Dynamic reliability : towards efficient simulation of the availability of a feedwater control system

In the reliability modeling of complex control systems, classical methodologies such as eventrees/fault-trees or Petri nets may not represent adequately the dynamic interactions existing between the physical processes (modeled by continuous variables) and the functional and dysfunctional behavior of its components (modeled by discrete variables). This paper proposes a framework for modeling and simulation of a water level control system in the Steam Generator (SG) of the secondary circuit of a nuclear power plant. We have developed a complete benchmark case. The behavioral model of SG is obtained from a linearized model published in 2000 by EDF [1,2]. Four physical variables (steam flow rate, water flow rate, steam-water level, water level) are modeled; they follow a system of linear differential equations with piecewise constant coefficients, coupled with a PID controller that regulates the water level in the SG. Detailed description of the components, failure modes and control laws of the principal components is presented. For modeling the system, we use the Piecewise Deterministic Markov Processes (PDMP) framework and for implementation we chose Simulink associated with Stateflow. PDMP offer a very general modeling framework to deal with dynamic reliability problems; Simulink is an appropriate tool to simulate non linear differential equations and their controller, while Stateflow implementation is appropriate for finite state machine descriptions of different components.