Effect of heat-up and oxidation on the PHWR channel under severe accident scenario-an experimental study

Abstract

During unmitigated postulated severe accidents, the coolant channels of the pressurized heavy water reactor are heated to elevated temperatures. The channels may further get exposed to oxidizing media during the accident progression. Both heat-up rate and oxidation reaction affect the sagging deformation of channels. This study explores the effect of initial transient heat-up rate on the channel deformation and the contact time between heated and un-heated channels experimentally. The effect of oxidizing environment on the structural integrity of the channels is also studied. As experimentation with multiple full-length channels with bundle weights is complex and expensive, the experiments are conducted with scaled-down channels with geometric scaling of 1:3. During experiments, the sagging deformation of the channel has increased gradually with a low heat-up rate, whereas the contact between the channels has been instantaneous at a high heat-up rate. It is thus suggested that the deformation of the heated channel and the contact propagation along contacting channels may be governed by the rate-dependent plasticity (creep). The oxidative reaction in oxygen-rich corrosive media has severely deteriorated the outer calandria tube of the coolant channel, exposing the inner pressure tube to corrosion. The oxidation of the pressure tube degrades its mechanical strength resulting in the failure of the whole coolant channel. The length of central debris supported by the unheated bottom channel is long (equivalent to 3–4 fuel bundles) compared to end debris. The insight generated from this experimental study will further be used to validate the computational models for the channel deformation and its associated structural integrity.