ABSTRACT:

Three-dimensional numerical rock mechanical stress/strength analyses for two nuclear waste repository design concepts are presented. The examined depths were between 300 m and 800 m. The stress/strength states were analysed based on the brittle rock strength criterion, where the resulting stress state is divided into four sections: elastic state, crack initiation stale, crack damage state and peak strength state. In the analyses, both horizontal and vertical deposition hole concepts are found to be feasible.

RÉSUMÉ:

Des analyses tridimensionnelles numeriques des contraintes et de la resistance mecaniques de la roche ont ete realisees pour les besoins d'etudes port ant sur deux depôts de dechets nucleaires, La profondeur des zones examinees a varie entre 300 et 800 m. Les etats de contraintes / resistance ont ete analyses sur la base du critère de resistance de la roche friable, dans lequel l'etat de contrainte resultant est divise en quatre sections: etat elastique, etat de debut de fissure, etat de fissure aggravee et etat de resistance maximum. II est ressorti de ces analyses que les orifices de depôt horizontaux comme verticaux etaient realisables.

ZUSAMMENFASSUNG:

Fuer zwei Planungskonzepte zur Lagerung radioaktiven Abfalls wurden dreidimensionale numerische gebirgsmechanische Spannungs/Festigkeits-Analysen durchgefuehrt. Die Untersuchte Tiefenregion lag zwischen 300 m und 800 m. Die spannungs/Fesrigkeits-Zustande wurden auf der Basis von Sprödgestein-Festigkeitskriterien analysiert, bei denen der resultierende Festigkeitszustand auf vier Sektionen, d.h. elastischer Zustand, Rißbildungsbeginn-Zustand, Rißbeschadigungs-Zustand und Spitzenfestigkeitszustand verteilt ist. In den Analysen wurden sowohl horizontale als auch vertikale Deponie- Bohrloch-Konzepte als brauchbar befunden.

INTRODUCTION

High horizontal in situ stresses in Finnish Precambrian bedrock are among the most important Parameters affecting the planning and understanding of the rock mass behaviour around an underground nuclear waste repository. The rock is mostly strong (peak strength ≥ 80 MPa) with good quality. When the magnitude of the secondary stresses is close to the strength of the intact rock, the occurrence of rock failure (spalling, rockburst, etc.) might be possible. This behavior occurs in some of the Finnish deep mines and, e.g., in massive granite at the Underground Research Laboratory (URL), in Canada. Posiva Oy (previously TVO) is planning a spent fuel repository in hard, crystalline rock at depths 300 - 800 m in Finland. The disposal concept consists of several parallel horizontal tunnels with vertical or horizontal deposition holes. Three sites were selected based on preliminary investigations conducted in 1987 - 1992. These are Kivetty (Kl), Olkiluoto (OL) and Romuvaara (RO) (Figure I). The area of the nuclear waste repository will be about 0.5 square kilometers and will contain different kinds of rock structures. The large size of the repository will make the evaluation of rock mechanical and geological parameters difficult. Thus, some assumptions are needed when the analysing repository facilities with numerical models. Even if modelings were always a simplification of reality, it helps to understand the mechanisms and general responses of a rock mass. The objective of the stress/strength analyses was to increase the understanding of the near field behavior of a nuclear waste repository. The analyses are based on the rock mechanical and geological information from Posiva Oy's site investigations made in 1987 - 1995. In the analyses of intact rock models, two different concepts for deposition holes are used; namely, the horizontal deposition hole and the vertical deposition hole (Figure 2). Examined depth levels are 300 m, 500 m and 800 m. It should be emphasized that the study does not consider the thermal loading from the waste nor the swelling pressure from the compacted bentonite used in backfilling.

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