Reliability and availability evaluation of subsea high integrity pressure protection system using stochastic Petri net

In this paper, a safety evaluation method of subsea High Integrity Pressure Protection System (HIPPS) based on a generalized stochastic Petri net model is proposed. Different test methods were used to detect different types of failures and to analyze the reliability of HIPPS components under the influence of common cause failures and incomplete repair. The reliability curve of a diagnostic system consisting of a transmitter system and a logic system under the influence of uncertainty over time is analyzed. The safety of HIPPS with diverse test methods were quantitatively analyzed. The results show a significant improvement in the performance of the system after testing and maintenance. Both partial-stroke testing and increased partial-stroke test coverage can be used to increase the HIPPS performance compared to traditional methods. The analysis of the Partial stroke test (PST) strategy can afford a academic basis for the selection of PST frequency and Functional test (FT) interval in practical engineering.

Optimal allocation of reliability improvement target under dependent component failures

We study the problem of apportioning the reliability improvement target of a series system to components by considering the failure risk and improvement cost when both common cause and cascading failures exist. To solve this problem, previous research has developed different allocation weights in which every component is improved independently in proportion to the allocation weight. In practice, however, allocation weights are not independent among components because several components improve simultaneously when the occurrences of common cause failures are reduced. Therefore, in this study, we partition dependent component failures into mutually exclusive sub-failures to express the system risk in terms of the risk of component sub-failures, where the common cause failures, and cascading failures are incorporated into occurrence and severity evaluations, respectively. Then, an optimization problem is considered to maximize the effectiveness of the system improvement, which is measured as the difference between the decreased failure risk and the increased improvement cost. Finally, a numerical example is presented to illustrate that a component selected for improvement at a low budget would not necessarily be selected at a high budget if different marginal improvement costs were associated with different component failures. In other words, components are selected for improvement only if the reduction in the risk is sufficiently large to offset the improvement cost.

Reliability Analysis of C4ISR Systems Based on Goal-Oriented Methodology

Hard-and-software integrated systems such as command and control systems (C4ISR systems) are typical systems that are comprised of both software and hardware, the failures of such devices result from complicated common cause failures and common (or shared) signals that make classical reliability analysis methods will be not applicable. To this end, this paper applies the Goal-Oriented (GO) methodology to detailed analyze the reliability of a C4ISR system. The reliability as well as the failure probability of the C4ISR system, are reached based on the GO model constructed. At the component level, the reliability of units of the C4ISR system is computed. Importance analysis of failures of such a system is completed by the qualitative analysis capability of the GO model, by which critical failures of hardware failures like communication module failures and motherboard module failures as well as software failures like network module application software failures and decompression module software failures are ascertained. This method of this paper contributes to the reliability analysis of all hard-and-software integrated systems.

Sensitivity Study on the Correlation Level of Seismic Failures in Seismic Probabilistic Safety Assessments

It is popular that correlated seismic failures spread over the fault tree of a seismic probabilistic safety assessment (PSA) for a nuclear power plant (NPP). To avoid the calculational difficulty of core damage frequency (CDF), the fault tree has been simplified by replacing correlated seismic failures with one typical seismic failure by assuming a full correlation among the correlated seismic failures. Then, the approximate seismic CDF of a seismic single-unit PSA (SUPSA) has been calculated for decades with this simplified SUPSA fault tree. Furthermore, current seismic multi-unit PSAs (MUPSAs) have been performed with imperfect seismic MUPSA models that were generated by combining such imperfect seismic SUPSA fault trees. The authors of this study recently developed a method that can calculate an accurate seismic CDF by converting correlated seismic failures into seismic common cause failures (CCFs). In this study, accurate and imperfect MUPSA models were created and their seismic CDFs were compared. The results of this study show that the seismic CDFs in SUPSA and MUPSA are drastically distorted and safety margins are accordingly distorted when the full correlation assumption is employed. Thus, this study shows that very careful attention should be paid to calculating and interpreting seismic CDFs for the single-unit and multi-unit NPP regulations.

Safety Method for the Pre-conceptual Phase of Sodium-cooled Fast Reactors

The ESFR-SMART European project (Contract number: 754501) focuses on the development of innovative safety design options for future Sodium-cooled Fast Reactors (SFRs). The goal of this paper is to show how the Objective Provision Tree (OPT) method, which is part of the Integrated Safety Analysis Methodology (ISAM) developed by Generation IV Forum (GIF), can help in defining safety guidelines for the definition and the study of ESFR-SMART innovative design options. The OPT method provides a view of the implementation of main safety functions in accordance with defence-in-depth principle and has been adapted to ESFR-SMART needs. Guidance for a consistent and homogeneous application has been developed. The method is applied to each safety function with a specific adaptation for the confinement function. Beyond the allocation of the main equipment ensuring the safety functions to the different levels of defence-in-depth, the adapted method proposes a systematic identification of the mechanisms likely to degrade them. Then, safety features to cope with these mechanisms are investigated as well as the potential common cause failures of main equipment belonging to different levels of defence-in-depth. This adapted method allows to study and then to improve the independence between the levels of defence-in-depth as required for future reactors in Europe by Western European Nuclear Regulators' Association (WENRA). The paper provides the outline of guidance for OPT method adaptation to ESFR-SMART project needs, examples of application and main outcomes of the use of the method.