Passive Systems Reliability: Application of MELCOR Simulations for PCCS Case Study

Increasing use of passive safety systems also increases the need for advanced methods of analyzing their reliability. Passive components work in a variety of ways. IAEA classifies these components and systems based on the level of passivity they exhibit. Reliability analysis of passive components relying mostly on physical phenomena require a combination of physics modeling and probabilistic methods. Challenges are to identify the mechanisms that can degrade or otherwise affect the systems stated safety function and the sources of uncertainty. Sources of uncertainty can be the geometric properties and layout of the system, material properties, design parameters and phenomenological analysis. A pilot case study of simulating a passive containment cooling system (PCCS) using MELCOR is applied within a simplified passive systems reliability framework. The aim of the framework is to explicitly state the interactions and information flow between deterministic physics modeling and probabilistic risk assessments, and the uncertainties related to them. The framework includes identification of main hazards, main sources of uncertainty both in the input data and the models, propagation of the uncertainties in physical models and finally the inclusion of the reliability assessment into the probabilistic risk analysis (PRA) model. The framework is partly based on a case study on a passive containment cooling system performed for this project. This paper includes the main results and lessons learned from the case study.