Demonstration of Gas-Permeable Seals for Radioactive Waste Repositories: Laboratory and In-Situ Experiments

Abstract

In low/intermediate-level waste (L/ILW) repositories, anaerobic corrosion of metals and degradation of organic materials produce hydrogen, methane, and carbon dioxide. Gas migration in a L/ILW repository is one of the processes evaluated in the safety assessment of deep geological disposal in low-permeability formations, in particular with respect to the development of gas pressures in the repository caverns which could negatively affect the host rock or the engineered barrier system (EBS). In order to restrict build-up of gas overpressures in the emplacement caverns, Nagra (National Cooperative for the Disposal of Radioactive Waste, Switzerland) has proposed design options aimed at increasing the gas transport capacity of the backfilled underground structures, compromising neither the low hydraulic conductivity nor the radionuclide retention capacity of the EBS (Nagra, 2008). They involve specially designed backfill and sealing materials such as high porosity mortars as backfill materials for the emplacement caverns and sand/bentonite (S/B) mixtures with a bentonite content of 20% to 30% for the seals themselves and for backfilling other underground structures. These increased gas permeability materials can supplement the gas flow that is expected to occur through the excavation damaged zone (EDZ) and avoid the creation of overpressures. Preliminary experimental studies have confirmed the gas transport capacity of the S/B mixtures and demonstrated the ability to design mixtures with specific target permeabilities for water and gas flow (Nagra, 2008). Two-phase flow modelling studies have shown that the gas transport capacity of seals is largely dependent on their permeability and length. More detailed models of sealing elements show a rather complex history of seal saturation during the early saturation phase and the later gas escape phase (Gaus et al., 2010). Note, however, that current modelling approaches are based on parameters and conceptual understanding of small-scale laboratory experiments. Two large(r) scale experiments which aim at validating and, if necessary, improving current conceptual models for the resaturation and gas invasion processes into S/B seals and the determination of up-scaled gas / water permeabilities of S/B seals (i.e. two-phase flow parameters for large-scale models) have been initiated and will be highlighted in the paper. The first one, a mock-up experiment, was set up in 2010 as part of the EU 7th FP project FORGE, aiming at demonstrating seal performance on an intermediate (decimetre scale). The second one is a large-scale experiment (metre-scale), the Gas-Permeable Seal Test (GAST), which was also initiated in 2010 at the Grimsel Test Site (GTS). For GAST, a seal will be emplaced at the GTS to demonstrate the effective functioning of gas-permeable seals on a realistic scale and with realistic boundary conditions (‘proof of concept’).

Phenomenological Model of Nonlinear Dynamics and Deterministic Chaotic Gas Migration in Bentonite: Experimental Evidence and Diagnostic Parameters

“…There are three principal clay-based repository projects ongoing in Europe, using the Opalinus clay rock (Early Jurassic) in Switzerland, the Callovian-Oxfordian clay rocks of the Middle to Late Jurassic in France, and the Boom clay rock (Early Oligocene) in Belgium (Grambow 2016). Over the past 30 years, a series of projects have been performed to understand gas migration-MEGAS (Volckaert et al 1995;Ortiz et al 1997Ortiz et al , 1998, PROGRESS (Rodwell 1999(Rodwell , 2000, and FORGE (Rueedi et al, 2011). The purpose of Task A (modeling Gas Injection ExpERiments-ENGINEER) of the International Project DECOVALEX-2019 (https:// decov alex.…”

A dual-porosity model for the study of chemical effects on the swelling behaviour of MX-80 bentonite

“…Bentonite has been widely considered as seal materials in nuclear waste disposal facilities [1,13,15,43,44,53]. Bentonite is primarily composed of montmorillonite minerals.…”