The assessment of the production and fate of gases in the repository is presented in detail in NTB 24‑23 (Nagra 2024o) and is summarised in Section 4.3 of NTB 24‑22 Rev. 1 (Nagra 2024u) and in the following paragraphs.

The assessment of gas pressure build-up consists of a description and quantification of (1) the potential gas sources, which include the waste, barrier components such as disposal canisters and other gas-generating repository components, (2) the processes leading to gas generation, including reactions related to the corrosion of metals and degradation of organics, and (3) gas transport mechanisms shaping the hydraulic evolution of the repository described in the previous section.

The evolution of the gas generation rate and of the cumulative volume of generated gas during the time period for assessment is calculated using a range of alternative assumptions and parameter values in a set of assessment cases that account for uncertainties (deviation in performance) (see Chapters 5, 6, 7 and 8 in NAB 24‑25, Nagra 2024k for more details). The assessment cases correspond to “PA scenarios” related to the production of gas, the properties of the closure system and backfill, and the barrier function of the host rock (PA scenarios are discussed further in Sec­tion 6.4). These cases are additionally supported by probabilistic simulations to account for parametric uncertainty, as discussed in Section 6.3. The outcome of these calculations is used as input to the modelling of gas transport and the resulting build-up of pressure and associated fluid flows in the repository and surrounding rock, which are used as performance indicators.

The main outcome of the gas synthesis in Chapter 6 in NTB 24‑23 (Nagra 2024o) is a consistent hydraulic evolution of the repository adhering to the same principles and transport regimes for all calculations variants. This is the foundation for performance assessment of gas-induced effects, elaborated in NTB 24‑22 Rev. 1 (Nagra 2024u), which demonstrates that gas production in the geological repository for both HLW and L/ILW does not compromise the post-closure safety functions of either the host rock or the engineered barriers. In all the cases explored, there is a safety margin with regards to the defined performance targets. Even under the most pessimistic assumptions, the safety functions of the repository system are not degraded.

A significant observation concerns the self-regulating nature of gas pressure in the repository: an increase in the amount of gas in the pores results in a higher effective permeability for gas, which increases gas transport along the repository structures and decreases pressure build-up. The performance assessment shows that partially saturated conditions will sustain gas pathways through the repository structures such that overpressures are mitigated. This is particularly relevant to the L/ILW repository section, where gas migrates axially along the tunnels. It is shown that fully water-saturated conditions, which could result in a loss of porosity and gas permeability due to potential interactions between materials and consequently lead to gas-induced over­pressures, can be excluded. By contrast, the HLW near field saturates with water within few hundreds of years, due to the high suction and thermal gradient. In all cases the saturated granular bentonite material limits the gas flow away from the corroding canisters. However, due to the low corrosion rate of the carbon steel canister under repository conditions, gas pressure build-up in the HLW drifts is not significant unless extremely conservative assumptions are made.

Another important insight from the results described in Chapter 6 of NTB 24‑23 (Nagra 2024o) is that the V3 shaft seal saturates faster than it takes for waste-generated gas to reach the shaft. This leads to gas accumulation beneath the shaft, with gradual dissipation of gas through dissolution and diffusion. Over the course of tens of thousands of years, some of this gas eventually permeates to the Malm aquifer above the repository. By then, however, gaseous radionuclide-bearing species, notably methane incorporating 14C, will have decayed within the repository system, such that there are no significant radiological consequences (see Chapter 8). The reasons for the fast saturation of the V3 shaft seals are the abundance of water from aquifers above the Opalinus Clay and the high capillary suction of the compacted bentonite material of the sealing elements.

PA modelling makes use of conservative assumptions and simplifications. For example, the corrosion and degradation of materials are conservatively decoupled from the availability of water for these reactions. In reality, gas generation ceases in dry conditions, or at low relative humidity, as water availability is the most important limiting factor for all processes that generate gas (principally the degradation of organics and corrosion of metals). As a further example, it is conservatively assumed that gas generation from the L/ILW repository section starts immediately after waste emplacement, despite the highly alkaline near-field conditions, which, in reality, are expected to decrease corrosion and inhibit microbial activity. The calculated build-up of gas overpressure could be further reduced by including in the calculations the expected thriving gas-consuming microorganisms, which are not currently accounted for, and actual gas pressure build-up could, if it were to prove necessary, be reduced by further tailoring the closure system.