Technischer Bericht NTB 85-23

Ionendiffusion in hochverdichtetem Bentonit

The diffusivities of K+, Cs+, Sr2+, UO22+, Th4+, Cl- and I- in the Na-bentonite MX-80 and in the Ca-bentonite Montigel have been determined under transient conditions using free-standing samples. For dry densities between 1.3 g/cm3 and 2.1 g/cm3 the diffusivities depend primarily on the water content of the bentonite (going from 7 % to 25 % in water content yields an increase of the diffusivity by a factor of 100), the influence of the dry density being fairly small.

There is no significant difference between the two materials investigated.

For a water content of 25 % the diffusivities of the investigated ions are:

Cl-, I-: 10-6 cm2/s; K+: 3·10-7 cm2/s; Sr2+: 10-7 cm2/s; Cs: 2·10-8 cm2/s; UO22+, Th4+: <10-9 cm2/s.

An order of magnitude for the retention factors of the cations can be estimated assuming that the anions Cl- and I- do not sorb:

K+: 3; Sr2+: 10; Cs+: 50; UO22+: 2000; Th4+: 12000.

For the actinides there is a fair agreement with literature values, whereas for K+, Sr2+, and Cs+ significant discrepancies exist.