Nagra’s safety assessment methodology has been developed in line with international guidance, advances, and publications. Based on these and on Nagra’s own experience, the following principles and objectives for the safety assessment have been identified:

• A systematic approach to information gathering and integration – The assessment relies on a systematic approach to information gathering and integration to assemble the assessment basis, the main elements of which are summarised in Chapter ‎5.

• Rigorous consideration and treatment of uncertainty – As far as possible, all potential sources of uncertainty and the various sources of bias must be considered and recognised when conducting and interpreting the analyses. Uncertainty management is described in general terms in Section ‎4.4 and is a feature of all the processes within the safety assessment methodology.

• Assurance of completeness – All reasonably foreseeable possibilities for the characteristics and evolution of the disposal system are considered in developing safety case arguments. The assurance of comprehensiveness, i.e., the minimisation of “completeness uncertainty” is discussed further in Section ‎4.5.

• Validation and verification of models and databases – Appropriate quality assurance and control measures are adopted to ensure that the models and databases developed and applied in the analyses are suitable for their intended purpose. Each computer code used to perform analyses must be verified. Model abstractions, including the use of simplifications and conservatism in modelling, must be justified.

• Use of stylised approaches – Stylised approaches⁸ are adopted for the modelling of the biosphere and the nature of future human behaviour and actions, on account of the largely irreducible, poorly quantifiable or unquantifiable uncertainties associated with predictions regarding these aspects, even over relatively short timescales.

• Multiple lines of argument for safety – Claims related to the performance and safety of the disposal system are supported, as far as possible, by multiple lines of argument and wide-ranging evidence. The development of arguments and evidence supporting a set of high-level claims is a key element of the performance assessment methodology (see Chapter ‎6). The multiple lines of argument underpinning the overall safety case are brought together in Chapter ‎10.

• Internal and external reviews – All relevant aspects within the safety assessment are subject to internal and/or external review.