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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Keywords: Calandria failure; core collapse; in-vessel retention; plastic instability; creep rupture

1. Introduction In a Nuclear Power Plant (NPP), a severe accident is a very low probability event resulting from the failure of multiple safety systems. If not contained and managed timely, it may lead to large scale degradation of the reactor core. Further deterioration of the conditions may lead to the failure of the barriers that are provided to prevent the release of radioactivity to the environment. In view of this, the Emergency Operating Procedures (EOPs) are defined to limit the progression of the accident and prevent any core damage under the design basis plant events and accidents. However, for the cases wherein the EOPs are not deemed adequate, Severe Accident Management Guidelines (SAMGs) are called for. These guidelines suggest the set of actions to be taken based on the plant state, to limit the core damage and maintain the containment integrity. Post Fukushima nuclear accident, there has been a greater emphasis on the preparedness towards severe accident management and robustness of the accident management guidelines (IAEA, 2008; IAEA, 2009).

Nomenclature ASTEC Accident Source Term Evaluation Code CSTS Calandria Side Tube Sheet EOP Emergency Operating Procedure FSTS Fuelling machine Side Tube Sheet LMP Larson- Miller Parameter LCDA Limited Core Damage Accident LOCA Loss of Coolant Accident LWR Light Water Reactor NPP Nuclear Power Plant PHWR Pressurized Heavy Water Reactor SAMG Severe Accident Management Guideline SBO Station Black Out SCDA Severe Core Damage Accident

The ‘ Defense-in-Depth ’ principle followed during the design of NPPs takes into account the events ranging from the day to day normal occurrences, operational transients (frequency ≥ 1 per reactor year) to rare events involving multiple failure leading to faulted conditions (frequency between 10 -4 to 10 -6 per reactor year). Accordingly, a number of design features and systems are provided to ensure the safe operation, and shutdown of the reactor. The limited core damage accident (LCDA) events, which lead to single channel/ multi-channel degradation but no disruption to the core geometry, are precursor to the severe core damage accidents (SCDAs). The LCDAs are generally considered under the design basis. When the accident progresses to the stage where the core geometry is lost, it is categorized as SCDA. The accident conditions, with still higher severity that are not considered for design basis, are generally termed as Design Extension Conditions (DECs) and are analyzed with best estimate criteria. These generally comprise of an initiating event and subsequent failure/ unavailability of one or more safety systems and accident management action. To study these events with very low frequency of occurrence, it is required to postulate the accident scenario. 1.1. Severe core damage accident and core collapse in PHWRs In PHWRs, the disruption of the core geometry would occur when the moderator inventory is partially or completed lost, leading to loss of the external cooling to the channels. The heated channels would deform and undergo disintegration/ disassembly, forming a suspended debris bed while resting on the lower channels. With further increase