A new method has been presented in this study, for evaluating the functional reliability of passive systems based on the fuzzy method. A case study has been presented, in order to analyze the passive residual heat removal system (PRHRS) of AP1000. The RELAP5 is used to analyze the thermal-hydraulic behavior. However, the failure criterion may not be suitable, since the local saturated boiling temperature is not only related to the pressure, but also related to the local surface conditions. In addition, it is not reasonable to categorize the failure, based on a single numerical value. There are no essential differences among these values which are near the critical numerical value. Hence, it is different to use the classical theory for calculating the failure probability. However, the fuzzy method has been founded to be appropriate for such a case. The failure criteria parameter is regarded as a fuzzy number, which can be described by a membership function. Finally, the failure probability of PRHRS is obtained as 7.26×10−8 which is well within the range of classical failure probability.

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