Application of REPAS Methodology to Assess the Reliability of Passive Safety Systems

Abstract

The paper deals with the presentation of the Reliability Evaluation of Passive Safety System (REPAS) methodology developed by University of Pisa. The general objective of the REPAS is to characterize in an analytical way the performance of a passive system in order to increase the confidence toward its operation and to compare the performances of active and passive systems and the performances of different passive systems. The REPAS can be used in the design of the passive safety systems to assess their goodness and to optimize their costs. It may also provide numerical values that can be used in more complex safety assessment studies and it can be seen as a support to Probabilistic Safety Analysis studies. With regard to this, some examples in the application of the methodology are reported in the paper. A best-estimate thermal-hydraulic code, RELAP5, has been used to support the analyses and to model the selected systems. Probability distributions have been assigned to the uncertain input parameters through engineering judgment. Monte Carlo method has been used to propagate uncertainties and Wilks' formula has been taken into account to select sample size. Failure criterions are defined in terms of nonfulfillment of the defined design targets.

Failure identification in a nuclear passive safety system by Monte Carlo simulation with adaptive Kriging

“…However, the operational experience with PSS is lower than with active systems: thus, a more thorough analysis of the possibly wide range of PSS operating conditions and a detailed exploration of the critical combinations that may lead to PSS functional failures are mandatory. A structured procedure for the reliability assessment of T-H PSSs (namely, REPAS) has been developed in the past (Jafari et al, 2003;Pierro et al, 2009): however, it does not explicitly include an important step, i.e., the detailed characterization of the PSS Critical Regions (CRs), which is necessary to identify the combinations of critical operation of the system (i.e., those combinations leading the PSS to fail providing its function). The identification of the states that lead a PSS to safety-critical conditions provides relevant insights for improving system safety, since it allows prevention and preparation.…”

“…The DHR system successful response to the accident is measured in terms of its heat removal function and, specifically, in relation to the amount of heat removed during the accidental transient, lasting about 8 h. If the heat is not removed adequately, the temperature and pressure may dangerously rise inside the PV and if the pressure increases beyond the Safety Relief Valve (SRV) set-point, considered at 75.5 bar, the valve opens to discharge the vapor inside the containment building. Two output parameters (Y 1 , Y 2 ) are considered as Transient Performance Indicators (TPIs) (Pierro et al, 2009) to evaluate the PSS functional response:…”

“…For this, the REPAS (Reliability Evaluation of Passive Safety Systems) methodology (Jafari et al, 2003;Pierro et al, 2009) has been developed to provide a structured procedure for conducting a T-H reliability assessment of PSSs accounting for uncertainties. However, in its original formulation REPAS does not explicitly include the detailed characterization of the PSS Critical failure Regions (CRs).…”

Passive Safety Systems in Advanced PWRs

“…Experimental studies and validation of numerical codes are presented in the following two papers [2,3]. The paper of Gou et al [4] deals with numerical analysis of the performance of passive safety systems; whereas the methodology for the reliability assessment of passive safety systems is investigated in the paper of Pierro et al [5]. The last three papers are devoted to hydrogen safety issue, which is a main topic of the workshop [6][7][8].…”