No guidance documents for risk-informed analysis and assessment of changes in Limiting Conditions for Operation (LCO:s) in TS (Technical Specifications) have been available in the Nordic countries. In the U.S. several guidance documents, primarily developed by the owners groups for PWRs and BWRs, exist. Guidance for analysis of TS changes in this report generally follows the U.S. Regulatory Guide 1.174.
The TS are developed for assuring safety during all operational modes and are part of the licensing basis for the plant. The original TS were based on deterministic analyses and engineering judgments (and to some extent risk evaluations). Existing PSA (Probabilistic Safety Assessment) studies are not primarily developed to be used for assessing TS conditions. Therefore it is important that all aspects valid for conditions in TS are either included in the plant specific PSA-study or taken care of by complementary means.
As the PSA has developed over the years, it has demonstrated to constitute a useful tool for evaluating many aspects of the TS from a risk point of view. However, PSA is only one of the tools that ought to be/can be used in an evaluation process of the TS conditions. It is an excellent tool to be used to verify the risk importance, and thereby possibly relaxation, of TS conditions. PSA is not sufficient in itself for defining which equipment that shall or shall not have TS conditions.