Qualification of methodologies for reliability analysis of passive system against experimental data

Passive systems deserve a special attention within the nuclear technology owing to their potential to increase the safety level of the power plants and to reduce the cost for the energy production. The intensive use of passive systems in the new nuclear technology needs a robust assessment of their reliability. The passive safety systems for their nature, because their functioning depends only by natural physical law and not by an external source of supplied energy, are more reliable than the active ones. Nevertheless the passive systems may fail their mission as consequences of components failure, deviation of physical phenomena, boundary and initial conditions, etc. The reliability of a passive system has to be evaluated not only from the point of view of the mechanical components, by mean the classical risk assessment tools as FMEA (Failure Mode and Effects Analysis), FTA (Fault Tree Analysis), HAZOP (Hazard and Operability analysis), but even form the point of view of the thermal hydraulic phenomena in terms of deviation from the expected behavior due to alterations in the environmental conditions. For this purpose, three methodologies have been developed: REPAS (Reliability Evaluation of Passive Safety System), RMPS (Reliability Method for Passive System), APSRA (Assessment of Passive System Reliability). In this paper a first attempt in the qualification against experimental data of the three methodologies mentioned above is described. The basic idea in the qualification strategy is to select an experimental facility where the natural circulation can be induced and the main parameters can be varied and controlled in order to observe the transition unstable/stable in the natural circulation phenomenon. The facility selected is a simple square pipe vertical loop working at atmospheric pressure where the fluid (water) is heated in the lower horizontal pipe and cooled in the upper horizontal pipe. Once the power at the heat source is fixed, the temperature at the heat sink is varied stepwise (but constant in each transient) in order to have the transition unstable/stable in the natural circulation behavior. The methodologies applied to the experimental data should correctly simulate the system behavior and at the end the reliability of the system experimentally evaluated have to be compared with the one coming from the simulation. This activity has been developed in the framework of the IAEA CRP I31018: “Development of Methodologies for the Assessment of Passive Safety System Performance in Advanced Reactors” started in the 2009 and planned to be completed in the 2011. The objective of this CRP is to “increase the capability in Member States to achieve progress in the development of a common approach to reliability assessment of passive safety system performance”. On the base of the results obtained is it possible to state that a common strategy has been established and a first example of qualification has been provided. This represent just a preliminary attempt that has to be considered as starting point for future qualification activities based on “real” passive systems used in the nuclear power plant.