Gas and water sampling from the FEBEX in situ test

Abstract

The FEBEX (Full-scale Engineered Barrier EXperiment in Crystalline Host Rock) full-scale heating in situ test was performed at the Grimsel Underground Laboratory in Switzerland with the general aim of studying the behaviour of near-field components in a repository for high-level radioactive waste. One of the objectives of the FEBEX in situ experiment was to analyse the gases generated and transported through the bentonite barrier because the accumulation, consumption and release of gases from a HLW repository system may affect a number of processes that influence the long-term safety (e.g., gas pressure build-up, changes in bentonite properties, influence of groundwater transport, radiological hazard, ….). In different phases of the FEBEX in situ test (First Operational Phase: 1996-2002, and Second Operational Phase: 2002-2015), various pipes were introduced inside the bentonite buffer for analysing the evolution of the gases and the chemical composition of the porewater. For this purpose, different gas/water sampling campaigns were performed since 1996 until 2011, before and after dismantling of Heater #1. Two additional campaigns were carried out in 2014 and 2015 during the FEBEX-DP Project for understanding the gas generation/consumption processes in the buffer and to establish the chemical conditions and redox state inside the bentonite barrier. The main gases detected in the FEBEX in situ during the 18 years of experiment were O2, N2, CO2, CO, H2, CH4 and other saturated and unsaturated aliphatic hydrocarbons (alkanes up to C5 and alkenes). Fluid pressures remained close to atmospheric pressure because the FEBEX in situ test cannot be considered as a gas-tight system, and any excess of gas pressure generated could be dissipated. The highest fluid pressures measured (between 2 and 4.5 bar) at the beginning of the experiment were related to water pressures because of the water inflow during the saturation process of the bentonite. Furthermore, part of the gases identified were consumed in different (bio)-geochemical processes. Although the concentration of oxygen decreased over time, oxygen was never depleted completely, showing minimum values between 3 and 0.2 vol.%. However, the presence of hydrogen, methane and other hydrocarbons indicate the presence of spatial microenvironments where anoxic/anaerobic conditions were established, as confirmed by the reducing redox potentials calculated from hydrogen contents and measured in some pore waters collected from the different pipes.