Experimental study on thermo-mechanical deformation of PHWR channel at elevated temperature

Abstract

The severe accidents in PHWR type reactors are less probabilistic events owing to extensive safety features, but the consequences of these accidents are serious if not mitigated. Severe accidents progress with the channel deformation due to thermal load, internal pressure and weight of its components. Channel’s sagging behaviour and its failure will define the accident progression under the event of low-pressure inside the channel. Sagging deformation determines the nature of early debris or suspended debris formation. Also, sagging of channel results in the pull-out force on the Calandria end-shield. Therefore, it becomes imperative to know the nature of the event in order to provide any mitigating countermeasures.The present paper aims to study the sagging of coolant channels of standard 220 MWe Indian PHWR under the hypothesis of postulated severe accidents with large break LOCA and loss of ECCS as an initiating event. The moderator cooling system is postulated as impaired with no make-up system. It is difficult to perform a full-scale channel sagging experiments in laboratory. Hence, a novel scale-down approach (1:3) has been adopted for laboratory experiments for the channel sagging study.Experiments have indicated a significant initial sagging with sudden power ramp. The sagging of channel was significant as the temperature of CT reached to 460 °C. The pull-force was not dominant during the transient heat-up, but significant pull or end-load has been observed at the end of the channel cooling. The rate-dependent plasticity (creep) is found to be a dominating mechanism for sagging of the channel.