Numerical investigations on heat transfer characteristics and melting behaviour of a PHWR pressure tube under combined LOCA and moderator voiding

Failure of the Pressure tube under severe Loss of Coolant Accident (LOCA) with Emergency Core Cooling System (ECCS) failure and complete moderator voiding is a serious issue in Indian Pressurized Heavy Water Reactors (IPHWRs). The present study investigates the thermal behaviour, heat transfer characteristics, and melting progression of a single fuel channel of the 220 MWe IPHWR using ANSYS 2023 R1. The steady-state simulations were performed for heat fluxes ranging from 900 to 4950 W/m 2 to determine the temperature distributions and to identify the melting initiation heat flux for the onset of melting, while the heat flux ranges 4400–4950 W/m 2 were used for the transient phase change simulations to determine the melting rate. The results show that there is a significant temperature asymmetry within the pressure tube-calandria tube assembly. The peak temperature of the pressure tube was raised from 396.25°C to 2080.60°C and the melting initiation heat flux was predicted to be approximately 4400 W/m 2 . With increasing heat flux, the complete melting time decreased from 80 s to 8 s, with an increase of about ten in the Melting Rate Index (MRI), thus representing very non-linear melting behaviour. The proposed PT-CT Thermal Coupling Index (TCI), Normalized Temperature Ratio (NTR) and Melting Rate Index (MRI) are quantitative assessments of the thermal vulnerability and melting progress. The proposed approach will be useful for severe accident analysis and safety assessment of Indian PHWR systems for engineering applications.