The response of granite to the environment of an underground nuclear fuel waste vault is examined through laboratory experiments using specimens of intact granite. Conventional creep tests and specially designed crack growth experiments suggest that void volume created through microfracture at 140 MPa may be less than one tenth of one percent. At higher stresses, the granite may enter into steady-state creep and eventually fail. Because stresses of this magnitude will either be unlikely or limited to very small areas, the creation of a significantly large high permeability zone in the geological barrier is not expected.
Die Reaktion des Granits zu der Umgebung von Felshohleraumen fuer Atomabfallprodukte wird durch Laborexperimente untersucht, indenen Proben von unversehrten Lac du Bonnet Granit verwendet werden. Normale Kriechversuche und spezial entwickelte Experimente fuer Rissvergroesserungen deuten daraufhin, dass das Gesamtvolumen der Mikrorupturen bei 140 MPa wehniger als ein zehntel Prozent ist. Bei groesseren Druckspannungen von solcher Groesse unwahrscheinlich sind,oder nur in sehr kleinen Flachen vorkommen, wird keine bedeutsam hohe Durchlassigkeitszone in der geologischen Barriere erwarted.
Grace à des experiences menseen en Laboratoire, où on utilise des echantillons de granit provenant de la regeion du Lac du Bonnet, Manitoba, on essaie de tester les reactions qu''un depôt de dechets nucleaires pourrait exercer sur la roche. Selon les tests conventionnels de fluage et des experiences plus specifiques destinees a mesurerl''extension des fissures, on est porte à croire que Ie volume du vide cree par la micrbfracture a 140 MPa n ''atteint que Ie dixieme d''un pour cent. Sous des compressions plus elevees, Ie granit peut se deformer lentement et, à long terme, il peut se fracturer. Mais il est peu probable que des contraintes de cette ampleur soient appliquees ou si elles devaient l''être, elles seraient reduites à des secteurs d''une superficie limitee. De ce fait, il y a peu de chances qu''une zone de haute permeabilite d''une etendue considerable Se forme dans la roche.
The Canadian nuclear fuel waste management program is administered by the Whiteshell Nuclear Research Establishment of Atomic Energy of Canada Limited. Field and laboratory investigations are now under way to locate and then develop an underground Waste disposal vault in a hard rock formation of the Canadian Shield. This paper examines the problem of the long-term response of granitic rock to the anticipated environment of a nuclear fuel waste management site. The rock used in the experiments is Lac du Bonnet granite obtained from a quarry near Lac du Bonnet, Manitoba, Canada. An underground research laboratory is now planned for location in the same granite.
Many aspects of the design and construction of the nuclear fuel waste vault have not been finalized yet so that the environmental conditions to which the granite will be subjected during its service-life are still subject to some speculation. Conventional wisdom, however, suggests that the relevant factors include the state of stress, including its time dependence, and the temperature and the chemistry of the groundwater.
The stress to which the rock in a disposal vault will be subjected is determined by the state of the presently acting tectonic stress (the primary state of stress), the geometry of the underground development, the method of excavation, and the thermal load generated by the radioactive decay of the fuel waste. There have been a number of measurements of tectonic stresses in the Canadian Shield (Herget, 1980), indicating that insitu stresses'' at 1 km depth (the maximum which is now under consideration) may be postulated as follows. The maximum principal stress is horizontal trending easterly with a magnitude of about 45 MPa. The vertical direction coincides with the minimum principal stress direction, its magnitude being equal to the gravitational load, i.e. 25 MPa. The third, the intermediate principal stress is again horizontal with its magnitude varying widely between 25 and 45 MPa. With its magnitude depending on the general outlay of the underground installations, a concentration of stress occurs at the rock perimeter as the openings are excavated (the secondary state of stress). The magnitude of the stress concentration, is influenced by the construction technique as well. Drilled tunnels in relatively unfractured rock, like the Lac du Bonnet granite, could fully develop the theoretical elastic stress concentrations. Conversely, a conventional construction method using explosives damages the rock walls and consequently shifts the stress concentrations inward into the rock with some loss in magnitude. The stress concentration factor around a single opening ranges, typically between one and three. Even after allowing for ther~al loading and the effect of overlapping stress fields between intersecting or adjacent developments, a stress concentration factor greater than three to four is unlikely to occur.