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

P.3 MINERAL TABLES
P.35
P.3.1 Hosts for Radionuclides
Table P.3.1 lists selected minerals which have potential as hosts for radionuclides.
The entries in Table P.3.1 were selected according to the criteria listed below.
Approximately 2500 mineral species have been identified. These have been compiled into
reference sources of which those of the Dana system (Palache et al. 1944, 1951), Deer, Howie
and Zussman (1962), Strunz (1970), and Roberts, Rapp and Weber (1974) were consulted. Each
of the 2500 minerals was reviewed and in a first sieving all minerals that were known to be
water-soluble, chemically undesirable or crystal-chemically unsuitable as radionuclide hosts
were eliminated. A much shortened list of about 100 minerals remained. A second sieving
eliminated minerals of great chemical complexity that would be difficult to synthesize. The
minerals that remained were separated according to the radionuclide for which they were to
serve as host and these groups were then roughly ranked with the best candidates listed
first.
Table P.3.2 is the final listing. In addition to mineral name and formula, the table
lists some available information of the occurrence of these minerals in nature, which pro-
vides clues to their stability in the repository environment, and on alteration processes
where known, it must be emphasized that the data on these later categories are very sparse
although this study does not claim to be an exhaustive literature survey. Table P.3.2 is
intended as a guide for future research rather than finalized data for engineering design.
P.3.2 Commentary on Table P.3.1
The lack of silicate minerals on the listing is perhaps unexpected. Silicates make up
the bulk of the rocks on the earth and many of them are very stable. However, the common
silicate structures utilize the most abundant elements of the earth and the critical radio-
nuclides from nuclear waste are, with the exception of 90 Sr, unusual elements, either too
large or too small to fit into available sites in the silicate minerals. Furthermore,
silicates are relatively less resistant to weathering and only a few, or which zircon is an
outstanding example, survive the weathering process to become detrital minerals. Even fewer
survive to become placer minerals.
Phosphates and oxides are the first and second most stable minerals in a wide variety
of geochemical environments from initial formation at high temperatures and pressures,
through weathering transport, contact with salt water in oceanic depositional basins,
burial, diagenesis, upheaval, and in some cases a complete second cycle of weathering.
A very large number of phases on the list occur in pegmatites or in alkaline rocks that
are closely related. The minerals, by implication, are stable in the presence of aqueous
solutions at temperatures to 600'C and pressures to several kilobars. Chemical compatibility
with granite rocks is implied. Whether many of these minerals are compatible with other can-
didate repository rocks, basalts, and shales require research. The fact that the minerals
do not occur in these rocks in nature means only that the chemistry for their formation was
not correct, not that the minerals are necessarily incompatible.