Preliminary probabilistic risk model for digital I&C architecture

This report presents a preliminary probabilistic risk assessment (PRA) model for the OECD/NEA WGRISK DIGMORE reference case representing digital instrumentation and control (I&C) systems in a simplified boiling water reactor plant. The reference case covers an I&C architecture with several systems, such as the primary and diverse reactor protection system, operational I&C system, hard-wired backup system, and prioritization and actuation control systems. The reference case has not been completed yet, and therefore, tentative modelling assumptions have been used in the PRA model. There are still several issues that need to be clarified, including the design of the operational I&C, spurious signals, reliability parameters and common cause failure assumptions.

In the preliminary results, certain priority and actuation control (PAC) units have a very high risk contribution. Even though the risk contribution is somewhat dependent on tentative parameter values, the logic of the model clearly implies that PAC is the most important part of the I&C systems, because it has no diverse alternative. The risk contributions of the other I&C system failures are small, because there are diverse solutions in each case.