Basic Examination of the Backfilling Material under Synthetic Seawater Condition

Kikuchi, Hirohito*; Tanai, Kenji ; Sugita, Yutaka 

In geological disposal of high-level radioactive waste, closure of repository is the technique of filling clearance using the backfilling material to preserve barrier performance of the engineered barrier system. One of the expecting behaviors of the tunnel backfilled with backfilling material is appearance of a clearance between backfilling material and concrete lining due to alteration of the concrete lining. Therefore, filling (swelling) performance is one of the required performances of the backfilling material. Then, basic examination of the backfilling material is performed. This examination focused on the feasibility of the backfilling material described in H12 report and the adequate bentonite/sand mixture to obtain conservative filling clearance performance.Results of the examination showed, under test conditions that 30% of the volume of concrete lining decreases due to alteration and such volume become clearance between the backfilling material and concrete lining, in distilled water condition, the specification (bentonite/sand mixture) of the backfilling material described in H12 report almost filled the clearance. However, in synthetic seawater, 50% and more bentonite was required to fill the clearance.Since this examination fixed the clearance, water stopping performance will be examined in next phase. Through the synthetic seawater examination, the basic clearance filling curve was obtained.