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Risk Informed Safety Margin Characterization Case Study: Selection of Electrical Equipment To Be Subjected to Environmental Qualification

Reference 1 discussed key elements of the process for developing a margins-based “safety case” to support safe and efficient operation for an extended period. The present report documents (in Appendix A) a case study, carrying out key steps of the Reference 1 process, using an actual plant Probabilistic Risk Assessment (PRA) model.

In general, the margins-based safety case helps the decision-maker manage plant margins most effectively. It tells the plant decision-maker such things as what margin is present (at the plant level, at the functional level, at the barrier level, at the component level), and where margin is thin or perhaps just degrading. If the plant is safe, it tells the decision-maker why the plant is safe and where margin needs to be maintained, and perhaps where the plant can afford to relax.

As discussed in Reference 1, the details of the safety case need to be considered collectively. Functional margin in a given system, structure, or component (SSC) matters (or not) depending on what alternative means the plant has to accomplish the given SSC’s function, should it fail. For that reason, it is necessary to have a clear picture of what SSC’s are being counted on collectively in order to assess the significance of a given SSC’s performance margin.

The case study in Appendix 1 focuses on environmental qualification (EQ) of electrical equipment in containment. Age-related degradation of cables (such that they become more susceptible to harsh environments) is cited in numerous discussions of extended operation as an important issue. Which cables matter? For which cables do we need ongoing assurance of performance (specifically under harsh environmental conditions)? Replacement of all cables is a daunting prospect. Being able to focus on a subset of cables, while still maintaining plant-level safety and efficiency even if the other cables degrade, would be very useful. The case study shows how to do this.