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

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The rankings, except for the top few entries, are almost arbitrary. Although available
mineralogical evidence suggests that these minerals are stable in the temperature and pres-
sure regimes generally thought to exist around nuclear waste repositories, their relative
stabilities are not known. Likewise, the relative solubilities of these generally insoluble
phases are not known. Thus, detailed ranking or the construction of any sort of figure of
merit cannot be done under the present state of knowledge.
Many of the oxide minerals are highly stable and insoluble because of a particular oxidation
state. Lower oxidation states of the transition metals and of uranium form less sol-
uble compounds than do the high oxidation states. The state of oxidation in a repository
will be controlled by the oxidation potential and oxygen buffer capacity of the host rocks
since these will be present in vastly larger volumes than the volume of the waste. Likewise
the solubilities of many of the minerals are a sensitive function of the acidity of any cir-
culating solutions. The fluorocarbonates are an example of minerals with low solubilities
in neutral or alkaline solutions that become progressively more soluble as the ph decreases.
The host rock in which the repository is formed will play an important role in buffering the
oxidation potential and acidity of any circulating ground water that might contact the synthetic
minerals of the waste form.
The large number of minerals that are listed as occurring in pegmatites is to be
expected. Pegmatites are complex mineral assemblages that form from a residual high-water
content fluid that remains after the crystallization of granitic rocks. Ions that are too
big or too small or have the wrong charge or the wrong electronic structure to fit into any
of the common granite minerals--quartz, feldspars, micas, and amphiboles, are concentrated
in the residual fluid and finally crystallize into pegmatites. It is not the pegmatite-
forming temperature and pressure regime that is critical but rather the complex solution
chemistry that allows these minerals to be formed. Many of these minerals can be synthe-
sized by entirely different methods but their occurrence in pegmatites does imply a substan-
tial degree of mutual compatibility among the phases.