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{{OR/17/067}}
Clay-based engineered barriers are a vital part of the design concept for the geological disposal of radioactive waste. The corrosion of metallic canister materials in the subsurface in anoxic conditions, as could occur in the KBS-3 disposal concept, radiolysis of water and radioactive decay of the waste could all cause the production of gas. The accurate understanding of the processes governing the movement of repository gases through engineered barriers is therefore of importance in understanding their long-term performance and integrity. The migration of gas through the engineered barrier may occur either by diffusion alone or a combination of advection and diffusion. If the gas production rate is greater than the diffusion rate through the material, gas will accumulate as a free phase (Weetjens and Sillen, 2006<ref name="Weetjens 2006">WEETJENS, E, and SILLEN, X. 2006. Gas generation and migration in the near field of a supercontainer-based disposal system for vitrified high-level radioactive waste. 1–8 in ''Proceedings of the 11th International High-level Radioactive Waste Management Conference (IHLRWM), 30 April–4 May 2006''. (Las Vegas, United States). </ref>; Ortiz ''et al.'', 2002<ref name="Ortiz 2002">ORTIZ, L, VOLCKAERT, G, and MALLANTS, D. 2002. Gas generation and migration in Boom Clay, a potential host rock formation for nuclear waste storage. ''Engineering Geology'', Vol. 64 (2–3), 287–296. </ref>; Wikramaratna ''et al.'', 1993<ref name="Wikramaratna 1993">WIKRAMARATNA, R S, GOODFIELD, M, Rodwell, W R, Nash, P J, and AGG, P J. 1993. A preliminary assessment of gas migration from the Copper/Steel Canister. ''SKB Technical Report'', TR-93-31. </ref>) and the pressure will rise until the gas can advect through the material. The advection of gas will be influenced by the layout of the radioactive waste repository, therefore a consideration of the advection of gas in a repository will have an impact on both its design and layout.

Recent work has shown that plastic clays, such as bentonite, idealised two-phase flow through a porous medium often does not adequately explain experimental observations (Horseman ''et al.'', 1996<ref name="Horseman 1996">HORSEMAN, S T, HIGGO, J J W, ALEXANDER, J, and HARRINGTON, J F. 1996. Water, gas and solute movement through argillaceous media. ''NEA Report'', CC-96/1, OECD, Paris. </ref>, 2004<ref name="Horseman 2004">HORSEMAN, S T, HARRINGTON, J F, and SELLIN, P. 2004. Water and gas flow in Mx80 bentonite buffer clay. 715–720 in ''Symposium on the Scientific Basis for Nuclear Waste Management XXVII''. Materials Research Society (Kalmar) Volume 807. </ref>; Harrington and Horseman, 1999<ref name="Harrington 1999">HARRINGTON, J F, and HORSEMAN, S T. 1999. Gas transport properties of clays and mudrocks. 107–124 in ''Muds And Mudstones: Physical And Fluid Flow Properties''. APLIN, A C, FLEET, A J, and MACQUAKER, J H S. (editors). Geological Society of London, Special Publication No. 158.</ref>; Angeli ''et al.'', 2009<ref name="Angeli 2009">ANGELI, M, SOLDAL, M, SKURTVEIT, E, and AKER, E. 2009. Experimental percolation of supercritical CO<sub>2</sub> through a caprock. ''Energy Procedia'', Vol. 1, 3351–3358.</ref>; Harrington ''et al.'', 2009<ref name="Harrington 2009">HARRINGTON, J F, NOY, S, HORSEMAN, S T, BIRCHALL, D J, and CHADWICK, R A. 2009. Laboratory Study of Gas and Water Flow in the Nordland Shale, Sleipner, North Sea. ''AAPG Special Volumes'', DOI:10.1306/13171259St593394. </ref>). Further work is required to understand such processes as gas entry, gas breakthrough, gas flow, flow path homogeneity and pathway sealing, as well as the dilatant mechanisms in the clay that control them. In addition, the gas permeability is likely to be a time- and location-dependent variable rather than a material property because it relies on the quantity, size and connectivity of pressure-induced pathways through the material (Horseman and Harrington, 1997<ref name="Horseman 1997">HORSEMAN, S T, and HARRINGTON, J F. 1997. Study of gas migration in Mx80 buffer bentonite. ''British Geological Survey Technical Report'', WE/97/7.</ref>). It is also unclear what effect previous gas flow through a clay will have on any subsequent gas flow; the first incidence of gas flow may reduce the effectiveness of the barrier against further gas migration.

Task A of the DECOVALEX-2019 programme has been designed to address these questions, and to improve understanding of the advection of repository gases through clay-based materials. A 1-D gas injection test performed on compact Mx80 bentonite has been conducted at the British Geological Survey. This test represents the first test dataset for Stage 1A of the DECOVALEX Task A; the results of this experiment are presented in this report.

==References==

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[[Category: OR/17/067 The response of compact bentonite during a 1-D gas flow test | 03]]

OR/17/067 Introduction - Revision history

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Ajhil at 11:19, 28 June 2018