Conceptual model for safety requirements specification and management in nuclear power plants

Clearly stated requirements, systematic configuration management and traceability are a key prerequisite for the safety of industrial plants. In spite of the long research tradition and training, in particular in software engineering, poor requirements are still a major source of safety problems. Requirements engineering is a challenge also in nuclear power plant automation. The characteristics of requirements engineering, such as multi-disciplinary collaboration, uncertainties and abstract concepts, are difficult to engineers who prefer to think in terms of technical solutions. The working practices and tools for describing requirements are often vague. Even the standards and guidelines developed for the nuclear domain fail to provide a clear and consistent vocabulary for describing power plants. Well-defined terminology would, however, be needed for communicating the requirements between various stakeholders and engineering disciplines. The goal of this report is to foster mutual understanding among industrial professionals by providing clear terminology. Furthermore, the concepts form a basis for design guidelines and computer tools. Accordingly, the report is divided into two parts, the first one discussing general modelling principles and the second one suggesting a more practical data model for tool development. One of the starting points is that requirements cannot be discussed in isolation from other engineering activities and system descriptions. Even the boundary between requirements and design solutions is not always clear. Therefore, this report is not limited to requirements but discusses the principles of modelling complex socio-technical systems in a broader sense. The second starting point is that the number of requirements and dependencies requires computer tools. Computer tools, in turn, need consistent data models. This is why this report has taken influences also from international standardisation of product data modelling. It works towards this vision in a semi-formal, database-oriented way by defining concepts that might be used in future computer tools to describe power plant systems and their requirements.