Development of core disassembly model for PHWRs under severe accidents

This paper presents a model for disassembly of the calandria tubes in Pressurized Heavy Water Reactors (PHWR) under severe accident conditions. Such a model gives criteria to decide when under severe accident condition calandria tube will get disassembled. Presently available severe accident codes use simplistic criteria based on a pre defined temperature to predict disassembly of fuel channels. The coolant channel disassembly model developed in this paper is based on modeling the sag and pile up of channels. A uniform temperature distribution is assumed across the channel. The disassembly of the channel is assumed when the total strain at any location exceeds the failure strain for a given temperature. A 3D failure surface which is a plot of time to failure, temperature of the calandria tube and load on the calandria tubes (on account of no of channels piled up) is developed. This failure surface can be used as an input to severe accident codes to predict the progress of the core disassembly. The temperature transient of the calandria tube for a severe accident obtained from thermal hydraulic codes can be mapped onto the failure surface. The time at which the mapped transient crosses the failure surface gives the time at which the calandria tube is disassembled. It is also brought out that the calandria tube of 220 MWe Indian PHWR has enough pullout strength to avoid failure at rolled joint for the submerged channels avoiding sudden core collapse.