Severe Accident Modelling and Analysis of VVER 1000 (V-412) for Kudankulam NPP, India

This paper highlights both analytical and experimental activities taken up to gain more insight into the complex physico-chemical and thermal-hydraulic behaviour during severe accident scenario of pressurized light water reactor (VVER-1000 V-412) in India. Integral system codes like RELAP5/SCDAP and ASTEC are being used to analyse in-vessel phase of severe accident scenario of Kudankulam Nuclear Power Project (KKNPP) for VVER1000 reactor. A preliminary in-house standalone code has also been developed to model the complex ex-vessel phenomena of corium relocation and corium-sacrificial material interaction inside core catcher. Both In-vessel and ex-vessel (core catcher) analysis considering a Postulated Initiating Event (PIE) like Large Break LOCA with Extended station Blackout (ESBO) has been considered with and without passive hydro-accumulator injection into Reactor Pressure Vessel (RPV). Modelling of top flooding specifically Counter Current Flow Limitation (CCFL) phenomena still remains an issue. The core catcher model also needs to address stratification of molten pool and mass diffusion of sacrificial material into molten pool. Experiments are planned with VVER scaled down bundle configuration for both intact and degraded core geometry for low temperature (up to 1000 deg. C) and high temperature (up to 2200 deg. C) with bottom and top reflood conditions with different flow rates. This will augment the fundamental understanding of complex thermal-hydraulic phenomena of combined top and bottom flooding and generate data for validation models for top flooding. The current status and evaluation of severe accident simulation codes has been described and areas for modelling improvements have been identified. On-going experimental programs to understand the complex reflood phenomena for bottom and top reflooding has also been presented. (author)