The current safety and repository concept is set out in Chapter 3. The main barriers and other features contributing to the safety functions are termed “pillars of safety”. Fig. ‎3‑3 and Fig. ‎3‑4 illustrate schematically how the pillars of safety contribute to the various safety functions. PA provides detailed descriptions of the pillars of safety and their intended functionalities under relevant conditions.

In PA, the expected performance of the multi-barrier systems is expressed in terms of a set of claims concerning the contribution of each component of the multi-barrier system and of the repository system as a whole to the repository safety functions, in line with the current safety concept. Each claim is formulated based on, and examined through, a comprehensive analysis of empirical data, experimental results and modelling evidence. The most important claims, arguments, and evidence in the assessment of the individual pillars of safety (assessment by barrier) are presented in the following sections. A more detailed discussion of the corresponding claims, arguments, and evidence is given in Chapter 3 of NTB 24‑22 Rev. 1 (Nagra 2024u).