Availability assessment of a nuclear reactor limitation system by a Timed Petri Net

Abstract

The constant evolution of instrumentation and control systems (I & C) of nuclear power plants requires a robust reliability assessment, due to the constant interaction between different systems that are demanded under normal operating, transient or accident conditions. Several methodologies have been developed to model the reliability of digital I & C systems and their dynamic behavior to better model the interaction between several equipment. In nuclear power plant design, many I & C systems work together according to priority scales and safety importance. In some designs, a Reactor Limitation System (RLS) has an important function to preserve equipment during transients and because of this, it is classified as safety related. After discussing pros and cons of the available tools for the analysis (including failure mode definition and quantitative data gathering), this paper evaluates the application of a timed Petri net to simulate RLS functions of an analog I & C design with timing on transitions. The reason for doing so is because it is possible in this model to represent the response time of components, maintenance time and periodic test behavior besides the interactions mentioned above.The results for three scenarios modeled by a timed Petri net indicate an RLS availability between 87.38% and 99.99% based on pressure transmitter, pressurizer spray valves and heater features (like component response time, failure characterization, maintenance and periodic test). The results of this work indicate that a timed Petri net can better evaluate the model of pressurizer pressure control by an RLS classified, actually, as an analog system that can be digitalized around the plant lifetime. Two importance risk metrics, namely, Risk Reduction Worth (RRW) and Risk Achievement Worth (RAW) were applied to identify the contributions for safety function failure of each type of RLS component.