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ceptual design study includes the layout of the underground facilities, the engineered barrier system, and the dimensions of the major repository system components needed to support the reference repository system design. This study is not site specific, because there are no site characterization data requiring repository performance to a specific location. Many design parameters and criteria are necessarily general or assumed.

DESIGN BASES

These design bases provide the information that identifies the specific functions to be performed by the repository system, as well as the specific values or ranges of values chosen as controlling parameters to bound the design. As a general guideline for this study2 the conceptual repository will be located in granitic rock, the major rock type in Korea, at a depth nominally of 500 m between two large fault zones that extend from this depth to the surface. The rock quality varies from highly weathered surface deposits to competent rock at the repository horizon. For this conceptual design the distance between faults should be about 7 to 10 km. Therefore, the repository layout must be capable of fitting within these major structural features. The most stringent requirement for the base-case underground facility is that the temperature of the bentonite buffer material remains below 100°C throughout the lifetime of the repository.

The development of the underground repository system conceptual design requires specifications of (1) design constraints and criteria, (2) disposal canister design, and (3) waste form and throughput. Key design constraints are summarized in Box 1.

BOX 1
Specifications for the Conceptual Design of an Underground Repository

  • Total spent fuel inventory to be packaged for disposal: 36,000 tHM[2]

  • Decay heat (40-year cooling): 385 watt/assembly for PWR, 2.28 watt/bundle for CANDU

  • The container filling material for void space within the container: carbon steel

  • Emplacement sequence: CANDU spent fuel first, followed by PWR spent fuel

  • Backfill design: vertical boreholes with bentonite buffer

  • Repository layout: single-level repository

  • Throughput: dictated by a 50-year operational life of repository

  • Thermal constraints: surface temperature of disposal container limited to 100°C

  • Waste package configuration: 4 PWR assemblies and 297 CANDU bundles

  • Borehole spacing: 3 m (CANDU) and 6 m (PWR)

  • Drift spacing: 40 m



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