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

P.12
as a waste element fixation mechanism, because the mineral's stability may be better con-
trolled by the host composition. In other words, the waste ion would be sufficiently dilute
in the host structure that it does not substantially modify the stability of that host.
Isostructural Compounds
Crystals that allow solid solution necessarily have the same crystal structure for the
end memers. Compounds with the same structure may show no or very limited solid solu-
biflty, usually because of marked size differences of the ions involved. Sih isostructural
groups may have similar stability properties. Thus it may be useful to identify families
of compounds with certain structural properties that may predict the existence of a stable
compound of a particular waste element. Calcium compounds, for example, may indicate possible
strontium compounds. Bromides and chloride compounds may indicate possible iodide
compounds. Several isostructural possiblities are identified below.
P.1.4 Synthesis
Preparation of synthetic minerals requires that the desired elements from the waste
streams be mixed with other materials. The mixture is then reacted to form the synthetic
mineral. Considered here are the problems that may arise in the processing of nuclear
wastes into synthetic minerals.
The purity of the partitioned waste stream will determine whether side reactions will
lead to additional phases in the synthetic mineral assemblage. The controlling factors will
be the ionic size and the ionic charge of the additional cations present. Ions whose size
and charge are similar to those of the element being packaged will dissolve into the synthetic
mineral as a minor solid solution. Many of the mineral phases are very "forgiving";
that is, they will accept many elements into solid solution at least in small amounts. If
there is a large size or charge mismatch, the impurity elements in the waste stream will
react to form secondary minerals of their own. Whether this is detrimental to the proces-
sing will have to be evaluated in individual cases.
Three general methods of reaction are in common use among geochemists for the synthesis
of minerals: calcination, solid state reaction, and hydrothermal reaction. In each method,
it is necessary to mix the waste elements with the other components in the right proportions
to form the minerals. Many minerals are nearly stoichiometric, that is the components must
be mixed in exactly the proportions called for in the mineral formula. If this is not done,
some components will be left over to form additional phases. The stoichiometry of minerals
that form solid solutions is not quite so critical.
Mineral synthesis by calcination involves these steps:
* taking each component into solution (for example, as the nitrates)
* mixing the solutions in correct proportions using volumetric methods
* precipitating the solution as a gel, spray drying, or by another method forming a
calcine (a highly reactive fine-grained, often poorly-crystallized powder)