Uncertainty and Sensitivity Analysis of Severe Accidents Simulations at VVER in the Framework of the IAEA CRP I31033

The IAEA launched in 2019 the Cooperative Research Project (CRP) I31033 entitled "Advancing the State-of-Practice in Uncertainty and Sensitivity Methodologies for Severe Accident Analysis in Water-Cooled Reactors". The main goal is to advance the understanding and characterization of sources of uncertainty and investigate their effects on the key figure of merits (FOMs) of the severe accident codes predictions in water cooled reactors (WCRs). In the framework of the CRP four institutions NNEGC "ENERGOATOM", OKB "Gidropress" JSC, IBRAE RAN, and NRC "KURCHATOV INSTITUTE" preformed uncertainty and sensitivity analysis of VVER rectors. Two severe accident codes are used MELCOR 1.8.5 and SOCRAT together with in-house developed uncertainty tools. Each participant has selected a specific severe accident to investigate and developed a list of input uncertain parameters. NNEGC "ENERGOATOM" simulates a station blackout (SBO) at the first unit of South-Ukrainian NPP. OKB GIDROPRESS investigates a SBO scenario with account for accident management actions (opening of 3 PORVs 2 h since the initiating event).. IBRAE RAN has opted for LB LOCA (ACC injection line) that happens simultaneously with unmitigated SBO. NRC "KURCHATOV INSTITUTE" analyse LB LOCA (rupture of the pressurizer surge line) with simultaneous SBO. One paid attention to the justification of PDFs of uncertain input parameters giving preference to literary sources or technical documentation rather than expert judgment. The U&S analysis is mainly focused on in-vessel stage of severe accident progression and key FOMs at VVER-1000 reactor. Determined FOMs include hydrogen production, times of key events and other reactor and containment parameters. By now these institutions have obtained preliminary results on U&S analysis of selected FOMs and uncertainty associated with mesh refinement. The paper presents the description of the applied methodologies for U&S analysis, base case calculation results, and U&S analysis results performed by the group.