Annex A – Statement of Work
1. Objective
The objective of this research is to obtain laboratory data on the performance of bentonite seal materials in a brine environment. The data are will be used in the development of numerical models by the CNSC to assess the long-term performance of sealing materials for repositories in sedimentary rock.
2. Background
Two initiatives for the deep geological disposal of radioactive wastes are currently being proposed for Canada, and are being evaluated and investigated by the CNSC. The proposed repositories rely on both the surrounding host rock and engineered barriers to contain and isolate waste from the biosphere for hundreds of thousands to millions of years. The success of both projects depends largely on the long term performance of these barriers.
CNSC has developed a Coordinated Assessment and Research Program on the safety of geological repositories (CARP). This program examines the long term performance of both the geological and engineered barriers. Over the long term, the host rock is a natural and major barrier to contaminant migration from repository waste. Therefore, the previous phase of CARP focused on geoscientific aspects of the host rock. However, preliminary long term safety assessments performed by the Nuclear Waste Management Organization (NWMO) have shown that bentonite seals used in the repository galleries and shafts also play an important role by minimizing preferential pathways for contaminant transport. Therefore, the current phase of the CARP is being extended to examine the long term performance of those engineered barriers. The CNSC collaborates with the IRSN (the Institut de Radioprotection et de Sûreté Nucléaire, France) in the SEALEX project, which involves a series of in-situ tests on the long term performance of sealing materials at the IRSN’s Tournemire underground research laboratory. The SEALEX project provides the CNSC with a wealth of experimental data that allows the CNSC to develop mathematical models for the long term performance of those sealing materials.
One particular feature of Canadian sedimentary rocks is the existence of brine at anticipated repository depths of more than 500 m [1]. Experimental evidence shows that salinity plays an important role on the hydraulic, chemical and mechanical characteristics of bentonite seals. However, the experiments conducted so far were only performed at salinity concentrations much lower than the 200-300 g/l concentration levels of Canadian brines. For repositories proposed in Canadian sedimentary rocks, there is therefore a strong need to understand the interaction of the brine and the bentonite-based seals, in order to assess their long term performance as a barrier to water and gas flow.
3. References
[1] NWMO, 2011. OPG’s Deep Geologic Repository for Low and Intermediate Level Waste: Geosynthesis. Report NWMO DGR-TR-2011-11 (http://www.nwmo.ca/dgrsubmission)
[2] Barnichon J.D. and Deleruyelle F. Sealing experiments at the Tournemire URL: the SEALEX Project. Eurosafe 2009
[3] Barnichon J.D., Dick P. and Bauer C. (2011) The SEALEX in situ experiments: Performance tests of repository seals. In: Harmonising Rock Engineering and the Environment – Qian & Zhou (eds) © 2012 Taylor & Francis Group, London, ISBN 978-0- l415-80444-8, pp. 1391-1394
[4]DECOVALEX 2015- description of task A: http://www.decovalex.org/task-a.html
[5]Wang, Q., Tang, A.M., Cui, Y., Delage, P., Barnichon, J.D., Ye, W.M (2013). The effects of technological voids on the hydro-mechanical behaviour of compacted bentonite-sand mixture. Soils and foundations 53, 2 (2013) 232-245
4. Scope of Work
Perform laboratory tests to obtain hydro-mechanical characteristics and simulated long term evolution of a 70-30 bentonite-sand mixture, using brine as pore water. Laboratory testing will need to extend over a period of at least 2 years.
5. Tasks to be Performed
5.1 Design and produce a model water to represent porewater in the Cobourg limestone of the Michigan Basin, at depths of approximately 700 m.
5.2 Prepare MX-80 bentonite at a dry density of 1.61 and 1.41 Mg/m3. Prepare MX-80 bentonite/sand mixture similar to the ones used in the SEALEX project [2, 3] at dry densities of 1.65, 1.8 and 1.95 Mg/m3.
5.3 Perform oedometer and hydration tests on MX-80 and MX-80 bentonite/sand mixture with the model water to determine water retention, swelling pressure and other index properties (e.g. swell index).
5.4 Perform a parallel study of microstructure properties to observe changes over time.
5.5 Perform “mock-up” tests with a technological void similar to the one described in [4, 5]. The bentonite/sand mixture at an initial dry density of 1.95 Mg/m3 will be used in these tests. Instead of steel cells, the contractor should consider using hollow cylindrical samples of Cobourg Limestone as confining vessels. Perform gas/liquid injection test in one sample after the technological void has been filled and a steady-state swelling pressure is reached.
5.6 Perform chemical testing of pore waters over time and measure the cation exchange capacity after testing.
