ATWS SCOPING CALCULATIONS FOR A BWR PLANT PERFORMED USING RELAP5/MOD2 AND CONTAIN CODES

In the context of the licensing process for Alto Lazio nuclear power plant (a BWR/6, MARK III design) some calculatio have been performed for evaluating the plant response during an ATWS transient. Specifically, the ATWS related safety systems were simulated in order to evaluate their influence on the plant behaviour. Furthermore, the containment behavior and the effectiveness of hypothetical containment venting have been evaluated. Two best-estimate codes have been used for this study: • - RELAP5/MOD2 for the Reactor Cooling System (RCS) behaviour calculation; • - CONTAIN 1.1 for the containment response calculation. The initiating event chosen was a Main Steam Isolation Valve (MSIV) closure because the associated ATWS was found, by an Alto Lazio specific Probabilistic Risk Assessment study, to have the highest core melt frequency. In order to assess the corresponding effect, three different Stand-by Liquid Control System (SLCS) actuation modes have been considered: 1. (a) NOSLCS case: the SLCS is assumed to be unavailable; 2. (b) SLCS2 case: the SLCS starts two minutes after the initiating event; 3. (c) SLCS10 case: the SLCS is assumed to start 10 min after the initiating event. The paper contains the calculation results and the related analysis of the above-mentioned cases. © 1990.