Radioactive waste containment in indurated shales: comparison between the chemical containment properties of matrix and fractures

Abstract

Abstract The chemical containment properties of poorly consolidated, plastic clays have received considerable attention, particularly during investigations related to radioactive waste disposal. However, the chemical containment properties of indurated shales have received rather less consideration. This paper summarizes those properties of consolidated shales which make them suitable for the chemical containment of radionuclides, using the example of investigations at the Tournemire research site in the south of France. The problems associated with investigating such low-permeability media are highlighted, and solutions to these problems are presented. At this research site, tectonic processes have induced centimetre-scale fractures and decimetre-scale faults in the shales, whereas excavation of a tunnel has generated metre-scale fracturing. Some low water flows through the fracture networks have been detected. However, the Tournemire shales are characterized by very low matrix pore-water contents, and any limited fluid mobility through the matrix is caused mainly by diffusion. Slowly flowing waters in fractures are chemically analogous to these matrix pore-waters. The hydrochemistry is of Na-Cl-( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>HCO</mml:mi> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> ) type, and the waters are slightly alkaline, strongly reducing and of low ionic strength. The resemblance between the chemistries of pore- and fracture-waters arises from the buffering of the pore-water chemistry by matrix mineral/water interactions, and possible subsequent diffusion of the pore-water into the fractures. The chemical buffering mechanisms controlling the water chemistries (especially pH, alkalinity and Eh ), are discussed and modelled. The chemical conditions will minimize the mobility of many radionuclides in both shale matrix and in fractures. Chemical evidence from the fracture-filling calcite is consistent with this hypothesis and demonstrates the past immobility of U and Cs. Batch experiments carried out in gloveboxes under a N 2 atmosphere and with synthetic fracture-water show weaker retention of Cs by the calcite fillings than by the shale matrix. Due to its very low porosity, the shale matrix should behave like a membrane filter for colloids, further reducing the mobility of radionuclides. In fractures, interface structures between matrix walls and calcite veins should also hinder colloid migration (except perhaps in occasional centimetre-scale apertures).