Analyzing Defense-in-Depth Properties of Nuclear Power Plant Instrumentation and Control System Architectures Using Ontologies

The overall instrumentation and control (I&C) architecture of a nuclear power plant (NPP) is comprised of several I&C systems and their dependencies. The architecture needs to fulfil the principle of defense in depth (DiD). Defense-in-depth is the principal method for preventing accidents and mitigating the potential consequences of accidents. The levels of DiD should be independent of each other. The primary means to achieve independence are diversity, physical separation, and functional isolation. Approaches with extensive tool support for ensuring that the design solutions of nuclear overall I&C architectures realize relevant DiD properties are scarce. An ontology of the semantic web is a specification of a representational vocabulary for a shared domain of discourse, containing definitions of classes, individuals, and their relationships. An ontology-based knowledge base, built on named graphs, enables a computer to combine pieces of information into valuable knowledge based on queries. In this paper, we present an ontology-based approach for assessing that an NPP I&C architecture fulfils different DiD properties. In our approach, we aim at checking requirements related to physical separation, electrical isolation, communication independence, diversity, safety classification, and failure tolerance. We also discuss the developed work process and tool chain for ontology-based analysis. We demonstrate the use of the ontology and the work process based on two case studies.