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

P.1.2.1 Leaching rate
P.9
If the leaching is surface-controlled, the rate at which a cation is leached from a
mineral depends on: 1) the reactive specific surface area of the mineral and the solution;
2) the concentrations of the species or ions involved in the transition state of the rate-
determing step for surface reaction; 3) the free energy of activation of the activated com-
plex; and 4) the temperature of the solution-rock system. The effects of pH, Eh and com-
plexes enter via their effect on the numbers in 2). The role of temperature in kinetic
processes is much more prominent than its role in solubility calculations, due to the high
activation energies (10 to 100 Kcal/mole) often encountered. Thus it is crucial to measure
accurately the activation energies for the important leaching rates.
Leaching rates can also be controlled by the rate of transport (i.e., diffusion) of
leached cations from the weathering mineral-solution interface to the bulk of the solution.
In this case, temperature will play a much more minor role, since diffusion activation
energies are %4 to 5 Kcal/mole in electrolyte solutions. Experiments should decide which
mechanism is operative for each mineral (e.g., feldspars and calcite seem to weather according
to the surface-controlled mechanism, while olivine dissolves by a diffusion-controlled
mechanism). The leaching rate may sometimes be severely limited by inhibitors. These inhibitors
could be organic substances or ions such as P 4. 3- , which deactivate the active
sites on a surface (e.g., such as the effect of P04 3 on calcite dissolution). A
protective coating may sometimes also form on the surface of the weathering mineral. All
these factors add to the kinetic stability of a mineral.
Neither data on leaching rates of relevant minerals nor an understanding on their
mechanisms are now available. This gap certainly needs to be filled. The theoretical
framework to understand the kinetics of leaching or dissolution is developed to a reasonable
degree (Nielsen 1964, Hofmann et al. 1974); however, application to relevant geologic mate-
rials is needed.
P.1.3.1 Element Substitution
P.1.3 Crystal Chemical Criteria
In establishing which minerals are appropriate to contain the relevant nuclear waste
elements, one may use minerals that are known to contain the element or elements of interest
and satisfy the stability criteria. Many such examples will be identified, particularly for
Sr, lanthanides, and U. However, elements such Cs, I, actinides, and Tc are so rare in
nature that few known minerals contain them as essential elements. One can then use the
principles of crystal chemistry to predict the formation of mineral-like phases that will
contain the elements in question or mineral phases into which significant quantities may be
incorporated in solid solution.
The critical elements all behave essentially like ions in their compounds, so one can
use the principles of element substitution in ionic compounds as criteria for predicting