Layer of Protection Analysis (LOPA) is extensively used for safety analysis of non-nuclear facilities e.g., chemical processes. It is simpler and less expensive methodology comparing full probabilistic risk assessment methods. It evaluates the probability of failure per demand for the safety system failures and the resulting consequences. LOPA is widely used in chemical process safety analysis however it is introduced as a practical technique for risk assessing in many other industries. This research examines utilization of LOPA method for nuclear systems as a quick evaluation of the safety level. The changes are proposed to classic LOPA method by using event tree method for event scenario identification and development. The so-called modified layer of protection analysis (Modified-LOPA) methodology utilizes Bayesian updating for more precise quantification of the failure probabilities and updating data. A simple example of a fire protection system shows application of this method.

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