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Preliminary Uncertainty Assessment of a Severe Accident Scenario Using the ASYST Code

The ASYST code, developed as part of an international nuclear technology ASYST Development and Training Program (ADTP) managed by Innovative Systems Software (ISS) is an advanced analysis software for nuclear safety applications. It uses best-estimate SAMPSON THA two-fluid, non-equilibrium models and correlations for thermal hydraulic calculation, SCDAPSIM models for calculation of severe accident progression in the reactor core, and SCDAPSIM COUPLE module for the finite element thermal analysis of the reactor vessel lower head filled with molten corium and a variety of other member-developed computational packages. One of these packages is an integrated uncertainty package developed jointly by the Technical University of Catalunya and ISS. The uncertainty analysis enables the user to perform and post-process multiple computer runs to estimate uncertainty bands for desired output parameters. The code generates automatically the file with sampled values of the input parameters, based on the probability density function, the appropriate percentile and confidence levels (typically using the default Wilks’ formula). The uncertainty analysis is performed for a postulated severe accident scenario of a station blackout at a PWR plant. There are large uncertainties present in the severe accident issues, related to, among others, hydrogen generation during reflood or melt relocation into water, core coolability, in-vessel heat transfer in a damaged core, in-core molten pool behaviour, corium relocation to the lower head, lower head corium behaviour, vessel failure and material release, etc. Some of these issues are taken into account when determining the appropriate SCDAPSIM parameters to be modified and their probability density functions. The objective of the analysis is to determine uncertainty limits of the most important parameters for assessing the core integrity, quantify deviation of results and perform sensitivity study for hydrogen release.

ASYST, severe accident, uncertainty analysis, SBO

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