Significance of the input parameters selection and the nodalization qualification in the final results of an IBLOCA BEPU calculation

Abstract

In the framework of Design Basis Accidents (DBA) for Pressurized Water Reactor (PWR), and based on recent studies on pipe integrity combined with the Risk-Informed Decision Making (RIDM), the USNRC proposed the Intermediate Break LOCA (IBLOCA) scenario to become a design basis accident. These studies reported that the probability of a complete rupture of a pipe depends on the pipe size, and in particular that this probability is higher for smaller tubes. Therefore, the double guillotine break of smaller pipes connected to the primary side of a PWR such as the surge line or the ECCS lines should pose a more probable scenario than an integral rupture of the cold or hot legs. In order to assess the effectiveness of the safety strategies and the Emergency Operational Procedures (EOP) for a selected Nuclear Power Plant (NPP) and scenario, Best Estimates (BE) codes like RELAP5 are of great value because they allow simulating the overall behavior and response of the system under accident conditions. Furthermore, for licensing purposes, BE simulations should be combined with uncertainty analyses to yield the so-called Best Estimate Plus Uncertainty (BEPU) calculations. In the present paper a preliminary assessment of an IBLOCA BEPU calculation is carried out for the Asc ó -2 NPP (3-loop PWR Westinghouse design). The transient follows its reported EOPs with Emergency Core Cooling Systems (ECCS) failure assumptions for core uncover conditions. A preliminary evaluation of the effectiveness in the selection of the input uncertainty parameters is presented. Those that were concluded as relevant for LBLOCA and SBLOCA in previous works are analyzed to determine which are the most influential to be considered in IBLOCA. In addition, the influence of the nodalization qualification is also studied. Two different nodalizations are compared for assessing the significance of the modelling approach in BEPU analyses: the first one, a 1D vessel nodalization that was qualified with operational events reported in the actual NPP; and the second one, the same nodalization with a Pseudo 3D modelling of the vessel (core parallel channels with transversal flow paths and DC parallel channels with azimuthal connections). This second nodalization was qualified for SBLOCA accidents with experiments performed at Integral Test Facilities (G7.1 of PKL and Test 3 of LSTF ) b y the use scaling techniques (SCUP methodology).