An underground-undersea repository for the disposal of low-level radioactive waste is being considered for a small island offshore from Taiwan. The preliminary site characterization provided data to define intact rock properties, discontinuity characteristics, hydraulic conductivity, in situ stress, and rock mass quality using RQD and the Q system. The preliminary design includes a 4-km access tunnel and 28 disposal chambers. Rock reinforcement estimates are based on empirical guidelines provided by the Norwegian Method of Tunneling (NMT) support charts. The excavation stability was evaluated by wedge fallme analyses and by two-dimensional boundary element modeling. The performance assessment for evaluating radionuclide migration included groundwater model simulations with variable-density, equivalent-porous-media flow to calculate groundwater gradients and flow paths. Radionuclide transport was analyzed as onedimensional fracture flow through the rock mass, with diffusion dominating the migration through the engineered barriers in the disposal chambers.
An offshore subseabed repository for the disposal of low-level radioactive waste (LLRW) is being considered for an island near Taiwan. The island surface and subseabed consist mainly of competent granitic rock. The island presents several challenges to be accommodated limited useable area, remote location, and potential for high seawater elevations. The preliminary site investigation provided data from vertical and inclined boreholes (some drilled to a depth of 300 m), laboratory testing, surface geologic mapping, in situ stress testing, and geophysical surveys (ground surface, marine and borehole). Many different layouts and configurations were evaluated during the preliminary design. The design considered the best for this site has the repository located within 400 m offshore from the island and approximately 100 m below sea level. The disposal area consists of 28 disposal chambers capable of accommodating 1,030,000 drums. The chambers will be accessed from the island through a tunnel. The tunnel forms a large loop north of the island, with tunnel portals on the island surface. The layout design includes flexibility to accommodate phased construction, separation of construction and operations areas, and future expansion.
Excavation of the repository will use pre-splitting and smooth-wall blasting tunneling techniques to remove more than 1,000,000 m 3 of rock for the underground repository and preserve the intact properties of the rock mass. Closure will consist of grout backfill around the drums placed inside concrete cells, closing each end of the disposal chambers with thick concrete plugs, constructing high-integrity seals within the access tunnel, and backfilling the access tunnel with muck.