Atomic Energy of Canada Limited is examining the disposal of nuclear-fuel waste in a vault in plutonic rock. There is interest in the long-term stability of rock openings, and the surrounding rock, as changes could influence the transport of waste components to the biosphere. The dominant mechanism of deformation of intact rock is microcracking, at temperatures below 150°C. In time, the stress safety factor is reduced because of microcracking. Stress safety factor is defined as the ratio of the principal stress at failure to the applied principal stress. MCDIRC considers the perturbation of the rock-mass by a vertical-cylindrical opening and calculates the initial distribution of safety factor around the opening. Then, changes with time in the safety factor, caused by microcracking are estimated to find: the movement of the opening wall, the strain in the radial direction, and the extent of decrepitation at the opening wall.
Atomic Energy of Canada Limited are examining the possibility of disposal of nuclear-fuel waste in a vault in plutonic rock. The stability of rock openings and surrounding rock that comprise the vault is of interest. For example, changes in the dimension of an opening may transmit unwanted loads to the contents of the opening. Changes may also alter the local rock permeability. The mechanism of deformation of intact rock at temperatures below 150°C (including creep) is microcracking (Cruden 1974). Other mechanisms are insignificant at these low temperatures. Therefore, a model has been created to assess the stability of rock around an opening in terms of microcrack-driven creep. Rock joints are important to the stability of openings and to water flow. However, they are ignored here and the rock mass is assumed to be intact. The model assumes both pre-existing microcracks and the creation of new ones. Other than the additional assumption of a reasonably uniform distribution of microcracks no extra detail is required.
Essentially, plutonic rock is an elastic solid containing many cracks of various shapes and sizes. Its matrix is an aggregate of crystals of several minerals, which are incapable of plastic behaviour at low temperatures. Each crack alters the local stress state. The important parameter is the stress intensity factor (K), which is a measure of the magnitude of the stress at a crack tip. At a critical value (Kc) the energy required to grow the crack is less than the energy released in the growing process. Hence, the crack becomes unstable and grows rapidly. Values of A and n are available for several plutonic rocks, including Lac du Bonnet granite, and for other rocks types, see Table 1, (Wilkins, Reich and Vallace 1984, Henry, Paquet and Tancrez 1977, Atkinson, Rutter, Sibson and White 1979). In rock mechanics, it is common to refer to a strength 'safety factor' as the ratio of the principal stress at failure to the applied principal stress, i.e., (σ1f/σ1). In this paper, for convenience, safety factor is referred to as Y.
