An attempt is made to predict the thermomechanical stability of a conceptual disposal vault for immobilized irradiated fuel on the basis of numerical modelling and fieldobservations. For study purposes, it is assumed that the disposal vault would be located in a granitic pluton in the Canadian Shield, at a reference depth of 1 km from the surface. The results of in-situ stress measurements have revealed a state of high horizontal compressive stresses in many parts of the Canadian Shield. Field observations indicate that fracturing and spalling of surficial layers of rock could result if the combination of thermal, in-situ and axcavation stresses exceeds the strength values of the rock. The rock stresses are computed in two steps using the finite element technique. A simulation of the excavation of the disposal vault is first carried out, taking into account the in- Situ stresses. This is followed by a detailed analysis of the transient temperatures and thermal stresses generated by the radiogenic heat source at various times during the postemplacement period.
Es wird versucht, die thermomechanische Stabilitat einer sich noch im Planungsstadium befindenden Aufbewahrungsgrube fuer bestrahlten Brennstoff durch zahlenmassige Modellierung und Feldbeobachtungen vorauszusagen. Zwecks Untersuchung wiru angenommen, dass die Aufbewahrungsgrube in einem granitischen Pluton im "kanadischen Schild" in einer Tiefe von 1 Kilometer liegt. Die Ergebnisse der in-situ Spannungsmessungen haben einen Zustand hoher horizon taler Druckspannungen in vielen Teilen des kanadischen Schildes gezeigt. Feldbeobachtungen zeigen, dass sich Bruchbildung un Abblattern der Oberflachenschichten des Gesteins ergeben könnten, wenn die Kombination von thermischen, in-situ und durch Sprengungen verursachten Beanspruchungen die Widerstandswerte des Gesteins ueberschreitet. Die Felsbeanspruchungen werden in zwei Schritten mittels der Finite-Elemente-Methode berechnet. Eine Simulation des Ausbruchs der Aufbewahrungsgrube wird zunachst durchgefuehrt, die die in-situ Beanspruchungen in Betracht zieht. Es folgt eine ausfuehrliche Analyse der Übergangstemperaturen und der Warmespannungen, die von der radiogenischen Warmequelle zu verschiedenen Zeiten wahrend der Periode nach der Herstellung erzeugt werden.
On essaye de predire, sur la base de la modelisation numerique et des observations sur le site, la stabilite thermomecanique d''une caverne de stockage à l''etat de la conception, destinee à recevoir le combustible irradie immobilise. Aux fins de l''etude, on suppose que la caverne de stockage soit situee dans un pluton granitique du Bouclier canadien, à une profondeur de reference d''un kilomètre de la surface. Les resultats de mesures in situ des tensions ont demontre un etat de tensions compressives horizontales elevees dans plusieurs parties du Bouclier canadien. Les observations faites sur place indiquent que la fissuration et l''ecaillement des couches superficielles du rocher pourraient resulter si la combinaison des tensions thermiques, in situ et dues au creusement, depasse les valeurs de la resistance du rocher. Les tensions du rocher sont calcules en deux etapes, utilisant la technique des elements finis. D''abord on effectue une simulation du creusement de la caverne de stockage, en tenant compte des tensions in situ; puis on effectue une analyse detaillee des temperatures transitoires et thermiques generees par la source radiogenique de chaleur, à divers moments pendant la periode suivant l''implantation.
Of all the alternatives considered for the ultimate disposal of nuclear fuel waste, geologic disposal appears to be the most promising and most extensively studied. The geologic media currently being investigated include crystalline hard rocks, salt and argillaceous formations. By far, the majority of the research programs - including the Canadian program - has been devoted to the study of crystalline hard rocks. Specifically, the Canadian program focusses on the development of a disposal vault in a suitable pluton in the Canadian Shield, which is known for its high degree of stability over geologic times. Four categories of plutonic rocks are being considered, namely, granite, gabbro, syenite and anorthosite, with granite being the most abundant in occurrence. For study purposes, a reference depth of approximately 1 km has been assumed for this disposal vault. No decision has yet been made on whether to reprocess the irradiated fuel from Canada''s CANDU reactors. Hence, the nuclear fuel waste to be disposed of eventually could be in the form of either immobilized irradiated fuel or immobilized reprocessing waste. In the latter case, the plutonium would have been extracted for use in advanced fuel cycles. In either case, heat would be generated as a result of radioactive decay. This paper examines the mechanical effect of radiogenic heat on the rock mass around an irradiated fuel disposal vault. The thermomechanical response of the host rock has been generally recognized as a basic research requirement in the geologic disposal of nuclear fuel waste. In Canada, this has been a subject of investigation since the inception of the Canadian nuclear fuel waste management program. As a matter of fact, one of the few case histories of thermal spalling actually took place in the Canadian Sheild. In the following sections, the paper will give a brief description and interpretation of this case history, followed by some typical results of thermomechanical analyses of a conceptual disposal vault for irradiated fuel.
