Safety justification frameworks: Integrating rule-based, goal-based and risk-informed approaches

The reliability and safety of the digital I&C systems that implement safety functions are critical issues. In particular, software defects could result in common cause failures that defeat redundancy and defence-in-depth mechanisms. Unfortunately, the differences in current safety justification principles and methods for digital I&C restrict international co-operation and hinder the emergence of widely accepted best practices. These differences also prevent cost sharing and reduction, and unnecessarily increase licensing uncertainties, thus creating a very difficult operating environment for utilities, vendors and regulatory bodies. The European project HARMONICS (Harmonised Assessment of Reliability of MOdern Nuclear I&C Software) is seeking to develop a more harmonised approach to the justification of software-based I&C systems important to safety. This paper outlines the justification framework we intend to develop in HARMONICS. It will integrate three strategies commonly used in safety justifications of I&C system and its software: rule-based-evidence of compliance to accepted standards; goal-based-evidence that the intended behaviour and other claimed properties has been achieved; and risk-informed-evidence that unintended behaviour is unlikely. The paper will present general forms of safety case that can be adapted to a variety of specific topics