Backfilling of the underground facilities on a disposal site

Sugita, Yutaka ; Fujita, Tomoo ; Tanai, Kenji ; Hasegawa, Hiroshi; Furuichi, Mitsuaki*; Okutsu, Kazuo*; Miura, K.*

Regarding disposal techniques of high-level radioactive waste (HLW), the HLW is vitrified and then stored for cooling for a period of 30 to 50 years. After cooling, the HLW is isolated in the deep underground. The concept of geological disposal is based on the requirement to enclose the HLW in the deep underground for the long-term durability of the human's environmental safety. Backfilling of a repository is a unique activity on the geological disposal. If underground tunnels excavated to construct the repository are left, they may have significant influences on the barrier performance of an entire repository, such as: the mechanical stability of a tunnel may be damaged by rock stresses and a tunnel may provide a fast pathway for ground water flow. Therefore, the underground facilities are expected to be backfilled with a backfilling material after emplacement of the HLW and a buffer material. The material for the backfilling of the underground facilities is backfilling material. In this report, bentonite-aggregate mixture is considered, as one of the candidate materials for the backfilling material. Aggregate imitates the muck that is generated during construction phase of the underground facilities. The combination of backfilling, plugging and grouting is considered in some underground situations. Plug is composed of concrete material or clay-based one. Grouting material is concrete material or clay-based one, too. In this report, the concept of the backfilling, mechanical and hydrological characteristics of the bentonite-aggregate mixture, the function, work methods and a schedule of the backfilling materials, plugging and grouting are considered, and items of quality control for the bentonite-aggregate mixture, concrete material and grouting are listed.