The models used to calculate doses in the analysis of radiological consequences, as well as to evaluate the RT, RT fluxes and RT concentrations described in the previous sections, incorporate numerous conservative assumptions and simplifications to handle uncertainties that are not addressed by other means (e.g., through parameter variations).

Conservatisms include the deliberate omission of some phenomena that are considered likely to occur and that are beneficial to safety, from quantitative analysis because suitable models, codes, or databases are unavailable. Such phenomena are termed reserve FEPs, since they can be mobilised at a later stage of the waste disposal programme, provided the necessary models, codes, and databases are developed. Important reserve FEPs identified in the course of the Project Entsorgungsnachweis (Section 8.2.8.3 of NTB 02-05, Nagra 2002) that remain reserve FEPs in the present safety assessment are:

• the co-precipitation of radionuclides with secondary minerals derived from spent fuel, glass and canister corrosion (except for co-precipitation of radium, which is included in all cases),

• sorption of radionuclides on canister corrosion products,

• natural concentrations of isotopes in solution in bentonite porewater, which could further reduce the effective solubilities of some radionuclides,

• irreversible sorption of radionuclides in the near field or in the geosphere (surface mineralisation),

• long-term immobilisation processes (precipitation / co-precipitation) in the geosphere, and

• the delayed release of radionuclides, due to the slow corrosion rate of L/ILW metallic materials (e.g., hulls and ends), as well as a period of complete containment by L/ILW steel drums and emplacement containers.

Newly identified reserve FEPs that concern radionuclide release and transport in the aqueous phase are:

• the absence, for an extended period of time, of a continuous liquid phase connecting the L/LW cavern and the host rock, which would be needed for the release of radionuclides into the aqueous phase and further transport to the biosphere, and

• the increased sorption of some radionuclides due to elevated temperatures of the geological environment.

In addition, newly identified reserve FEPs that concern repository-generated gas and radionuclide release and transport in the gas phase are:

• the presence of microorganisms in the backfilled operations and construction tunnels that have the potential to reduce the hydrogen gas pressure to well below the hydrostatic pressure, and

• the impact on corrosion and degradation rates of the very slow resaturation of the repository, resulting in a very slow exchange of the original atmosphere with repository-generated gas such as hydrogen and slow build-up of gas pressures.

These reserve FEPs have the potential, in the future, to provide additional quantitative contributions to the evaluated performance of the disposal system. One specific reserve FEP that was identified in the Entsorgungsnachweis²⁷, but, due to site characterisation activities carried out in the intervening years, was mobilised in the present safety assessment, is:

• retardation in the confining geological units of the Opalinus Clay host rock.

This reserve FEP was conservatively neglected in the reference conceptualisation of the geo­sphere in the radionuclide retention and transport calculations for the Entsorgungsnachweis, but is included in the present analysis of radiological consequences, which considers retention and transport through a broader CRZ.

Even when they are not mobilised, the presence of reserve FEPs constitutes, in effect, an addi­tional qualitative argument for safety, since it indicates that the actual performance of the disposal system will, in reality, be more favourable than that evaluated in the analysis of assessment cases given in Chapter 8, or with more detail in Chapter 7 of NTB 24‑18 (Nagra 2024p).

In addition to the reserve FEPs, there are further reserves due to several simplifying pessimistic or conservative assumptions that had to be made for the quantitative analysis of some assessment cases because of the limitations of the available models and codes as well as uncertainty in the underlying scientific understanding. Conceptual assumptions and simplifications of the models used in the analysis of radiological consequences are described in Chapter 3 and Appendix of NTB 24‑18 (Nagra 2024p). Many of those that are currently highly conservative are expected to be replaced by more realistic assumptions in future safety assessments, which will yield even lower calculated radiological consequences than in the present assessment.