The major components within the L/ILW near field are cement-based, compatible with each other and contribute to an alkaline environment upon contact with water from the Opalinus Clay (Sections 3.3 – 3.5 and Appendix A in NTB 23-03, Kosakowski et al. 2023 and Chapter 5 in NAB 20-11, Kosakowski et al. 2020 and references therein).
The cementitious near field contributes to the retention of radionuclides because, by design, the cement paste contains C-S-H phases that sorb radionuclides (Nagra 2021c, 2024u, Tits & Wieland 2023, Kosakowski et al. 2023).
The concrete disposal containers and cementitious container infill mortar contribute to the initial structural integrity of the waste packages because:
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by design, the concrete containers are reinforced and constructed of high-quality concrete, and
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filling of the disposal container with a container infill mortar contributes to limiting void space within the L/ILW cavern.
The grain-supported cavern backfill mortar (Fig. 6‑6) contributes to the mechanical integrity of the L/ILW caverns by filling the void space and because, by design, the backfill mortar has grain-to-grain contact (Jacobs et al. 1994, Nagra 2021a, Nagra 2024u).
Finally, the porosity and chemical composition of the cavern backfill mortar contributes to the compatibility of the multi-barrier system. Specifically, the backfill mortar provides storage volume for repository-generated gas and a continuous porosity to transport that gas (Nagra 2024o). The highly alkaline conditions present in the backfill and throughout the near field also limit gas production by supressing corrosion rates and microbial activity (Section 3.1.2 in NTB 21-02, Nagra 2021a; Section 3.3.1 in NTB 23-03, Kosakowski 2023; Chapter 6 in NTB 13-04, Warthmann 2013 and Section 3.4.3 in NTB 24-22 Rev. 1, Nagra 2024u).
Fig. 6‑6:Grain-supported cavern backfill mortar for the L/ILW caverns
The figure shows results from a large-scale test of backfilling techniques and properties of the mortar.