Gas-Permeable Seal Test - Introduction
National Radioactive Waste Management Agency (ANDRA), France
National Cooperative for the Disposal of Radioactive Waste (Nagra), Switzerland
Korea Radioactive Waste Agewncy (KORAD), South Korea
Nuclear Waste Management Organization (NWMO), Canada
The degradation of organic substances and metals leads to gas production and accumulation in the emplacement caverns of radioactive-waste repositories. A concept for gas-permeable plugs and seals was developed in the framework of phase 1 of the Swiss sectoral plan process for deep geological repositories. The main aim of these plugs and seals is to increase the gas transport capacity of the backfilled underground structures without compromising the radionuclide retention capacity of the engineered barrier system. The design option is called "engineered gas transport system" (EGTS) and involves specially designed backfill and sealing materials e.g. seals made of sand/bentonite (S/B) mixtures which are part of the L/ILW repository concept.
Figure 1: Concept of the designed gas path (red arrows) for the L/ILW repository with inlay showing the detailed design drawing of repository seal mod. V4 (after Nagra, 2008).
The GAST experiment is a large-scale demonstration and validation experiment which focuses on the specific issue of S/B seal behavior during saturation, and the gas transport capacity in the later gas invasion phase. The consolidation behavior and the gas transport capacity of the EDZ and the intact host rock formation are disregarded intentionally. They are investigated in the HG-A experiment performed in the Mont Terri rock laboratory. The GAST experiment demonstrates the efficiency of the gas permeable seal and enables providing high-quality datasets at realistic scale to validate and, if necessary, improve the existing models in order to make reliable long-term predictions of the EGTS behavior.
Figure 2: Schematic layout of the GAST experiment
Previous experimental studies confirmed the high gas transport capacity of the S/B mixtures. These experiments have shown as well the ability to design S/B mixtures with specific target permeabilities for water and gas flow. First in-situ experiences were gained through the Gas Migration Test (GMT) at Grimsel Test Site (GTS), but with in-silo emplacement.A comprehensive laboratory program on combined gas/water transport in S/B mixtures has been conducted as part of the EU project FORGE.
Gas-Permeable Seal Test (GAST)