Engineering Properties Of Granular Bentonite Materials For HLW Disposal

Granular bentonite materials are some kinds of buffer/backfill materials with good performance,which are constituted by bentonite pellets and bentonite powder.The materials can be used to backfill pores between bentonite blocks and surrounding rocks,transportation tunnels and so on.The materials have several advantages,such as,easy to compaction,strong expansibility,low permeability and self-healing ability.The materials can also fill irregular pores.However,as a result of limited sizes of tunnels,some granular bentonite materials will not be compacted,and stay in their packing state.In order to evaluate overall performance of HLW depository,as a component of engineering barrier,the performance of packing granular bentonite materials need to be studied solely.In order to study expansibility,permeability and compressibility of packing granular bentonite materials,four kinds of granular bentonite materials,as the compacted/packing bentonite powder had been selected.Including K5F(70%pellets sized between 5-2mm,mixed with powder),K2F(70%pellets sized between 2-1mm,mixed with powder),KAF(Mixed by pellets sized smaller than 5mm with Fuller grain size distribution),K(Mixed by pellets sized between5-0.5mm),F(Bentonite powder).Free swelling ratio tests,swelling strain tests,permeability tests and compressibility tests had been conducted on these four granular bentonite materials as powder the reference.The results showed that,under natural moisture conditions,granular bentonite materials had higher packing dry densities than bentonite powder.Among these four selected materials,K5F material and KAF material had almost the same dry densities and their densities higher than the other two materials.Free swelling ratio of granular bentonite materials was proportional to packing dry density.The volume of full-expanded granular bentonite materials was in reverse proportion to pellet ratio.Gradation influenced the final swelling strains through the packing dry densities of granular bentonite materials.The final swelling strains were in proportion to packing dry densities of granular bentonite materials.However,initial structures of granular bentonite materials had some influences on final swelling strains.When packing dry density of K5F is 0.003 g/cm~3 smaller than dry density of compacted bentonite powder,the final swelling strain of K5F is l2.9%larger than bentonite powder.Saturated permeability of four granular bentonite materials had been in the order of 10~-99 cm/s.Dry density is no longer the control factor.When the moisture contents were smaller than 13%,compacted dry densities of granular bentonite materials under a constant load were larger than those of bentonite powder.Compressibility of granular bentonite materials was smaller than that of bentonite powder.Among these four granular bentonite materials,K5F and KAF had higher packing and compacted dry densities.When the moisture contents were higher than 13%,under lower pressure,compacted dry densities of granular bentonite materials were smaller than those of bentonite powder.Under higher pressure,compacted dry densities of granular bentonite materials were almost the same as those of bentonite powder.