Basic characteristic test of buffer/backfill material under Horonobe groundwater condition

By the second progress report (H12) on research and development for the geological disposal of high-level radioactive waste (HLW) in Japan, Japan Nuclear Cycle Development Institute (JNC) extended the data base of basic properties of compacted bentonite which were mainly obtained by using distilled water as test fluid. This report presents influence of Horonobe groundwater on the basic properties of buffer and backfill material. The Horonobe groundwater is a type of saline groundwater. The groundwater was sampled at GL-300 m or deeper by using bore hole HDB-6 of the underground laboratory of Horonobe site. In addition, basic properties are also obtained by using distilled water, synthetic seawater, and NaCl solution. Experimental results are as follows; 1) Swelling characteristics, hydraulic characteristics and mechanical characteristics of the buffer material and backfill material decrease by the influence of saline water. The relationship between effective clay density and swelling stress is described by the following equation. σ = exp (2.5786ρb3 - 12.238ρb2 + 21.818ρb - 14.035) where σ is swelling stress [MPa], ρb is effective clay density [Mg/m3]. The relationship between effective clay density and intrinsic permeability is described by the following equation. κ = exp (-41.466 + 4.316ρb - 4.069ρb2) where κ is intrinsic permeability [m2], ρb is effective clay density [Mg/m3]. The relationship between effective clay density and unconfined compressive strength is described by the following equation. qu = 1.4 x 10-4 exp (5.637ρb) where qu is unconfined compressive strength [MPa], ρb is effective clay density [Mg/m3]. 2) Saline water doesn't influence the thermal characteristic of the buffer material. The thermal conductivity and specific heat are derived by using the relationship that was obtained so far. (author)