Reliability Analysis of Digital Reactor Protection System

Abstract

The reliability analysis of the digital reactor protection system (RPS) is one of the essential parts in the probabilistic safety assessment (PSA) of the advanced boiling water reactor (ABWR). In this study, the reliability model and methodology were modified to evaluate the reliability of the digital RPS installed in the Japanese ABWR plant. The hardware failure rates in the foreign data source of digital components were applied, based on the similarity of the function of the digital components. The hardware failure rates of the digital components were estimated to range from 1.0E−5 (/hr) to 1.0E−7 (/hr), according to the types of the components. The software error events and their recovery factors in the design and fabrication stages were evaluated, considering the verification and validation process provided by the Japanese industry guideline on the digital reactor protection system. Then, the software failure probability of the programmable digital component was evaluated, utilizing the probability of software error events and their recovery factors. The software failure probability was estimated to be 3.3E−7 (/demand), which was about one order higher than that of our previous estimation. These models and results were applied to evaluate the reactor trip system (RTS) and the engineered safety feature (ESF) actuation system of the ABWR plant, both of which are the subsystems of the RPS. The unavailability of the digital RTS was estimated to be the mean value of 7.2E−06 (/demand). If both an alternate rod insertion (ARI) and a manual scram were considered, the unavailability was estimated to decrease to 1.6E−09. This value was nearly equal to the mean value of the previous study, 1.1E−09 (/demand), even though the quantification model and data were considerably modified, including the software failure probability. The system unavailability of the emergency core cooling system (ECCS) was also evaluated in conjunction with the ESF actuation system, in order to investigate the effect of the model and data modification. The ECCS unavailability was estimated to be also nearly equal to the same values as the previous estimation, because the system unavailability was dominated by the unavailability of the mechanical components, such as pumps, valves, etc. The sensitivity analyses were conducted systematically, in order to evaluate the effect of the modeling uncertainty on the digital RTS unavailability. The results indicated that the unavailability of the digital RTS only changed within the range of factor 2, even though the various assumptions were used on the hardware and the software failure of the digital components.

Dynamic Modeling of Reactor Protection System in Nuclear Power Plant for Reliability Evaluation Based on State Transition Diagram

“…The trip signal is received by Output Logic Unit (OLU) and then sent to the Load Drivers (LDs). The difference in ESFAS that voting between signals is carried out by the Safety Logic Units (SLUs) to create the trip signal [16]. Upon failure to accomplish the required function of the reactor hardware design, the digital RPS automatically generates the trip signal by executing the following "2oo4" voting rules.…”

Development of dynamic fault tree model for reactor protection system

“…The PSA analysis includes several steps first, a detailed understanding of system design and operation, then the construction of a fault tree (FT), finally the mathematical calculation of fault tree event probabilities to establish the likelihood of the top undesired event. Static FT models of RPS have been conducted by many researchers as in 2‐6 . A most important limitation of the conventional SFT analysis is the inability to model component interactions such as sequence‐dependent failures or configuration changes in time.…”

“…Static FT models of RPS have been conducted by many researchers as in. [2][3][4][5][6] A most important limitation of the conventional SFT analysis is the inability to model component interactions such as sequence-dependent failures or configuration changes in time. The DFT introduces a powerful modeling method that considers these interactions thus it has gained the attention of many reliability engineers working on safety-critical systems.…”