Sand/Bentonite Barriers and Gas Migration: The GMT Large-Scale In-Situ Test in the Grimsel Test Site
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Sand/Bentonite Barriers and Gas Migration: The GMT Large-Scale In-Situ Test in the Grimsel Test Site S. Vomvoris1, B. Lanyon2, P. Marschall1, K. Ando3, T. Adachi3, A. Fujiwara3, S. Yamamoto4 1 Nagra, Hardstrasse 73, CH-5430 Wettingen, Switzerland 2 Fracture Systems Ltd., Tregurrian Ayr, St. Ives, Cornwall TR26 1EQ, UK 3 RWMC, No. 15. Mori Bldg. 2-8-10, Toranomon, Minatu-ku, Tokyo, 105-001, Japan 4 Obayashi, Civil Engineering Technology Division, Shinagawa Intercity Tower B, 2-15-2, Konan, Minato-ku, Tokyo 108-8502, Japan ABSTRACT The Gas Migration Test in the engineered barrier system (GMT) investigates the migration of waste-generated gas from low and intermediate level waste in a silo-type disposal concept. The EBS has now been emplaced and saturation was initiated in August 2001. The saturation patterns show heterogeneity within and between different layers of the EBS. Plans for the remaining test sequence are also presented. INTRODUCTION The Gas Migration Test in Engineered Barriers (GMT) is performed in Nagra's Grimsel Test Site in Switzerland under the leadership of the Radioactive Waste Management Funding and Research Center in Japan (RWMC). The project focuses on issues related to waste-generated gas migration through the engineered barriers in a silo-type disposal. The GMT experiment addresses three critical events associated with repository gas generation and migration: • Gas pressure pushes contaminated water out from the waste packages and the disposal cavern into the geosphere. • Escaping gas transports volatile nuclides through the EBS and the surrounding geological barrier. • Excess gas/water pressure causes stresses on the concrete silo and other EBS components leading to a loss of integrity and/or mechanical failure. The objectives of GMT are [1]: • Assess the function of the system 'EBS and adjacent geosphere' as a whole with respect to migration of waste-generated gas. •
Evaluate models (conceptual and numerical) applicable to gas migration through barriers under realistic in-situ conditions.
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Provide data for further improvement of the EBS design with respect to gas migration (including the presence of a vent).
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Demonstrate the construction and the emplacement of an ILW-silo system under realistic insitu conditions.
The components of the field experiment (see Figure 1) are: a silo cavern 4.5 m deep and 4.0 m in diameter, a concrete silo (height of 2.5 m, outer diameter of 2.5 m) emplaced within the cavern, a gas vent in the top of the concrete silo, sand/bentonite backfill around the silo (20% bentonite
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and 80% sand) and, finally, sand/gravel backfill in the upper cavern above the silo cavern. Gas will be injected at the center of the concrete silo and its migration through the EBS and the adjacent geosphere will be monitored. The sand/bentonite was emplaced as a series of “lifts” which were compacted in situ. Instrumentation was emplaced in the compacted material at specific levels which have been used to define 11 layers (as shown in Figure 1) within the sand/bento
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