DIFFUSION COEFFICIENT TEST OF ~(237)Np IN BENTONITE BACKFILL MATERIALS

摘要

This paper describes the work on diffusion coefficient test of Np in bentonite backfill materials. Due to its very low permeability, diffusion is the dominant migration mechanics in bentonite. The bentonite comes from Inner Mongolia of China. ~(237)Np was used as tracer. The special apparatus for diffusion test was setup, the diffusion coefficient of Np in pure bentonite and sand-bentonite mixture were tested. The tracer was introduced between two bentonite columns. After a specific contacting period, the bentonite columns were taken out and cut to very thin slices. The radioactivity in bentonite slices was analyzed to give the nuclide concentration versus distance curves. The diffusion coefficient could be estimated. The diffusion coefficient of pure bentonite at different density was tested. When the density of pure bentonite samples varied from 1.1, 1.3, 1.5 to 1.7g/ml, their diffusion coefficient were 1.36xl0~(-13)m~2/s 1.16xl0~(-13)m~2/s, 1.07xl0~(-13)m~2/s and 8.26x10~-~(14)m~2/s respectively. The diffusion coefficient of Np in sand-bentonite mixture sample was 4.13xl0~(-13)m~2/s. To estimate the distribution coefficient (K_d) value of Np in mixture sample by diffusion method, the diffusion coefficient of Br was measured. The concluded K_d value was 77ml/g for the sand-bentonite mixture. The K_d value obtained by batch test methods was 30ml/g. The reason is related with the error of Br diffusion coefficient and solid-liquid ratio.