FEBEX-DP onsite analyses report.

Abstract

The aim of FEBEX (Full-scale Engineered Barrier Experiment) was to study 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, this allowed the evolution of the temperature, total pressure, water content, water pressure, displacements and other parameters to be obtained continuously in different parts of the barrier and the host rock. This information was used as a contrast to the predictions of the thermo-hydro-mechanical (THM) and thermo-hydro-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 (Fig. 2); the heating system, made up of two heaters placed inside a liner installed concentrically with the gallery and separated one from the other by a distance of 1.0 m, with dimensions and weights analogous to those of the real canisters; the clay barrier, formed by blocks of compacted bentonite; the instrumentation and the monitoring and control system for data acquisition and supervision and control of the test both autonomously and remotely from Madrid. The FEBEX test initially contained 632 sensors of very diverse types, installed to monitor 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 was made 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). 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 with three concentric rings. In the heater areas the interior ring was in contact with the steel liner, whereas in the non-heater areas a core of bentonite blocks replaced the heaters (Fig. 3). The geometry and dimensions of the blocks are shown in Fig. 4 and Tab. 1. The thickness of the bentonite barrier in the heater areas was 65 cm. The blocks were obtained by uniaxial compaction of the FEBEX clay at its hygroscopic water content using pressures of between 40 and 45 MPa. These blocks had dry densities of 1.69 – 1.70 g/cm3, a density preselected 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).

This report compiles and analyses the results of the on-site measurements performed by AITEMIN during the dismantling of the FEBEX in-situ test (FEBEX-DP project). In particular the water content and dry density determinations carried out in samples taken from selected sections evenly distributed along the gallery (Fig. 6) are provided. Additionally, their relationship with the blocks' measured dimensions, the x-coordinate changes and other field observations are assessed.