In connection with studies for evaluating the teachability of Cs from solidified radioactive waste, the surface alteration of pollucite was studied under hydrothermal conditions. Pollucite exposed to NaCl or KCl at 300°C formed on its surface a precipitate phase of analcime in the case of NaCl or leucite in the case of KC1. Cubo-octahedral analcime built up not only on the outermost surface of sample but also in the altered layer underneath. This crystal penetration was attributed to precipitation and isomorphous substitution of Na by Cs in the pollucite. The leachability of Cs proved to increase in keeping with salt concentration. Exposure to CaCl2 or to MgCl2 formed a thick precipitate layer of anorthite in the case of CaCl2 or clinochlore in the case of MgCl2 over a thin reaction layer. This precipitate layer had the effect of inhibiting the migration of Cs. Si and A1 to restrain Cs leaching into solution, which caused the amount of Cs leached to level off beyond 0.01 mol-dm-3 salt concentration in the case of CaCl2, and beyond 0.1mol.dm-3 in the case of MgCl2.
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