PSI - Issue 60

Keshav Mohta et al. / Procedia Structural Integrity 60 (2024) 36–43 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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in suspended debris weight, the lower channels become more prone to give up. Failure of a number of channels in the cascading manner would lead to core collapse, and formation of a terminal debris bed at the bottom of the Calandria (IAEA, 2008), as illustrated in Fig.1. Such an accident scenario would come under the rarest of the rare events. Severe accident analysis and determination of the in-Calandria corium retention time for a PHWR core-collapse scenario requires the postulation of the accident scenario wherein the structural integrity of Calandria is threatened. One such postulated accident scenario is unmitigated, total loss of heat sink which comprises of the large Loss of Coolant Accident (LOCA) followed by a prolonged Station Black Out (SBO), i.e. loss of moderator cooling, end shield water cooling and Calandria vault water cooling, leading to the complete collapse of the reactor core on to the Calandria bottom. This scenario has been considered for present study.

(a)

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Fig. 1. (a) Reactor core block under normal operating conditions, (b) formation of terminal debris bed after the core collapse under accident conditions in a typical PHWR

1.2. In-vessel retention of core debris by Calandria The retention of core debris would limit the physical spread of the accident. It would prevent it from falling on to the concrete vault floor, thus preventing the molten corium- concrete interaction. It would also facilitate the cooling of debris bed through the Calandria vault water, and delay the accident progression, providing more time to operators for the SAMG actions. Thus, the in-vessel core debris retention by Calandria would become a crucial aspect for the accident management for a core collapse scenario. However, it would be subjected to the structural integrity of Calandria assembly under the large thermal and mechanical loads arising during the accident progression that are not considered during the design. These loads may cause large damage to Calandria leading to its structural failure and ejection of core debris/ molten corium. Thus, the estimation of the in-Calandria corium retention time would be useful for formulation/ validation of the SAMGs. 2. Comparison with Light Water Reactors (LWRs) and need for analysis for PHWR A number of detailed studies such as Rempe et al. (1993), Koundy and Hoang (2008), Bal Raj Sehgal (2012), Mao et al. (2017), Yamaguchi et al. (2017) etc. have been conducted on the accident scenarios involving core collapse in Light Water Reactors (LWRs). However, owing to the differences in geometry, material and loading conditions, and design features such as large water inventory etc., the insights gained from these studies are not directly applicable for PHWRs. The severe accident analysis for PHWRs requires considerations of relevant PHWR specific loads, geometry and features. In this regard, several experimental and analytical studies have been conducted in the recent past by Kulkarni et al. (2013), Singh et al. (2017), Prasad and Nayak (2017) etc. However, these studies provide limited insight owing to inadequate consideration of many factors affecting the Calandria response such as realistic boundary conditions, flexibility provided by annular plate and diaphragms to accommodate the thermal deformation, high temperature tensile and creep deformation behavior etc, necessitating the detailed assessment.