The objective of the study was to estimate the largest shear displacement that could be expected on a pre-existing fracture located in the repository area, due to the heat release from deposited waste. Two-dimensional numerical analyses using UDEC have been performed. The maximum shear displacement, at the fracture centre, amounts to 0.2–13.8 cm depending on fracture parameters. The fracture extension, friction angle and shear stiffness are found to be the most important.

Le but de cette etude est de determiner le deplacement maximal en cisaillement qui puisse se developper le long d'une fracture preexistante en reaction à la chaleur degagee par les dechets radioactifs enfouis dans la roche. Les analyses numeriques ont ete realisees à l'aide du code 2D UDEC. Le deplacement maximal en cisaillement enregistre au centre de la fracture varie entre 0.2 et 13.8 cm en fonction des proprietes de la fracture. L'ouverture, l'angle de friction et la rigidite au cisaillement semblent avoir une influence dominante sur les resultats.

Zielsetzung der Studie war die Abschatzung der maximalen Scherverschiebung, die auf einer innerhalb des Endlagergebietes existierenden Rissflache infolge der Warmeabstrahlung des eingelagerten Materials zu erwarten war. Es wurden zwei-dimensionale numerische Berechnungen mit UDEC durchgefuehrt. Die maximale Scherverschiebung im Zentrum der Rissflache betragt 0.2–13.8 cm, abhangig von den Risseigenschaften. Dabei hat sich herausgestellt, dass Rissgrösse, Reibungswinkel und Schersteifigkeit den grössten Einfluss haben.

Introduction

The primary function of a deep repository for spent nuclear fuel is to isolate the waste. If this isolation should be breached, a second function is to retard the transport of radionuclides from the fuel. The canister, the buffer and the host rock work in conjunction to provide these two functions. This study relates to the requirement of having mechanical stability of the rock around the repository. One of the concerns in this respect is the possibility of a large displacement occurring on a pre-existing fracture intersecting a deposition hole. Such a large displacement might jeopardise the intactness of a waste canister, i.e. break the isolation. A large fracture displacement might also result in increased ground water transport from the deposition hole to the surrounding rock mass, i.e. reduce the retardation.

The main difficulty in estimating the movements of large rock discontinuities, such as fracture zones or minor faults, is the lack of knowledge concerning their mechanical properties (Leijon, 1995). For the purpose of safety analyses it thus makes it inevitable to choose parameter values within wide ranges.

If it is assumed that a discontinuity can be approximated as a singular planar structure where the shear displacement is mainly controlled by friction, the most conservative case is a zero friction angle. An analytical calculation of a two-dimensional case (Figure 1) indicates that, with the most unfavourable orientation, the size of a discontinuity should be on the order of 100 m to get a displacement of 0.1 m. The intention with the analyses performed in this study was to add to the current state of knowledge by performing numerical analyses of a two-dimensional case.

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