Clay is a candidate host rock for high radioactive waste disposal. The understanding of the behaviour of natural elements in clay formations is an unavoidable step towards the understanding of the fate of such radionuclides. Further laboratory studies are also needed to help forecast, by analogy the behaviour of radioelement released from deep radioactive repositories.
The ARCHIMEDE project aims at three major goals
1. understanding the mechanisms of acquisition and regulation of formation water chemistry in clay environment, as a result of water rock interactions;
2. testing and validating the physico-chemical parameters that describe the clay formation and its properties and that feed geochemical models;
3. applying these results to model a real clay environment : Boom Clay formation (Belgium).
Rock sample characterization is generally realized in the laboratory after sampling. Sampling techniques usually applied can sometimes significantly disturb the original geochemical and microbial state of the sediment. In this study special attention is paid to prevent such a drawback: the Boom clay formation (solid, fluid and gaseous phases) is sampled from the Underground Research Laboratory facility in Mol (Belgium) under anaerobic and aseptic conditions, and experimental works are designed in order to measure non conservative (pH, Eh) or potentially versatile parameters (interstitial fluids, trapped gases, cation exchange capacity) directly in the formation. Microbiological techniques will be developed for natural sample bacteriological characterization, in conjunction with conventional bacteriological investigations.
Seven drill works in the HADES Underground Research Facility (URF) at Mol were completed in 1992, apart from one which encountered technical problems. Solid and fluid sampling: a 20 m long horizontal core was drilled in the URF for solid and fluid sample collection. The work was performed under sterile and anaerobic conditions.
Fluid sampling: 2 15 m long horizontal piezometres were drilled on both sides of the previous work in order to collect large amounts of interstitial fluid. As previously they were completed under sterile and anaerobic conditions. One of them is equipped to house and optical fibre dedicated to in situ pH determination.
Experimental work: In situ deep freezing and coring (2 cores 2 m long), active geochemistry piezometre setting, and pH/Eh probe setting for in situ measurement have been completed.
The four main topics of the project are as follows
1. fluid and solid sampling, in situ measurements.
2. fluid and solid characterization.
3. microbial investigations.
4. fluid-rock equilibria modelling will integrate the results obtained after the characterization step.