Clay Club Catalogue of Characteristics of Argillaceous Rocks

Boisson J.-Y
OECD / NEA / RWMC / IGSC (Working Group on measurement & Physical understanding of Groundwater flow through argillaceous media) august 2005 Report NEA n° 4436 (Brochure & CD-Rom including data base). OECD / NEA Paris, France 72 p

A wide spectrum of argillaceous media are being (or were) considered in NEA member
countries as potential host rocks for deep geological disposal of radioactive waste, i.e. from plastic,
soft, poorly indurated clays to brittle, hard mudstones or shales.
Among the favourable characteristics that are generally considered for argillaceous media are:
• thickness and continuity;
• low permeability and low hydraulic gradients;
• chemical buffering capacity;
• propensity for plastic deformation and self-sealing of fractures;
• geochemical characteristics that favour low solubility of radionuclides; and
• high capacity to retard the migration of radionuclides towards the accessible environment,
e.g. through sorption capacity and due to a diffusion-dominated transport.
For evaluating performance of deep geological formations, a site characterisation programme helps to provide the required and specific data such as the hydromechanical characteristics vis-à-vis
underground excavation works and operation, or hydrogeochemical characteristics with respect to the ability of the formation to limit the potential migration of radionuclides to the environment.
In that context, the OECD/NEA Working Group on the Characterisation, the Understanding and the Performance of Argillaceous Rocks as Repository Host Formations, namely the “Clay Club” examines the various argillaceous rocks that are being considered for the deep geological disposal of radioactive waste. The Clay Club promotes:
• a continuing inter-comparison of the properties of different argillaceous media;
• an exchange of technical and scientific information on clay properties and behaviour and on testing being carried out in underground research facilities; and
• a detailed review of the available and most promising investigation techniques for site characterisation.
Considering its overall objectives, one of the first initiatives of the Clay Club was to gather in a structured way the key geoscientific characteristics of the various argillaceous formations that are – or were – studied in NEA member countries with regard to radioactive waste disposal. The effort resulted in an internal catalogue of characteristics in the beginning of the 1990s. After several internal updates
(1995 and 1998) and restructuring resulting from end-users’ feedback, the NEA Clay Club considered it necessary and timely to prepare an open version of the catalogue. The present publication represents the outcomes of this Clay Club initiative.
Other past and current Clay Club activities cover the following topics:
• hydraulic and hydrochemical characterisation of argillaceous rocks (see Ref. 1); 4
• understanding of the basic concepts and mechanisms which control the movement of water, transport of solute and gas across a whole spectrum of argillaceous media being considered for radioactive waste disposal (see Ref. 2);
• evaluation of the occurrence of fluid flow through faults and fractures in argillaceous settings (see Ref. 3);
• assessment of the available pore water extraction methods and their respective advantages and limitations (see Ref. 4 & 5);
• description and assessment of current knowledge and relevance regarding the self-healing of argillaceous rocks under repository conditions (see Ref. 6);
• compilation of relevant “features, events and processes” specific to the disposal of long-lived waste in argillaceous formations and their current relevance for system understanding, namely “FEPCAT” (see Ref. 7);
• establishment of a list of relevant scientific references dealing with argillaceous media (see Ref. 8).