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.