Diffusion of La3+ in Compacted GMZ Bentonite Used as Buffer Material in HLW Disposal

Abstract

It has been studied that the GMZ bentonite which has a chemical barrier property can play an important role in retarding the migration of radioactive nuclide. In order to understand the diffusion properties of nuclide in GMZ bentonite, a series of experiments were conducted to obtain the apparent diffusion coefficient of La 3+ in GMZ bentonite under different dry density and pH value conditions using a autonomous ion diffusion instrument. The results show that the apparent diffusion coefficient of La 3+ decreases significantly with the increasing dry density or pH value. The influence of dry density on the diffusion is very apparent for a low dry density, and the influence gradually slow down with the increase of dry density. At last the influence mechanism of the dry density and pH value on diffusion coefficient was also analyzed. KeywordsGMZ bentonite Á Diffusion property Á Lanthanum ion IntroductionThe deep geological disposal which based on a multi-barrier system is the most appropriate means for safe disposal of high-level radioactive waste (HLW). Bentonite has been chosen as the buffer/backfill material for this disposal because of good mechanical buffer resistance, low permeability, better swelling property and adsorption property (Wen 2006).The chemical barrier properties of bentonite are mainly manifested in retarding the migration of radioactive nuclide. Considering most of the nuclide uranium, thorium, plutonium in disposal repository are actinide elements, and lanthanide and actinide elements have similar electronic configuration inside and outside layer which determine their similar physical and chemical properties. Therefore the lanthanide ions are always selected instead of radionuclide in the study. In this work, effects of the dry density and pH value on the diffusion property of La 3+ in GMZ bentonite were studied. Experimental MaterialsThe GMZ bentonite derived from inner mongolia China was used in this experiment. Its major mineral is (Wang and Su 2006): montmorillonite 75.4 %, quartz 11.7 %, crystaballite 7 %, feldspar 4.3 %, calcite 0.5 %, kaolinite 0.8 %. It has been studied that GMZ bentonite has strong cation exchange capacity, hydration ability and strong adsorption ability.