A prime concern in safety analyses and in safety assessment is the issue of phenomenological “completeness”, that is, the need to demonstrate that:

  • features, events, and processes (FEPs) and their interactions that could affect post-closure safety have been thoroughly identified, and

  • these FEPs have been adequately accounted for in the safety assessment.

Although it is impossible to prove beyond all reasonable doubt the completeness of a safety assessment, measures can be taken to ensure the inclusion of a comprehensive set of potentially relevant phenomena and, equally importantly, to ensure that the safety-relevant phenomena have been represented appropriately in the assessment.

In the safety assessment that supports the general licence application, an assurance of phenomenological completeness and appropriate FEP management is provided by:

  • the development of a comprehensive database of FEPs that could be relevant to the safety assessment, and

  • a careful audit of the phenomena included in the safety assessment against the FEP database.

Nagra’s FEP database has been developed iteratively over many years, largely independently of the main safety assessment process. Each version has been reviewed by internal and external experts and compared with other relevant FEP databases compiled by national organisations in other countries and by international bodies.

A FEP audit has been performed and documented in a supplementary volume of NAB 24‑20 Rev. 1 (Nagra 2024l). It considers whether all the FEPs in the FEP database have been adequately treated in the safety assessment and whether omission of any FEPs is well justified. In particular, the report indicates which FEPs can be excluded a priori, based on the existing scientific understanding or their negligible likelihood and/or consequences, or on the grounds that they are outside the scope of the safety assessment, and which FEPs are considered either as part of the performance assessment, or directly in the analysis of radiological consequences. The audit report can thus be viewed as a “filter” as to which FEPs are to be considered in detail in the safety assessment and which can be excluded from the outset.

FEPs can be included in the safety assessment in different ways, for example:

  • in the description of the safety scenarios, and/or

  • in the models used in performance assessment and in the analysis of radiological consequences, either explicitly, or implicitly through the selection of parameter values for those models.

Where FEPs are excluded, it is shown that these are either outside the scope of, or irrelevant to, the safety assessment in support of the general licence application. This helps to ensure that no relevant FEPs have been overlooked and that there is a sound justification for the exclusion of any FEPs.