Passive systems deserve a special attention within the nuclear technology owing to their potential to reduce the cost for the production of electricity and to enhance the safety level of power plants. Drawbacks for their use come from the larger difficulties in the thermal-hydraulic design (compared with active design) and from reliability considerations specifically connected with the system operation. The research, carried out in ENEA in collaboration of University of Pisa and Polytechnic of Milano since 1999, is finalised to the development and validation of a methodology allowing the evaluation of the passive system reliability, which operation is based on moving working fluids ( type B cf. IAEA). This document describes this methodology and reports the results of an exercise performed on a system, which operation is based on Natural Circulation. The selected system is a loop including a heat source and a heat sink where the condensation occurs. It includes also all the main features of the Isolation Condenser, part of the SBWR design. Selected ranges of operating conditions (pressure, fluid velocities, heat transfer coefficients) include values expected for the same system.
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