Developing PRA computer code requirements based on probabilistic risk analysis practices

In the development and analysis of probabilistic risk analysis (PRA) models, dedicated computer codes play an important role. These codes handle complex mathematical models. Because results of PRA analysis are used in safety decision making, a PRA computer code should fulfill rigorous quality and performance requirements. PRA standards/safety guides provide mainly recommendations in performing and managing a PRA project. Requirements for computer codes used for PRA analysis are generally only very briefly addressed. In this paper a case study on developing requirements specification for an existing Level 1 PRA computer code called FinPSA is presented. The goal was to demonstrate that FinPSA conforms to related international standards and to link FinPSA specific requirements to the framework represented in the standards. As a starting point in the development of the requirements, the following viewpoints were chosen; applicability and reviewability, usability and acceptability. The process for developing the requirements was fairly straightforward. First generic requirements for PRA level 1 and PRA level 1 computer code as presented in a PRA standard were expressed, and then the computer codespecific requirements that serve the attainment of the generic requirements follow. Additionally, computer code specific requirements were derived from general experience related to earlier FinPSA use and from quality attributes relating to the different viewpoints. The experiences from the case study demonstrated that obtaining a necessary set of requirements for a PRA computer code is not a trivial task and that managing requirements specification is an iterative process.