Writing ‘usable’ Nuclear Power Plant (NPP) safety cases using bowtie methodology

Abstract

Historically, the majority of Nuclear Power Plants (NPP) Safety Cases have been produced and implemented under highly prescriptive regulatory regimes, with emphasis placed on demonstrations of the robustness of the facility’s design basis against a set of deterministic criteria and technical standards and rules set by the Regulatory Body. This has resulted in Safety Cases that are technically sound, but at the same time too complex and not easily accessible to and usable by persons responsible for ensuring safe operations; i.e. operations and maintenance staff who are in direct control of the plant as well as managers who are accountable for safety – the key end-users.Shortcomings regarding the ‘usability’ of the Safety Cases are not new and have been the subject of discussion in recent years. They are deeply rooted in the way these documents are produced and implemented. It means that in order to overcome these difficulties attention should be focused primarily on the Safety Case process, affording it the same importance that is given to the final product – the documented Safety Case. This paper explores the advantages that incorporation of the Bowtie risk management methodology into the Safety Case (production) strategy can bring to the delivery of a fit-for-purpose, accessible and usable Safety Case, supporting current efforts undertaken by the nuclear industry to ensure ‘Right First Time Safety Cases’.The paper also suggests that the conduct of Bowtie workshops - in the author’s opinion, the most important part of the Bowtie building process – provides vital input from the people who have most knowledge and experience of the plant and its current operational status. It enables the Bowtie diagrams to reflect the ‘operational reality’ in the plant, by providing a real picture of the actual barrier condition and, more importantly, to identify those barriers that need to be strengthened. The latter will have a direct effect on the usability of the resultant Safety Case product during plant operation.A generic Advanced Boiling Water Reactor (ABWR) design has been taken as an example to highlight the claimed advantages. The paper reflects how Bowtie diagrams can be built within a workshop setting that depicts how a particular accident scenario can be managed through appropriate barriers and controls. It also shows how the use of Bowtie diagrams can make the Safety Case process a true ‘aid’ to thinking and deliver a final product that is accessible and easy-to-understand by key end-users.