Review of Various Dynamic Modeling Methods and Development of an Intuitive Modeling Method for Dynamic Systems

When a new system is developed, an evaluation of the system reliability should be carried out in order to determine whether the system is acceptable for an actual spot. Reliability of systems in a nuclear power plant is also analyzed to compare with regulatory criteria designed to ensure the availability of high-quality qualifications. There are several reliability analysis methods: the fault tree and event tree method, reliability graphs, reliability block diagrams, Markov chains, and Monte Carlo simulations. Each method has its own unique features and those features should be considered when determining the most suitable method. As digital systems are introduced to nuclear power plants, issues related with reliability analyses of these digital systems are being generated. The conventional static modeling methods, such as the event tree and fault tree methods, present significant shortcomings when used in the reliability modeling of digital instrumentation and control (I&C) systems in nuclear power plants, as they cannot properly account for dynamic interactions between the digital systems and components [1]. To overcome the limitations of the conventional static modeling methods, dynamic modeling methods should be adopted for the reliability analyses of digital systems in nuclear power plants. A dynamic fault tree method based on the introduction of additional dynamic gates to a static fault tree was proposed [2] and has been widely used. However, since it is not a perfect method, other modeling methods for dynamic systems have been proposed based on improvements to existing conventional methods, such as Bayesian networks, Petri nets, reliability block diagrams, and Monte Carlo Simulation. Nevertheless it has been concluded that no single available methodology satisfies all the requirements [1]. In this paper, several dynamic modeling methods are reviewed and a new modeling method that has intuitive modeling power is introduced.