Publications Details

EXPLORING VITAL AREA IDENTIFICATION USING SYSTEMS-THEORETIC PROCESS ANALYSIS

Sandt, Emily S.; Clark, Andrew; Williams, Adam D.; Cohn, Brian C.; Osborn, Douglas M.; Aldemir, Tunc

Vital Area Identification (VAI) is an important element in securing nuclear facilities, including the range of recently proposed advanced reactors (AR). As ARs continue to develop and progress to licensure status, it will be necessary to ensure that safety analysis methods are compatible with the new reactor designs. These reactors tout inherently passive safety systems that drastically reduce the number of active components whose failures need to be considered as basic events in a Level 1 probabilistic risk assessment (PRA). Instead, ARs rely on natural processes for their safety, which may be difficult to capture through the use of fault trees (FTs) and subsequently difficult to determine the effects of lost equipment when completing a traditional VAI analysis. Traditional VAI methodology incorporates FTs from Level 1 PRA as a substantial portion of the effort to identify candidate vital area sets. The outcome of VAI is a selected set of areas deemed vital which must be protected in order to prevent radiological sabotage. An alternative methodology is proposed to inform the VAI process and selection of vital areas: Systems-Theoretic Process Analysis (STPA). STPA is a systems-based, top-down approach which analyzes a system as a hierarchical control structure composed of components (both those that are controlled and their controllers) and controlled actions taken by/acted upon those components. The control structure is then analyzed based on several situational parameters, including a time component, to produce a list of scenarios which may lead to system losses. A case study is presented to demonstrate how STPA can be used to inform VAI for ARs.