots and local melting of fuel in normal operational and ATWS conditions that need to be identified. According to the results, the occurrence of the void fraction above zero is locally expectant and an appropriate monitoring system should be used to identify weakness points of the system.