In NPP, the digital control system which integrated software and hardware are increasingly used to improve dependability and introduce new functionality. Traditional safety analysis can get a good result when handling accidents caused by component failures, but software does not fail in this way. STPA is a new hazard analysis technique based on systems theory rather than reliability theory. It considers the system as a whole (include the hardware and software) to analyze failure and causality of systems and treats safety as a control problem rather than a failure problem. Being a safety-critical system, RPS in NPP needs to be considered carefully in system safety. So, we adopt this new approach to analyze the design process. From the analysis results, we found that causal factors leading to safety accidents identified by STPA included all the hazards identified by the fault tree analysis. Furthermore, there are some causal factors that were identified by STPA only. We utilize these results of the analysis on causation factor to refine the safety requirements and reduce the occurrences of the hazardous scenarios.
