Preliminary Investigation On Reliability Assessment Of Passive Safety System

With the development of Nuclear power technology and application, nuclear safety becomes more and more essential. The introduction of passive system is an important means to improve the NPP safety performance. Among nuclear safety studies, probabilistic safety assessment (PSA) has been widely used. However, reliability evaluation of passive safety system still remains in its primary stage.For passive safety system failure, both equipment and physical process failure are needed to be considered. Physical process failure probability calculation of passive system is quite different with that of active system. Since the operation reliability of passive system are closely related with accident scenarios, it is necessary to make the reliability analysis with respect to sequence of events. According to the object of reliability assessment of passive safety system, procedures of quantifying the reliability estimation include identifying relevant parameters which significantly affect fulfillment of system function, establishing corresponding functional failure criteria, and then assessing system reliability with accident scenario.Applying passive residual heat removal system is an important feature of AP1000 passive safety system. A practical example of reliability assessment is delivered in the thesis for the case of AP1000 passive heat removal system under loss of normal feedwater accident.Applying the best estimate code platform RELAP5/MOD3, modeling and simulation analysis on AP1000 loss of normal feedwater accident scenario, as well as observation of critical parameter variation, is carried out. Key operational parameters are selected via Analytic Hierarchy Process (AHP). The parameter ranges and probability distributions are identified according to design stage. Functional reliability criteria are defined by the function of PRHR system. Within the engineered possible range, sampling is carried out through Latin Hypercube Sampling (LHS) method with respect of the selected parameters. The sampled data are used as input combination of RELAP5 for uncertainty propagation calculation.Furthermore, calculations, sensitivity and reliability estimation on PRHR system function and behavior were presented, which might provide AP1000 PSA with PRHR system reliability.