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.

中文翻译：

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PHWR通道高温热机械变形实验研究

摘要 PHWR型反应堆的严重事故由于具有广泛的安全特征，属于概率较小的事件，但如果不减轻这些事故的后果，则后果很严重。由于热负荷、内部压力和其组件的重量，通道变形会导致严重的事故。通道的下垂行为及其失效将决定通道内低压情况下的事故进程。下垂变形决定了早期碎屑或悬浮碎屑形成的性质。此外，通道的下垂会导致 Calandria 端罩上的拉出力。因此，必须了解事件的性质以提供任何缓解对策。本文旨在研究标准 220 MWe 印度 PHWR 冷却剂通道下垂的问题，假设严重事故假设为大断裂 LOCA 和 ECCS 损失作为起始事件。调节剂冷却系统被假定为在没有补给系统的情况下受损。在实验室中很难进行全面的通道下垂实验。因此，一种新颖的按比例缩小方法 (1:3) 已被用于通道下垂研究的实验室实验。实验表明，突然的功率上升会导致显着的初始下垂。当 CT 温度达到 460 °C 时，通道的下垂很明显。在瞬态加热过程中拉力不占主导地位，但在通道冷却结束时观察到显着的拉力或端部负载。