Atomic Energy of Canada Limited is studying the underground storage of nuclear waste in a vault in hard rock in the Canadian Shield. As part of this work, models are being developed to assess the possible rates of escape of varlgus waste components through the rock to the biosphere. At the temperatures expected «150 C) ln the vicinlty of the vault, the response to stress perturbation will be creep, via macro- and microfracturing. As rock permeability and the overall stability of the vault will be affected by the extent of fracturing, it is important to establish the possible extent and kinetics of fracture processes. This paper describes some aspects of this work. It presents a two-dimensional finite element model that relates stress relaxation to microfracturing using linear elastic fracture mechanics. The kinetics of the process are derived from slow crack extension data for a local granite.


Atomic Energy of Canada Limited untersucht gegenwartig die Möglichkeit radioaktiven Abfall in einem unterirdischen Gewölbe, das in das harte Gestein des kanadischen Schildes gehauen werden soll, zu lagern. Als Teil dieser Arbeit werden Modelle entwickelt, die es erlauben das mögliche Ausmass des Entweichens der verschiedenen Abfallkomponenten durch das Gestein in die Biosphare zu bestimmen. In diesem Vortrag werden einige Aspekte dieser Arbeit beschrieben. Es wird ein zweidimensionales Finite Element Modell vorgefuhrt, welches mit Hilfe der linear elastichen Bruchmechanik eine Relation zwischen Spannungsrelaxation und Microbruch-sowie Reissverhalten aufstellt. Die Kinetik dieses Prozesses wurde aus experimentell ermittelten Daten abgeleitet, die der Untersuchung des langsamen Bruchvergrösserungsmechanismus galten. Fur diese Untersuchungen wurde lokal vorkommender Granit im trockenen Zustand bei 20°C und im mit Wasser gesattigten Zustand bei 80°C untersucht.


L''Energie Atomique du Canada Limitee etudie l''evacuation souterraine des dechetsnucleaires dans une enceinte pratiquee dans la roch dure du bouclier canadien. Dans le cadre de ces travaux, on met au point des modèles pour evaluer les taux possibles d''echappement de divers composants de dechets à travers la roche et vers la biosphere. Ce rapport decrit certains aspects de ces travaux. Il presente un modele a deux dimensions, par elements finis, reliant la relaxation des contraintes a la microfissuration. La cinetique des processus est derivee des donnees de propagation lente des fissures, exprimees sous l''angle de la mecanique des fissures lineaires elastiques. Le resultat de l''analyse est une evaluation de la facon cont, pour la roche intacte, les contraintes, la densite des fissures et le facteur de securite varieront en fonction de l''espace et du temps.


AECL is studying the possible disposal of solid nuclear waste in a vault in hard rock (Boulton 1978). Part of this work is to develop models that assess the rate of escape of various waste components to the biosphere. As active material can reach the biosphere through the rock mass, via water transport, a knowledge of rock permeability is essential. Hence, information is needed on rock fracture populations and on factors (stress state, etc.) that affect the flow of water through these fractures. Excavation of the vault and heat generated by active waste will disturb the original stress and temperature distributions in the rock mass. The major response to these disturbances in hard rock at the anticipated temperatures «150oC) will be microfracturing that occurs both i~~ediately and over a long time period. ~; the general stability of openings and rock permeability depend on the degree of fracturing, we must establish the kinetics and extent of the process. Also, because nuclear waste is involved, it is important to develop a sound theoretical basis for extrapolation to long times. Rock is a complex solid. It consists of an aggregate of different constituents each with different, and usually anisotropic, mechanical and physical properties. Heating produces fracturing due to elastic strains generated by differential expansion and the inherent brittleness of the material (Davidge 1981). It occurs with or without the presence of thermal gradients. Deviatoric stress and confining pressure also cause microfracturing, which in hard rock results in stress relaxation, modified displacement and a reduction in effective elastic modulus (decrease in stiffness). This paper presents a two-dim~nsional analysis that relies on linear elastic fracture mechanics (LEFM) to relate stress relaxation to microfracturing. The kinetics of the process are obtained from experimental data for slow crack growth in hard rock (Wilkins 1980).


In LEFM the important parameter is the stress intensity factor (K). It represents the magnitude of the stress occuring at a crack tip in an elastic solid (Rooke and Cartwright 1976). There are three modes of cracking, each characterised by a different stress intensity factor, i.e. tensile-opening (KI), in-plane shear (K II) and anti-plane shear (KIll)'' Above a critical value of the factor, for example KIC'' a crack is unstable and will propagate rapidly. In hard rock, crack shapes and orientations are widely distributed; hence, all cracking modes are possible.

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