Zonal structure showing differential leaching in hydrothermal alteration of Cs- and Sr-bearing borosilicate glasses

In order to assess the stability of high-level radioactive waste form, several compositions of borosilicate glasses have been treated under a hydrothermal condition (T=200degC; p=1.54 MPa), and chemically mapped by electron microprobe. Chemistry of altered glass grains are mostly heterogeneous and shows zonal patterns of seven types: 1) homogeneous; 2) decreasing from center outwards, 3) increasing, 4) local maximum, 5) local minimum, 6) oscillating, and 7) random. Among them, the decreasing and local maximum types indicating a simple migration model are not common. Some grains have a hole at center of a grain. Other grains have a channel, by which an original grain is separated into several sectors. These patterns and structures suggest that the migration of elements from inside to outside of a grain is not simple, and that hence the change of solid phases should be taken into account to assess the stability reliably. (author)