Dynamic Risk Evaluation of Systems with Multiple Protective Systems
Volume 3, Number 4, October 2007 - Paper 6 - pp. 453 - 466
TAKEHISA KHODA1 and MASAKI NAKAGAWA21 Dept. of Aeronautics and Astronautics Graduate School of Engineering, Kyoto University Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
2 Mitsubishi Chemical Group, Science & Technology Research Center, INC. 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
(Received on March 23, 2006)
To prevent a system accident, several types of protective systems are installed based on the concept of "defence in depth" in such a system as nuclear and chemical plants. In the risk evaluation of a system with multiple independent protective systems, the accident occurrence probability is obtained as the occurrence probability of an abnormal event multiplied by failure probabilities of its related protective systems. Since failure probabilities are conventionally evaluated as its time average unavailability over the operating period independently, their variation during the operation cannot be considered well. This paper proposes a dynamic evaluation method of the accident probability with the consideration of inspections and maintenance. Using the decomposition of a protective system into detection, diagnosis, and execution parts, the on-demand failure can be easily analyzed even for protective systems composed of both hardware and operators. An illustrative example of a simple reactor system with several protective systems including operator recovery actions shows the details and merits of the proposed method.
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