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Sato, Haruo

PNC TN8410 97-075, 29 Pages, 1997/04

PNC-TN8410-97-075.pdf:1.52MB

In the performance assessment of geological disposal of high-level radioactive waste in Japan, redox condition in deep geological environment is considered to be reducing, and Se is one of the important redox sensitive elements. However, no studies on diffusion of Se in bentonite under reducing conditions have been reported yet. This paper describes the results of apparent diffusion coefficients of Se in compacted sodium bentonite obtained as a function of bentonite density under reducing conditions and discusses its diffusion behaviour. Apparent diffusion coefficients of Se in compacted sodium bentonite, Kunigel V1 (constituent montmorillonite 46 49wt%), were obtained in a range of dry densities of bentonite, 800 1800 kgm under reducing conditions (Eh vs. SHE -373 -363mV) at room temperature (23.6 23.7C) by in-diffusion method. All the experiments were carried out in an N-atmospheric glove box (O 1ppm) and the reducing conditions of the porewater were maintained by continuous contact between compacted bentonite and reducing solution including 5.7 10 M-NaSO through a sintered metal filter. The Eh of reducing solution was continuously monitored. Furthermore, a through-diffusion experiment of NaSO was also carried out at a dry density, 1800 kgm in order to check the reducing condition of the porewater. The Eh in the measurement cell was confirmed to be the same as that in the tracer cell. The apparent diffusion coefficients of Se were in the range, 6.110 4.310 ms and showed a tendency of slight decrease with increasing dry density of bentonite. The dominant species of Se in the porewater under reducing conditions is predicted to be HSe, and the apparent diffusion coefficients of HSe in the bentonite were approximately the same as those of TcO ...