STPA-Based Safety Approach on the Emergency Ventilation System in Nuclear Power Plant

Instrumentation and control (I&C) systems have modernized replacing the hardwired hardware with digital elements. In the past, numerous hazard analysis techniques have been applied to analyze the safety of DI&C systems. However, underlying traditional methods normally do not consider a large extent of the unsafe interactions among system components, human mistakes, and software requirement deficiency. Systems theoretic process analysis (STPA) is a new hazard analysis technique that provides a potential solution to describe how unintended outcomes can occur due to inadequate implementation of constraints on the design, development, and operation of systems. In this paper, we have discussed the STPA-based safety approach to evaluate the safety of the emergency ventilation systems (EVS) in NPPs. We have considered the control structure and process model to identify the unsafe control actions (UCAs), including different controllers (human operator/reactor protection system), types of controls (manual/automatic), and various controlled processes. This approach is implemented on a conceptual EVS inspired by the Halden safety fan (HSF) design. The STPA based safety approach helps to identify safety constraints for the EVS that need to be enforced and ensure that they are adequately enforced in the EVS operation. Moreover, it identifies the process model that the controller needs to provide adequate control and the information required.

Keywords: STPA, Safety, EVS, Human operator.

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