Operational Vulnerability Identification Procedure for Nuclear Facilities Using STAMP/STPA

The nuclear facilities are operated to give safety the utmost priority and all possible scenarios that may lead to hazardous states must be evaluated. To date, the probabilistic safety assessment has been used as one of the standard tools for the safety evaluation; however, concerns have been raised about its capability to treat the complex interaction between human operators, digital systems, and diverse plant processes. This paper proposes an operational vulnerability identification procedure based on STAMP/STPA (System Theoretic Accident Model and Process/Systems-Theoretic Process Analysis) which allows us to derive unsafe control action (UCA) leading to the unwanted consequence of a system, such as a spurious reactor trip. The effectiveness of the proposed procedure is demonstrated with the case study of a cold neutron source system installed in High-Flux Advanced Neutron Application Reactor (HANARO). In result, 127 UCAs were derived for 51 control actions regarding spurious trip scenario. The UCAs were reviewed by the HANARO operators and found new scenarios that requires further investigation for reducing the possibility of a spurious trip. The proposed procedure is expected to provide a holistic viewpoint for operational vulnerability identification and further used to suggest recommendations for the safety enhancement of nuclear facilities.