FEBEX-DP: Thermo-hydro-mechanical postmortem analysis of bentonite performed at CIEMAT.

Abstract

This document collects the results obtained by CIEMAT referring to the THM characterisation of the samples. Many of these results are also summarised and included in the synthesis report collecting results of the bentonite characterisation obtained by all the project partners NAB-16- 017 (Villar 2018). The results concerning the mineralogical and geochemical characterisation are reported in the NAB-16-025 (Fernández et al. 2018). The FEBEX project (Full-scale Engineered Barriers Experiment) studies the behaviour of components in the near-field for a high-level radioactive waste (HLW) repository in crystalline rock. The project was based on the Spanish reference concept for disposal of radioactive waste in crystalline rock (AGP Granito): the waste canisters are placed horizontally in drifts and surrounded by a clay barrier constructed from highly-compacted bentonite blocks (ENRESA 1995). As part of this project, an “in-situ” test, under natural conditions and at full scale, was performed at the Grimsel Test Site (GTS, Switzerland), an underground laboratory managed by NAGRA (ENRESA 2000, 2006). The thermal effect of the waste was simulated by means of heaters, whereas hydration was natural. The test was monitored, thereby obtaining data on the evolution of temperature, total pressure, water content, water pressure, displacements and other parameters continuously, in different parts of the barrier and the host rock. This information is then used to compare with the predictions of the thermo-hydro-mechanical (THM) and thermohydro- geochemical (THG) models. The basic components of the test (Fig. 1) were: the gallery, measuring 70 m in length and 2.3 m in diameter, excavated through the Aare granite; the heating system, made up of two heaters placed inside a liner installed concentrically with the gallery and separated from each other by a 1.0 m distance, with dimensions and weights analogous to those of the real canisters. The clay barrier is formed by blocks of compacted bentonite; the instrumentation, the monitoring and control system for data acquisition as well as supervision and control of the test both run autonomously and remotely from Madrid. Up to 632 sensors of very diverse types were initially installed for monitoring the different thermo-hydro-mechanical processes that occurred in both the clay barrier and the surrounding rock throughout the entire life of the test. The gallery was closed by a concrete plug. The clay barrier consisted of FEBEX bentonite, which was extracted from the Cortijo de Archidona deposit (Almería, Spain). The physico-chemical properties of the FEBEX bentonite, as well as its most relevant thermo-hydro-mechanical and geochemical characteristics obtained during the projects FEBEX I and II were summarised in the final reports of the project (ENRESA 2000, 2006) and later documents (Villar & Gómez-Espina 2009). To build the clay barrier, various types of blocks were manufactured from the bentonite in the shape of 12-cm thick circular crown sectors. The blocks were arranged in vertical slices consisting of concentric rings. In the heater areas the interior ring was in contact with the steel liner, whereas in the nonheater areas a core of bentonite blocks replaced the heaters (Fig. 2). The geometry and dimensions of the blocks are shown in Fig. 2, Fig. 3 and Tab. 1. The thickness of the bentonite barrier in the heater areas was 65 cm (distance from liner to granite). The bentonite slices were numbered from the back of the gallery towards the front, and those in which sensors were installed were called “instrumented sections” and given a distinctive reference letter. The backfilled area was sealed with a plain concrete plug placed into a recess excavated in the rock. The blocks were obtained by uniaxial compaction of the FEBEX clay with its hygroscopic water content at pressures between 40 and 45 MPa, resulting in dry densities of 1.69-1.70 g/cm3. The initial dry density of the blocks was selected by taking into account the probable volume of the construction gaps and the need to have a barrier with an average dry density of 1.60 g/cm3 (ENRESA 2000).