Technical Report NTB 08-07

Effects of post-disposal gas generation in a repository for low- and intermediate-level waste sited in the Opalinus Clay of Northern Switzerland

Within the framework of Stage 1 of the "Sectoral Plan for Deep Geological Repositories" Nagra has proposed Opalinus Clay as a possible host rock for a repository for low- and intermediate-level waste (L/ILW). Opalinus Clay is characterised by a low permeability and is, therefore, an excellent barrier against radionuclide transport. Because significant amounts of gas are generated in a repository for L/ILW a demonstration is required that despite the low gas permeability of the Opalinus Clay the gas can escape without compromising long-term safety. The present study provides a comprehensive assessment of the question how gas generation and transport in a L/ILW repository affects system behaviour. For the purpose of the present study a geological repository for L/ILW in the Opalinus Clay of Northern Switzerland with a depth of about 300 – 400 m below the surface is assumed. The report provides relevant information regarding the layout and the operation of the L/ILW repository as well as a brief survey of the waste inventories and the expected amounts of gas generated. Furthermore the state of geoscientific understanding of gas transport processes in the underground structures of the repository and in the surrounding host rock is presented and the impact of gas generation on the isolation capacity of the repository is considered. The modelling activities described in the present report started in 2005 and were completed by the end of 2007. The results of the model calculations were used to optimise the layout of the L/ILW repository with respect to the effects of gas generation and transport. Specifically a design option was studied in which, by an appropriate choice of backfill and sealing materials, the gas can escape along the access ramp into the overlying rock formations without creating undue gas overpressures.  

The estimates of the gas generation rates for the L/ILW repository are based on a waste inventory accounting for the existing nuclear power plants, with an assumed operation period of 50 years, and for wastes from medicine, industry and research with a collection period up to the year 2050. This inventory includes a total mass of approximately 40'000 tons of steel and other metals and about 2'200 tons of organic matter. Complete corrosion / degradation of all gas-generating materials yields a gas volume of approximately 20 to 30 million cubic meters (STP). The highest gas generation rates are expected in the early post-closure period up to several hundreds of years, followed by a steady decline. The expected total duration of the gas generation phase is in the order of 200'000 years. 

The total volume of voids in the backfilled repository is in the order of 58'000 m3 for the assumed waste inventory. If the total amount of corrosion and degradation gases were enclosed hermetically in this void volume, a high gas pressure would result. In the real system, however, at least a part of the gas will be released through the host rock, resulting in much lower pressures. In order to keep the gas pressure low even in the case of a very low host rock permeability and / or an increased gas production, specially designed backfill and sealing materials could be used such as high porosity mortars as backfill materials for the emplacement caverns and sand/bentonite mixtures with a bentonite content of 20 – 30 % for backfilling other underground structures and for the seals ("engineered gas transport system" – EGTS). The EGTS is aimed at increasing the gas transport capacity of the backfilled underground structures without compromising the radionuclide retention capacity of the engineered barrier system. Sand/bentonite mixtures with a low bentonite content exhibit a low permeability for water and a relatively high permeability for gas due to their (micro)structure. 

The development of gas overpressures in the backfilled emplacement caverns is unavoidable due to the large amount of corrosion and degradation gases. Numerical simulations show that, for the expected gas generation rate, the planned repository layout and a typical gas permeability of the host rock, the gas pressure in the emplacement caverns remains below the threshold pressure for the onset of pathway dilation (approximately 6.5 MPa for the assumed site conditions). For such conditions, no additional design measures are needed to mitigate gas impacts. For the case of conservative gas generation rates, or the case of a very low gas permeability of the rock (kOPA ≤ 10-21 m2), the gas pressure could rise above the critical threshold pressure for the onset of pathway dilation. Consequently, the use of appropriate backfill and sealing materials that ensure a release of a part of the gas along the access ramp would be a suitable design measure to limit gas pressure. Calculations indicate that such an approach could limit pressures in the emplacement caverns so that even in the case of a very low permeability host rock overpressures above hydrostatic pressure would remain within a range of 3 – 4 MPa. 

As a result of the elevated gas pressures in the emplacement caverns, pore water containing dissolved radionuclides will be displaced into the geosphere. The gas pressure build-up as an additional driving force for mass transport also tends to increase the path length for radionuclide transport in the host rock, an effect which is further enhanced by the anisotropy of the intrinsic rock permeability. The displaced water is widely spread over the footprint area of the repository towards the adjacent rock formations above and below the host rock. The numerical simulations indicate specific water fluxes in the host rock of up to 10-11 m/s in the very early gas generation phase (< 1'000 years after repository closure). The fluxes decline steadily with time until the regime of diffusion-dominated transport is reached in the late times of the gas production phase (specific water flux typically < 10-13 m/s after several 10'000s of years). A comparison of these results with those from safety calculations using a wide range of specific water fluxes leads to the conclusion that pore water displacement caused by elevated gas pressures will not compromise the long-term safety of a L/ILW repository in Opalinus Clay.