Coupled thermo-structural analysis for in-vessel retention in PHWR using ABAQUS

Abstract This paper presents simulations which give insights about the thermo-structural interaction of molten core debris and calandria vessel during severe accident scenario in a pressurised heavy water reactor. The insights developed are required to devise appropriate accident mitigation strategies including in-vessel retention and source term evaluation. Internationally, the severe accident assessment research has been focussed on RPV lower head failure studies and a limited and simplified approach has been attempted for analysing calandria vessel behaviour with core debris in pressurised heavy water reactor. This paper presents a 3D finite element based simulation of the thermo structural interaction of molten debris and calandria vessel. During a postulated severe accident, core debris would relocate into the lower portion of calandria vessel and may threaten the integrity of calandria vessel. The heat transfer between the molten core debris and the vessel may cause localised overheating (or partial melting) or inelastic strain accumulation which may result in vessel failure. In this paper coupled thermo structural analysis of calandria vessel with debris is carried out in a finite element framework to evaluate integrity of the calandria vessel. In this analysis boundary conditions include availability and non-availability of water outside calandria vessel. Initial conditions assume molten debris with different percentage of zirconium oxidation. Structural behaviour of calandria vessel is simulated considering elasto-plastic material behaviour including creep deformations. A failure criterion based on inelastic strain is used in the simulation. The analysis shows that the calandria vessel may not undergo inelastic strain failure as long as the calandria vault water is available. The failure of calandria vessel may occur due to the localised melting when the vault water is not available. Further the paper also presents experimental studies planned to investigate the heat transfer from calandria with core debris to calandria vault water. Finite element simulation is presented which is used to design the proposed direct heating arrangement to simulate the debris heat flux.