Management of Commercially Generated Radioactive Waste - U.S. ...

K.3 ENGINEERED SORPTION BARRIERS
K.26
In addition to retardation of radionuclide migration with an appropriate canister
design or inert coating of the waste form, certain materials can be used to absorb or other-
wise slow radionuclide migration from the package and the repository.
Possible retardation mechanisms include surface adsorption, ion exchange, coprecipita-
tion, and redox effects. The use of coprecipitation appears impractical as a retardation
mechanism because of its rather selective nature and because a wide range of radionuclide
chemical species must be retarded.
K.3.1 Performance Requirements
Solids selected for radionuclide adsorption, ion exchange, and redox effects in several
combinations can be used for repository backfilling, for an overpack in the immediate vicin-
ity of the canister exterior, and/or for a protective packing between the waste form and the
interior surface of the canister (Karn-Bransle-Sakerhet 1978). The sorption material must
be mechanically, thermally, and chemically stable in the repository environment. Also, it
must be dry when in contact with the canister interior and in the waste form radiation field
to prevent accelerated canister corrosion or pressurization. Good heat conducting proper-
ties and relatively low permeability to ground water also are desirable sorption material
characteristics. If the material is used for repository backfilling, it should have suffi-
cient loadbearing capacity to prevent cavern roof collapse onto stored wastes and to prevent
major movement of the waste canisters. The organic contents of the filling material should
be very low, probably less than 1%, to avoid radionuclide complexing and enhanced migration
rates. Materials may be added to affect oxidation-reduction changes that retard radionu-
clide migration. Radionuclide migration rates of the elements antimony, iodine, neptunium,
plutonium, ruthenium, technetium, and uranium may be affected by changes in the redox poten-
tial.
K.3.2 Sorption Materials Performance
Research sponsored by the Office of Nuclear Waste Isolation (ONWI) is determining sorp-
tion coefficients of many minerals and rocks that may be of interest for sorption barrier
use. Swedish (Allard et al. 1977, Haggblom 1977) and Canadian (Acres Consulting Services,
Ltd. 1977) workers also have ongoing programs to investigate sorption of radionuclides in
clays and rocks. Sorption investigations involving 19 radionuclides that are of interest
in waste disposal operations were summarized in a 1976 EPA literature search (Ames and Rai
1978).
The solution species formed from the radionuclides of the various elements are a pri-
mary control on their adsorption by a potential retardant. Possible solution species, based
on existing thermodynamic data, are shown in Table K.3.1. Rocks, soils, and sediments are
predominately cation exchange materials. The inorganic anion exchangers include the amor-
phous hydrated oxides from iron, aluminum, and manganese, which are found naturally, and
other synthetic anion exchange materials such as zirconia or titania. The environmental
factors reported to effect radionuclide adsorption are summarized in Table K.3.2.