A nonlinear finite element analysis is presented for a radioactive waste repository room assumed to be located at a depth of 1,000 metres in the Canadian Shield. The loading of the finite element model is both due to in situ stresses which exist prior to excavation and thermomechanical stresses arising from the radiogenic heat dissipation of the waste assumed to have a half life of 30 years and a gross thermal loading of 32 watts/m2 The influence of in situ stress, joint cohesion and joint friction angle on the isolation room stability and support requirements is examined for a simulated period of 30 years. For the range of in situ stress conditions, properties of the jointed rock mass, and the thermal loading considered, the extent of the rock failure is within the capability of conventional rock support measures.
This paper describes the results of a detailed rock mechanics study of the stability of radioactive waste isolation rooms using a baseline design geometry (Figure 1). The 5.0 m high x 6.3 m wide x 190 m long rooms are separated by is m thick pillars and are assigned to be located at 1,000 m depth in a granitic rock mass in the Canadian Shield. Radioactive waste canisters are assumed to be placed in holes drilled in the floor. The decay heat production is specified as 2S0 W per canister at the time of emplacement, with a gross thermal loading of 32 W/m2. The functional requirement assumed for an isolation room is that access for isolation and terrier ability of waste canisters should be available for about 25 to 30 years. The requirement does not necessarily exclude local failure or fracture of rock around the room. However, any failure resulting from long-term heating must not be violent or other than a local failure. The failed rock mass should be self-supporting, or capable of being supported over the life of the room. In a previous study (Acres, 1976), stress concentrations around the baseline configuration of an isolation room were obtained using a linear, two dimensional continuum, finite-element analysis approach. Stress levels around the periphery of the room were compared with the uniaxial compressive strengths and found to be generally acceptable. Several important rock mechanics considerations which were not included in the previous study are:
rock joints and their strength
possible mechanisms and criteria for failure of rock mass, including the phenomenon of thermal spalling
nonlinear behavior of rock mass under prolonged thermo-mechanical loads--modeling of progressive failure of rock around the room.
stabilization pressures for long-term Stability
parametric analyses--influence of in situ stress, rock mass strength and temperature conditions on room stability.
The objective of this study is to assess the rock mechanics feasibility of the baseline configuration (Figure 1) in view of the aforementioned consideration. The failure conditions that can be tolerated include local fracture or all-around fracture of rock to a limited distance (less than a metre) beyond the perimeter of an isolation room such that support requirements are minimal. However, the failed rock should either be self-supporting, or capable of being supported over the life of the room.