A coupled THMC model of the geochemical interactions of concrete and bentonite after 13 years of FEBEX plug operation

Abstract

FEBEX (Full-scale Engineered Barrier Experiment) was a demonstration and research project for the engineered bentonitic barrier of a radioactive waste repository in granitic rock. The FEBEX in situ experiment was performed with two heaters at Grimsel (Switzerland) in two operation periods. The 1st period lasted from 1997 to 2002 when heater #1 was dismantled. The 2nd period started after the emplacement of a metal cylinder and a shotcrete plug in the place of heater #1 and ended on June 2015. Here we present a coupled thermal (T), hydrodynamic (H), mechanical (M) and chemical (C) model of the geochemical interactions of the shotcrete plug and the bentonite buffer during the 2nd operation period (2002–2015). The model was performed normal to the concrete/bentonite interface (CBI) and was solved with the THMC code INVERSE-FADES-CORE V2. The bentonite near the CBI is fully saturated after 13 years and has a temperature of 30 °C. Model results show that dissolved Cl− diffuses from the bentonite into the concrete where it is sorbed on C–S–H phases. The high pH front (pH > 8) penetrates 0.1 cm into the bentonite after 13 years. Portlandite dissolves in the concrete and calcite, brucite and sepiolite precipitate in the concrete and in the bentonite at both sides of the CBI. C1.8SH dissolves in the concrete, especially near the CBI while C1.2SH precipitates. Smectite dissolves leading to the precipitation of analcime in the bentonite. The calculated porosity in the concrete near the CBI decreases strongly due to the precipitation of brucite, calcite, sepiolite and C1.2SH. Pore clogging is predicted in the concrete near the CBI after a few years. The computed porosity in the bentonite, on the other hand, increases slightly. Model results reproduce the measured Cl− concentrations, the pH in the concrete and the concentrations of exchanged cations in the bentonite. The computed mineral volume fractions reproduce the experimental mineralogical observations of portlandite, calcite, ettringite and C–S–H in the concrete and calcite, smectite, sepiolite, brucite and ettringite in the bentonite. There are model discrepancies, however, in the patterns of gypsum, brucite, Friedel's salt and C-A-S-H phases in the concrete and gypsum and C-A-S-H phases in the bentonite.