When an HLW disposal canister is breached and sufficient porewater enters, the breached inner containment allows water to contact the waste matrix. This interaction initiates the release of radionuclides from the waste packages. Some of the radionuclides, located in cracks in the SF UO₂ matrix or in the annular gap between fuel pellets and rods, may be relatively rapidly released as fission gas or as dissolved nuclides (the instant release fraction, or IRF (Chapter 4 in NAB 23-10, Johnson et al. 2023). However, most radionuclides are released much more slowly congruently with the dissolution of the UO₂ matrix and the corrosion of the cladding or con­gru­ently with the dissolution of the RP-HLW (Chapter 7 in NAB 23-09, Curti 2022 and Chapter 5 in NAB 23-10, Johnson et al. 2023).

In the case of L/ILW, water entering the waste containers and packages initiates the release of radionuclides from the waste packages.

Fission gas release (FGR)

The quantification of FGR from SF rods is relevant to short-term as well as to post-closure safety and safety assessment. In the short term, it is relevant to the safe handling and encapsulation of damaged fuel assemblies in the final disposal canisters. In the context of post-closure safety, FGR from SF is known to correlate with the IRF for I-129 and Cs-137 as explained in Section 4.1 in NAB 23-19 (Johnson et al. 2023). Trapped fission gas also contributes to gas pressure build-up that can affect the integrity of the cladding. Thus, campaigns to calculate the FGR for all Swiss reactor fuel assemblies, except for Mühleberg, have been carried out to obtain reliable average FGR data. These data (Nagra 2010, AREVA 2012, Oldberg 2009), combined with measurements of radionuclide release from SF carried out by Nagra in collaboration with PSI, Switzerland, and SKB and Studsvik in Sweden, for fuel with burnups from 45 to 65 GWd/tHM, have provided the basis for a radionuclide release model. FGR has been further investigated within the framework of an international project (Evins et al. 2021) that includes modelling and analogue studies, as well as sampling campaigns carried out for fuel in hot cells. The derivation of the IRF relevant to the analysis of radiological consequences is documented in NAB 23-10 (Johnson et al. 2023).

Dissolution of radionuclides

Once sufficient water is present inside the SF canister and has reached first the cladding and, after cladding failure, the spent fuel pellets, radionuclides start slowly to be released and dissolve in the in-canister water (Section 9.2 in NTB 23-02 Rev. 1, Nagra 2024v). They then diffuse through the breached canister shell and into the bentonite buffer in the bentonite porewater (see Chapter 7 in NTB 23-02 Rev. 1, Nagra 2024v for details). Most of the radionuclides present in the Swiss waste inventory are expected to be released in solution, see, e.g., arguments in Section 3.5 in NTB 24‑18 (Nagra 2024p), with release rates constrained by the solubility limits of the corresponding elements, which are, for most elements, in the nano- to micromolar concentration range (see Section 9.2 in NTB 23‑02 Rev. 1, Nagra 2024v and Chapter 4 in NTB 23-04, Hummel et al. 2023).

Upon contact with porewater, RP-HLW also dissolves extremely slowly according to the rates provided in Chapter 7 in NAB 23-09 (Curti 2022) and the released radionuclides diffuse along microcracks in the glass, first through the breached inner stainless-steel coquille and then through the breached canister shell into the bentonite buffer (see Section 9.1 in NAB 24-20 Rev. 1, Nagra 2024m and NAB 23-09, Curti 2022), where solubility limits for most elements are in the nano- to micromolar range (Section 9.3 in NTB 23-02, Nagra 2024v and Chapter 4 in NTB 23‑04, Hummel et al. 2023).

Resaturation of the L/ILW emplacement caverns is a slow process. When gas pressure increases, it limits the inflow of water from the host rock into the cavern. This, in turn, limits gas production. However, once the L/ILW caverns are sufficiently saturated, radionuclides may dissolve in the porewater and start to diffuse away from the waste (see Section 9.1 in NAB 24-20 Rev. 1, Nagra 2024m and NTB 23-03, Nagra 2024v for details). Within the L/ILW caverns, sorption pro­cesses are mainly controlled by secondary minerals formed from the degraded cementitious back­fill in the emplacement caverns (see Appendix D in NTB 23‑07, Tits & Wieland 2023 for details).

Most of the radionuclides released to the buffer decay before they reach the surrounding host rock and most of those that do reach the host rock decay there or in the wider CRZ, as is illustrated in Chapter 8 and in more detail in Section 7.1.1.1 of NTB 24‑18 (Nagra 2024p). However, some negatively charged, non-sorbing and long-lived radionuclides (such as I-129 and Cl-36) may eventually reach the biosphere.

Radionuclides released as volatile species

The most relevant radionuclide released as volatile species is ¹⁴C, with a half-life of about 5,700 years, predominantly in the form of methane (¹⁴CH⁴). Transport mechanisms for ¹⁴CH⁴ within the repository structures vary with time, with the degree of saturation and between the disposal areas (i.e., HLW vs. L/ILW). While the HLW near field saturates with water within a few hundred years, the L/ILW near field is expected to remain only partially saturated for a much longer period. Moreover, the HLW bentonite buffer exhibits relatively homogenous and isotropic porous media properties relevant to transport, whereas the cementitious near field of the L/ILW repository section is more heterogeneous. Volatile radionuclides migrate along the repository structures through advection and diffusion in the gas phase and also eventually dissolve in pore water,  as described in more detail in NAB 24‑07 Rev. 1 (Nagra 2024w).