Findings from the Perspective of the Aims

The specific aims of the Safety Report are listed as follows [see page 12 of Nagra (2002a)]:

1. To determine the suitability of the Opalinus Clay of the Zürcher Weinland as a host rock for the repository for SF/HLW/ILW from the point of view of long-term safety.

2. To enhance the understanding of the multiple safety functions that the proposed disposal system provides.

3. To assess the robustness of the disposal system with respect to remaining uncertainties and the effects of phenomena that may adversely affect the safety functions.

4. To provide a platform for the discussion of a broad range of topics related to repository development. More specifically, the findings from the safety assessment, together with those from the regulatory authorities' review thereof, will provide guidance for future stages of repository planning and development.

The findings of the IRT that are pertinent to these aims are summarised below.

5.1.1 On the suitability of the Opalinus Clay of the Zürcher Weinland as a host rock

The IRT finds that Nagra has presented strong evidence that the Opalinus Clay of the Zürcher Weinland is a suitable host rock for a geological disposal repository. In particular:

i) Multiple arguments have been presented that the Opalinus Clay of the Zürcher Weinland is a tight, self-sealing material that would provide strong isolation, retention, delay and dispersion of any radionuclides released from a disposal facility located in it. Natural analogue studies, laboratory and field experiments as well as theoretical analyses corroborate this.

ii) The Opalinus Clay of the Zürcher Weinland exists at sufficient depth and in sufficient thickness to host a geological repository. It is located in a seismically stable region of Switzerland and its properties are not sensitive to changes in the surface environment.

iii) The geometric model of the Opalinus Clay of the Zürcher Weinland is well founded. From the evidence presented the IRT finds it reasonable to treat the Opalinus Clay of the Zürcher Weinland as a homogeneous entity in the safety assessment. It is also reasonable to conclude that the safety-relevant properties can be extrapolated over a wide region.

5.1.2 Understanding of the multiple safety functions of the disposal system In accordance with international practice, Nagra cites three main safety functions for a geological repository, namely:

1. Isolation from the human environment.

2. Long-term confinement and radioactive decay within the disposal system.

3. Attenuation of releases to the environment

These functions are accomplished by a system of features that are shared amongst the natural and man-made barriers, as has been discussed in Chapter 2 and more extensively in Chapter 4. The IRT finds that the depth of the repository (about 650 m) and the properties of the geological strata provide a high level of isolation from the accessible environment.

Long confinement is assured due to functions of the barriers that include:

• Thick-walled steel canisters for SF and HLW that should provide absolute containment of radionuclides for at least 10 000 years, except possibly for a very small number of canisters that may contain defects.

• Durable waste forms – spent fuel and vitrified HLW – that will dissolve and release radionuclides very slowly (i.e. over periods of tens to hundreds of thousands of years or even longer for spent fuel) under the geochemical and solute transfer conditions expected to exist in the repository.

• A bentonite buffer that, in addition to providing a favourable chemical environment, a strong transportation and isolation barrier and heat transfer path, also provides a well understood mechanism for ensuring the self-sealing of the excavation disturbed zone surrounding the emplacement tunnels in which the waste containers are placed.

• The Opalinus Clay stratum, which is geologically stable and has an extremely low permeability and high retention properties for many radionuclides.

Nagra’s analysis shows that most nuclides present in the wastes would be retained within the geosphere for timescales of one million years or longer.

Only long-lived, non-sorbing and weakly sorbing radionuclides such as ¹²⁹I,

79Se, ¹⁴C and ³⁶Cl would be released from the geosphere and these would be diluted by Quaternary groundwaters before reaching the accessible environment.

The IRT finds that the understanding of the performance of the components of the multi-barrier system proposed by Nagra is based on sound science and that the multi-barrier system has complementary and redundant features that should ensure that radionuclide releases to the environment would occur at very long times in the future and would be small in comparison with natural levels.

5.1.3 To assess the robustness of the disposal system with respect to remaining uncertainties

Robustness can be considered to be a measure of insensitivity to remaining uncertainties. Nagra has provided robustness through their disposal concept, which has multiple barriers with an adequate degree of redundancy.

Nagra has assessed the robustness of the concept through analysis of a wide range of cases, including “what if?” cases, which are outside the range of possibilities supported by scientific evidence. As discussed in Section 3.2, the IRT concludes that Nagra has effectively demonstrated robustness for the current phase of the project.

5.1.4 To provide a platform for the discussion of a broad range of topics related to repository development

The IRT agrees that the findings of Nagra’s safety assessment provide useful guidance for deciding on the next and subsequent stages of repository planning and development in Switzerland. Despite the clarity of the documentation, the IRT considers that the very large volume of information would make it difficult for the general public to obtain an informed view. The IRT encourages Nagra, therefore, to provide a brochure of ~ 50-60 pages encompassing the Entsorgungsnachweis project. This document should also provide a road map to identify the various documents where the reader will be able to find additional, more detailed information.

Overall, the IRT observes that the Safety Report and associated documentation provided by Nagra clearly reflect discussions and considerations that have taken place over the past decade at the international level on the subject of what constitutes a modern safety case. It will provide a useful benchmark for other national studies.

5.2 Findings from the Perspective of Its Terms of Reference