The repository layout builds on the information and key geologic constraints which define emplacement tunnel dimensions and borehole separation distances, borehole layouts and waste-receipt schedules. The basic design concept for the repository consists of various underground openings including the disposal area of spent fuels and expansion area, the service shaft complex and the ventilation exhaust shaft complex.

SATURATED ROCK

(Courtesy of KAERI-HLW Geological Disposal System Development Department)

The design concept for the saturated rock is based on the deposition of the HLW canister in the vertical hole. Prior to the receipt of the container, the compacted bentonite buffer is placed in the deposition hole. After container emplacement, the gap between the container and the buffer is filled with bentonite powder. The emplacement tunnel will not be backfilled during the operational period of 50 years for monitored storage. After the designed monitored-storage operation, all access and deposition tunnels will be backfilled with a mixture of crushed rock and bentonite and a concrete bulkhead will be constructed at the tunnel entrance to complete sealing for repository closure.

The base case repository for the saturated rock may include backfilling, particularly of the emplacement tunnels, immediately after waste emplacement or 50 to 100-years following the monitored storage period. Because the backfilling is intended to provide additional support for the closure of underground openings, the backfill timing and method are important parameters from the aspect of disposal safety, cost and political issues (e.g. retrieval for reuse or change of safety constraints). The backfill composition should be determined through more detailed performance assessments and engineering trade-off studies.

UNSATURATED ROCK

The basic design concept for the unsaturated rock comes from the U.S. HLW project (Yucca Mt.). The canisters of spent fuels will be placed in horizontal emplacement drifts without buffer materials. A high thermal load generated from the HLW will drive the moisture in the rock away from the canisters. However, once the repository cools down, water vapor condenses. The engineered barriers, including double layered canister, invert in the floors, and drip shields, will prevent the waste package from dripping water.

Yucca Mountain Science and Engineering Report describes the following design concepts based on two different operating temperature modes.

For the higher-temperature operating mode, the packages would be spaced about 10 cm (4 in.) apart. The base design includes 58 horizontal emplacement drifts excavated to a 5.5-m (18-ft) diameter at a center-to-center drift spacing of 81 m (266 ft). The total subsurface area required to accommodate 70,000 MTHM is about 1,150 acres. For the lower-temperature operating mode, a larger area (up to about 2,500 acres) may be required. In the present design, the underground facility would be constructed over a period of about 23 years.