Final report on THM laboratory tests

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, − a “mock-up” test, at almost full scale, 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). The engineered barriers (waste, canister and clay barrier) are key elements in the final disposal concept for HLW. The clay barrier has the multiple purpose of providing mechanical stability for the canister, by absorbing stress and deformations, of sealing discontinuities in the adjacent rock and retarding the arrival of groundwater at the canister and of retaining/retarding the migration of the radionuclides released, once failure of the canister and lixiviation of the spent fuel have occurred. The behaviour of a HLW repository is determined, to a large extent, by the characteristics of the design and construction of the engineered barriers, and especially by the changes that may occur in the mechanical, hydraulic, and geochemical properties, as a result of the combined effects of heat generated by the radioactive decay and of the water and solutes contributed by the surrounding rock. Therefore, in FEBEX I, it was considered of as a main issue that the processes taking place in the near-field be understood and quantified, for the evaluation of long-term behaviour. As a consequence, the program of laboratory tests was designed to study and comprehend the processes that take place in the clay barrier under simple and controlled conditions and to develop the governing equations. These laboratory tests enabled to isolate the different processes, making their interpretation easier, and they provided fundamental data concerning the parameters to be used in the THM and THG models. The laboratory tests served also to support the checking of the predictive capacities of the THM and THG numerical models. FEBEX I showed that the THM and THG models are very sensible to small variations in some of the parameters that represent the properties of the materials. It became also clear that such parameters are not constant, but they may vary as a result of the changing conditions in the clay buffer. For these reasons, the FEBEX II project includes also a wide laboratory tests programme, which should focus on those aspects not sufficiently covered during FEBEX I, and in the new processes and scenarios that are being considered in this second phase of the project. This information will help in the interpretation of the results obtained in the “Mock-up” and “In situ” tests. Specifically, the objective of the THM tests to be performed during FEBEX II is to cover those aspects considered most significant or less known on the basis of the previous work concerning the two large-scale experiments. Sensitivity analyses have underlined the fact that the critical parameters are those included in the retention curves of the granite and bentonite, and the permeability to water of the bentonite for different states of saturation (relative permeability). Although considerable effort was made to determine these parameters through the tests performed within the framework of the FEBEX I project, the fact is that little information is currently available on the variation of these parameters with temperature and with the changes occurring in the clay microstructure during the swelling process.