Conceptual Design Of A Radioactive Waste Isolation Facility In Plutonic Rock

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A program was recently initiated by Atomic Energy of Canada Limited (AECL) to develop techniques and procedures for the ultimate disposal of high-level radioactive wastes in geological formations (1). One segment of this program involves the design, construction, and operation of an underground repository which will satisfactorily isolate the waste (2). In 1976, a preconceptual design study for such a repository situated in plutonic rock in the Canadian Shield was undertaken (3-11). This paper summarizes those aspects of the study that were concerned with the underground facility layout (5,8). The schedule for the repository development and operation, as provided by AECL (1), involves the operation of a test facility for 15 years, beginning in 1986, followed by the operation of a production repository facility for a period of 25 years. Based on an upper-limit estimate of the nuclear power generating capacity which could be operating in Canada to the year 2025, a repository facility must be able to contain all the wastes from approximately 700,000 tonnes of spent fuel. This would require disposal of approximately 1,082,000 canisters of high-volume reprocessing waste, with a heat production of 250 watts per canister at the time of emplacement. As an alternative, consideration was given to the disposal of 108,200 canisters with a heat production of 2,500 watts per canister at the time of eraplacement. For the purposes of this current study, a gross thermal loading of 32 watts/sq. meter (130 kw/acre)was utilized (4). The repository facility was supposed to be situated at a depth of 1,000 meters in a plutonic rock mass in the Canadian Shield. Available properties data from the literature (3) indicate that the plutonic rock will have a compressive strength (at ambient temperature) ranging from 138 to 275 MPa (20,000t o 40,000 psi); a Young?s Modulus of elasticity of 2 to 8 x 10⁴ MPa( 3 to 12 x 10⁶ psi); a thermal conductivity of 4 to 8 mcal/cm-sec. -0B0;C (1 to 2 Btu/hr.-ft.- °F); and a coefficient of linear thermal expansion of 6 to 8 x 10^-6 / °C. The rock mass is expected to have equivalent pemeabilities in the range of 10^-8 to 10¹¹ m/sec. due to systematic joints, although singular features with high local pemeabilities may exist. The purpose of this paper is to summarize the details of a repository facility layout for both single-level and dual-level construction, including the requisite excavation requirements. Consideration is also given to the various aspects of ventilation, mining equipment, excavated rock and radioactive waste handling and haulage, and safety. In addition, preliminary capital cost estimates for the facility are presented.

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During the course of the study, preliminary consideration was given to five concepts for the emplacement of radioactive waste canisters in excavations in a plutonic rock mass. These were:

• Multiple drillholes on a square grid pattern in the floor of a room, with one canister per drillhole (baseline emplacement concept);

• Multiple, long drillholes in clusters in floor of a room, with each drillhole containing multiple canisters;

• Multiple free-standing canisters of the floor of a room;

• Multiple canisters emplaced in a backfilled trench in the floor of room;

• Clusters of canisters emplaced in shafts or raises between rooms on two levels.