Event Sequence Diagram Method And Its Application

Since the publication of WASH-1400 Report, Probabilistic Risk Assessment (PRA) has become an important method of risk assessment and management of complex engineering systems. As supplement to PRA, lots of theories and methods of dynamic risk assesment have been raised and widely studied. These theories and methods specially focus on a human-machine-software system and its interactions with the environment. They emphasize that safety risk is the reflection of comprehensive nteractions of system factors, and it changes as these factors change. It has great advantages of using the dynamic PRA methods to evaluate large-scale complex systems, especially the human-machine-software systems over the traditional ETA/FTA method. But these methods have not been widely used in pratical engineering domains, because they still have the problems of being highly theoretical, highly complicated in technics and hard to master. Therefore, in order to do the PRA of the complex systems more effectively, we need to study some methods more practical in engineering domains. The Event Sequence Diagram (BSD) studied in the paper is such a method that has a broad future in engineering applications.The ESD is a visualized method to describe sequences of related events and can be seen as a general Event-Tree (ET). The ESD has the features of directness, trueness, easiness for mastering, being able to be qualitatively and quantitatively processed, and etc. The study and application of the ESD has great value in pushing dynamic PRA forward in engineering domains. While in pratical applications, the ESD method still has drawbacks in graphical modeling capability, connections with products' structures, size control of ESD models, and etc. Through in-depth study of the ESD method, this paper systematically describes the framework and its basic modeling elements of ESD, explains the principles of risk modeling, gives the mathematical model of the basic modeling elements of the ESD framework to support the quantitative analysis of the ESD model. Based on the research above, this paper expands the ESD framework, which includes: To overcome the drawback in graphic modeling capability of the ESD method, the paper invents some new logical modeling symbols like "N/K" gate, sequence gate, expandable gate and constraint, those enrich the modeling capability of the ESD method. To overcome the drawbacks in connections with products' structures and size control, the paper explains the multi-layered modeling principles based on the ESD. In order to mix the ESD with other methods, this paper explains ESD/FT and ESD/M methods, which increase the flexibility of the ESD modeling. Focusing on solving the dynamic PRA problem, this article invents analytical /semi-analytical quantification of ESDs.Finally, an accidental transient of fast reactor is studied. The accident scenario is represented by means of ESD. Once the ESD is drawn, the scenario definition is to guide a Monte Carlo simulation to solve the problem. This application improves the efficiency of the ESD method in doing dynamic PRA for large dynamic systems.