Passive systems are playing prominent role in the design and development of innovative reactor systems because of generally perceived enhanced safety and reliability on account of reduced human intervention and ample grace period for the operator in case of accidental conditions. These systems are considered to be more reliable than the active systems, due to their dependence solely on the natural phenomena based on simple physical laws. However, assessing their reliability in a transparent manner is an unresolved issue as the natural phenomena based on simple physical laws too undergo the degradation and may not be able to fulfil the desired function for the mission time in a satisfactory manner. Currently existing methodologies for the assessment of passive system reliability suffer the lack of universal acceptability due to unrealistic assumptions to account for uncertainty and over-dependence on the expert elicitation. This paper provides a general perspective on the evolution of state-of-art methodologies and examines the critical issues pertaining to the evaluation of passive system reliability which need to be considered to resolve the ambiguities surrounding the issue of passive system reliability assessment.
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