Hitoshi Muta et al., Department of Nuclear Safety Engineering, Faculty of Engineering, Tokyo City University, announced that they have developed a quantitative evaluation method for the accident rate in consideration of various dynamic changes over time in safety-related systems of nuclear facilities.

 Probabilistic risk assessment (PRA) is used for risks in nuclear facilities.However, this method mainly targets events caused by equipment failures and human error, and does not evaluate the effects from the outside (natural disasters, etc.) in detail.Therefore, it is assumed that the safety function is in a steady state with no change over time, and the probability of loss of function is calculated quantitatively using the average probability of loss.However, following the accident at the Fukushima Daiichi Nuclear Power Station, such preconditions have come to be considered inadequate.

 In this research, we model dynamic changes such as the occurrence of equipment failures and the progress of restoration work over time in the safety-related systems of nuclear facilities using stochastic processes.After that, we developed a method to quantitatively evaluate the probability of accident occurrence by finding the probability of loss of safety functions such as reactor shutdown, core cooling or damaged core cooling, and decay heat removal.

 In this method, the change in failure probability over time is taken into consideration when calculating the core damage frequency, and the probability of occurrence of each event is treated as a function of time.The state of the device is normal, failure, repair, and manual stop, and the simultaneous differential equations are solved to obtain the probability of each time and each state.Furthermore, based on this solution, the probability of core damage occurring over time is determined by combining devices that are in a state of loss of function.

 This method can also be applied to events that require consideration of the effects over time, such as the risks of earthquakes and tsunamis.It is expected that this will enable not only strict risk understanding but also risk judgment such as confirmation of effectiveness considering various variable factors for safety measures equipment.

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