Task 141: Post mortem bentonite analysis.

Abstract

The aim of FEBEX (Full-scale Engineered Barriers Experiment) is to study the behaviour of components in the near-field for a high-level radioactive waste (HLW) repository in crystalline rock. The experimental work consists of three main parts: - an “in situ” test, under natural conditions and at full scale, performed at the Grimsel Test Site (GTS, Switzerland); - a “mock-up” test, at almost full scale, performed at CIEMAT facilities (Madrid); and - a series of laboratory tests to complement the information from the two large-scale tests. The project is 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). In the two large-scale tests, the thermal effect of the wastes is simulated by means of heaters, while hydration is natural in the in situ test and controlled in the one performed on the mock-up. Both tests are monitored, this allowing 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 being used as a contrast to the predictions of the THM and THG models. The in situ test is performed in a gallery excavated in the granite of the underground laboratory managed by NAGRA at Grimsel (Switzerland). The basic components of the test are (Figure 1): the gallery, measuring 70 m in length and 2.3 m in diameter; 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 gallery is closed by a concrete plug. To build the clay barrier, various types of blocks were manufactured from the bentonite in the shape of a circular crown sector, with certain dimensional variations between the different types and with weights of between 18 and 23 kg. Figure 2 shows the geometry of the barrier in the heater and non-heater areas. In both areas, the three exterior crowns are equal; in the heater area the interior crown is in contact with the steel liner. The blocks were obtained through uniaxial compaction of the FEBEX clay with its hygroscopic water content at pressures of between 40 and 45 MPa. The heating stage of the in situ test, known as operational stage, began on February 27th 1997. After five years of operation (February 2002), the heater closer to the gallery entrance was switched off. After cooling of the system during four months, the bentonite barrier in front of and around the heater was dismantled and the heater extracted (Bárcena et al. 2003).