Addressing the uncertainties related to passive system reliability

Uncertainties involved in passive system operation play a crucial role in the relative reliability assessment endeavor. These uncertainties concern the deviations of the underlying physical principles from the expected behavior and are related to the onset of physical phenomena infringing the system performance or to the changes in the initial/boundary conditions. This issue is addressed by the development of subjective probability distributions relative to the physical modes of failure for a typical passive system for reactor decay heat removal, operating in natural circulation. Because of the limited availability of data, both experimental and operational, expert judgment and engineering assessment are used to characterize the present status of uncertainty with regard to a set of critical parameters driving the failure modes. This allows deriving a set of natural circulation failure probability distributions to quantitatively get information on system reliability.