Reliability assessment of pulse coded safety logic system for PFBR

Full text: Reliability of Instrumentation and Control (I and C) systems should be high for safe and economic operation of a nuclear reactor. Defense in depth, diversity, redundancy, safety are the various design criterion for designing I and C systems. The I and C systems are provided to assure safe shutdown of the reactor during any deviation from normal operation, removal of heat from the reactor core and containment of radioactivity during accident conditions. The Safety Critical systems require a very high reliability. Failure of a Safety Critical System to perform its function may result in the loss of life, damage to the equipment and severe environmental effects. Pulse Coded Safety Logic (PCSL) System is one of the safety critical systems of PFBR. It was designed as a fail-safe system to meet the reliability requirements. The electronic circuits are designed for long life. The components used are adequately de-rated. To meet the intended reliability, the system was qualified for environmental conditions. Further 'Failure Modes and Effects Analysis (FMEA)', which is an inductive analysis technique, was used to study the effects of component failure modes on a system. It is the first activity performed during reliability analysis of any system. It identifies the effects of various components on the functionality of the system. It starts from the knowledge of component failure modes and considers the effects of each failure on subsystems and the system. It involves the study of failure modes of all components in the system and is often applied also to higher level assemblies and systems. It checks whether proposed components, with their known failure modes, fulfill system-level safety requirements. This method proceeds from known cases of failure to unknown effects. With this analysis, the percentage of safe and unsafe failures of various components is identified. FMEA provides guidance for the evaluation of qualitative and quantitative reliability and availability of safety related nuclear systems. Total failures of the system can be divided into 4 main classes, viz, safe detected faults, safe undetected faults, unsafe detected faults and unsafe undetected faults. FMEA was performed on PCSL systems in accordance with MIL-1629A standard, which gives guidelines to systematically perform and document the effects of failure modes of components in a system. Next reliability of the system was calculated using the 'Relex' version 7.7 software based on MIL217F Notice 2 Standard and the calculation made on parts stress method. To find the failure rate of each component, all factors, such as failure rate, temperature factor, environment factor and quality factor etc have been taken care of. By using this software, the failure rate, danger failure rate, safe failure rate and the probability of failure on demand were calculated