ASSESSMENT OF NEAR-FIELD ROCK MASS FRACTURING AROUND A POTENTIAL NUCLEAR-FUEL WASTE REPOSITORY IN THE CANADIAN SHIELD

Atomic Energy of Canada Limited (AECL) is developing a concept for the permanent storage of Canada's nuclear fuel wastes. As of 1992, the co ncept was based on indoor emplacement of waste canisters within an eng ineer ed repository constructed at depths of 500-1000m in plutonic roc ks of the Canadian Shield. Canadian law requires that the Federal Envi ronmental Assessment Review Office (FEARO) undertake a formal review o f the concept before AECL proceed with site selection and design. In p reparation for this upcoming reuiew, the federal ministry which is res ponsible for the environment, Environment Canada, commissioned a small panel to undertake an overview of the concept. The sole purpose of th e panel's work was to identify issues of potential importance to the s afety and acceptability of the storage concept, so that the future rev iew process could be appropriately focused on these key issues. Althou gh fracture mechanics research has made much progress in under standin g rock mass failure in terms of fundamental fracture initiation, propa gation and interaction processes, there is still no generally accepted method by which to evaluate the actual fracturing response of a real rock mass in situ. Nevertheless, public review of the repository conce pt must address the issue of potential rock mass fracturing around the underground openings. The challenge is to do so despite the lack of a generally agreed methodology, in a manner which is practical within t ight budgetary limits, acceptable in the context of normal engineering practice, and sensible from a fracture mechanics perspective. The key question is whether simple rock engineering analyses, based on conven tional stress-strength comparisons, can be used to assess the potentia l for near-field fracturing of the rock mass. The goal of the work rep orted in this paper was to undertake a realistic but limited pie-feasi bility level engineering evaluation of the potential for near-field fr acturing of the rock mass, particularly in the vicinity of the caniste r emplacement holes. The approach chosen was based on simple parametri c modelling using boundary element analyses. Input parameters were sel ected to conditions appropriate to batholiths of the Canadian Shield, The results, based on conventional rock engineering stress-strength an alyses, were evaluated within the context of current fracture mechanic s research views and of engineering judgement derived fi om experience with rock mass fracturing around deep underground openings in the min es of the Canadian Shield. Results indicate that stress concentration effects will cause fracturing beneath the room floors, and experience suggests that rock damage will be mainly in the form of horizontal sla bbing. With time, rock damage may extend over the full 5m depth of the emplacement holes. These findings suggest that the concept of in-floo r storage of the waste canisters must undergo detailed critical review before being accepted as an inherent part of the AECL disposal concep t.