The case study employed many conservative assumptions. In particular, the waste was placed close to a fracture zone and the critical group used a water supply well drawing water from the fracture zone at the center of the contaminant plume from the repository. At any actual future disposal site, the waste would be located so as to allow the characteristics of the groundwater flow systems, the groundwater chemistry, and the rock structure to enhance the safety of the disposal system.17 Therefore, a larger margin of safety than indicated by the case study would be expected.
A technically suitable site would be one at which it could be demonstrated that nuclear fuel waste disposal would meet all applicable criteria, guidelines, and standards for protecting human health and the natural environment. We believe that technically suitable disposal sites are likely to exist in Canada because
(i) the qualitative considerations of the performance of a repository indicate that a disposal system would be safe in the long-term;
16 K. Mehta, G. R. Sherman, and S. G. King, ‘Potential Health Hazard of Nuclear Fuel Waste and Uranium Ore’, Atomic Energy of Canada Limited Report, AECL-8407, 1991.
17 C. C. Davison, A. Brown, R. A. Everitt, M. Gascoyne, E. T. Kozak, G. S. Lodha, C. D. Martin, N. M. Soonawala, D. R. Stevenson, G. A. Thorne, and S. H. Whitaker, ‘The Disposal of Canada’s Nuclear Fuel Waste: Site Screening and Site Evaluation Technology’, Atomic Energy of Canada Limited Report, AECL-10713, COG-93-3, 1994.
(ii) the numerical results of the postclosure assessment case study show a very large margin of safety between estimated effects and the regulatory limit, even though many conservative assumptions were made;
(iii) the geological conditions assumed for the case study are based on conditions at an actual location in plutonic rock of the Canadian Shield and are not unusual; and
(iv) plutonic rock is abundant on the Canadian Shield.
Disposal of nuclear fuel waste would proceed in sequential stages: siting (possibly\20 years), construction (\5 years), operation (at least \20 years and possibly more than 80 years to dispose of 5 to 10 million fuel bundles), decommissioning (\10 years), and closure (\2 years). Decommissioning and closure could be delayed to permit extended monitoring. The stages and activities of concept implementation are briefly described below.
The siting stage would involve identifying a site for a repository through community participation and technical investigation. From the large regions of plutonic rock available on the Canadian Shield, a small number of areas would be identified. These areas would be investigated in detail, a preferred site would be identified, and approval would be sought for construction of a disposal facility at that site.
The construction stage would involve constructing the facilities and systems needed to begin disposing of nuclear fuel waste. These would include transportation facilities and equipment, access routes, utilities, surface facilities, shafts, tunnels, underground facilities, and some or possibly all of the disposal rooms. All systems would undergo testing in preparation for full operation in accordance with legislative requirements.
The operation stage would involve transporting nuclear fuel waste to the repository, putting the waste into long-lasting containers, and emplacing the containers and sealing materials in the repository. At the same time, construction of disposal rooms would continue if all rooms had not been completed during the construction stage. The operation stage could begin with a demonstration of operation, during which the disposal rate or repository size might be limited. The construction schedule might also be affected.
After the operation stage, decommissioning would be delayed to allow for extended monitoring if the implementing organization, the regulatory agencies, or the host community required additional data on the performance of the filled, partially sealed repository. Similarly, after the decommissioning stage, closure would be delayed to allow for extended monitoring if the implementing organization, the regulatory agencies, or the host community required additional data on the performance of the sealed repository.
The decommissioning stage would involve the decontamination, dismantling, and removal of the surface and subsurface facilities. It would also involve the sealing of the tunnels, service areas, and shafts, and of the exploration boreholes drilled from them. The site would be rehabilitated and markers could be installed to indicate the location of the repository. Access to any instruments retained for extended monitoring would continue to be controlled.
The closure stage would involve the removal of monitoring instruments from
any exploration boreholes that could, if left unsealed, compromise the safety of the disposal system, and then the sealing of those boreholes. During the closure stage, the objective would be to return the site to a state such that safety would not depend on institutional controls.
Activities such as obtaining approvals, characterization, monitoring, design, assessment of environmental effects, and management of environmental effects would be ongoing. Implementation of the disposal concept would entail a series of decisions about whether and how to proceed,18 and the involvement of potentially affected communities would be sought and encouraged throughout all stages.7
A preclosure assessment case study of a hypothetical disposal system indicates that a disposal facility and transportation system similar to those assessed could be implemented while protecting the public, the workers, and the natural environment.19
6 Conclusions
Although current storage practice is a safe interim measure for the management of used fuel, eventually nuclear fuel waste must be disposed of. Research and development spanning more than 15 years indicates that the concept of disposal in plutonic rock of the Canadian Shield meets the general requirements for an acceptable disposal concept, and that implementation of this concept represents a means by which Canada can safely dispose of its nuclear fuel waste.
Acknowledgment
The Canadian research and development described in this article was performed as part of the Nuclear Fuel Waste Management Program, which is funded jointly by Atomic Energy of Canada Limited and Ontario Hydro under the auspices of the CANDU Owners Group.
18 C. Allan, ‘Building Confidence in Deep Geological Disposal of Nuclear Fuel Waste: Canada’s Approach’, Presented to the International Nuclear Congress INC93, October, 1993.
19 L. Grondin, K. Johansen, W. C. Cheng, M. Fearn-Duffy, C. R. Frost, T. F. Kempe, J. Lockhart-Grace, M. Paez-Victor, H. E. Reid, S. B. Russell, C. H. Ulster, J. E. Villagran, and M. Zeya, ‘The Disposal of Canada’s Nuclear Fuel Waste: Preclosure Assessment of a Conceptual System’, Ontario Hydro Report N-03784-940010 (UFMED), COG-93-6, 1994.
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