Ability of Current Advanced Codes to Predict In-Vessel Core Melt Progression and Degraded Core Coolability: Benchmark Exercise on the Three Mile Island-2 Plant – Final Report

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Based on the conclusions of the previous benchmark exercise, the NEA Working Group on Analysis and Management of Accidents (WGAMA) felt it worthwhile to extend the accident analysis scope by examining the capability of the codes to predict core melt progression and the effects of severe accident management (SAM) actions under a variety of severe accident situations in order to challenge them to the full extent of their capabilities, recognizing, however, that they are less reliable in late phase core melt progression. As the current research in the Severe Accident Research Network of Excellence 2 (SARNET2) WP5 is focused on late phase phenomena and debris coolability, the WGAMA and the SARNET2 WP5 jointly proposed a benchmark as a follow-up to the benchmark exercise (ATMI) which includes late phase core degradation during different severe accident sequences. This proposal was approved by the NEA Committee on the Safety of Nuclear Installations (CSNI) in December 2010.

Starting from the previous benchmark exercise on an alternative Three Mile Island unit 2 (TMI-2) accident scenario, the present benchmark is aimed at examining different severe accident sequences involving safety system operation failure and various SAM measures, e.g. depressurisation, delayed start of high pressure injection (HPI), loss of auxiliary feed water (AFW), etc. The impact on hydrogen production, core coolability, corium relocation into the lower plenum and vessel failure will be examined.