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

P.34
In nature, autunite, Ca(U0 2) 2(PO 4) 2.8-12H 20, and meta-autunite I, Ca(U0 2) 2 (P0 4 ) 2.6-8H 20,
are very common anywhere uranium is found. They are found as a secondary mineral in all
climates; and have been mined as ore minerals in several locations because of their abun-
dance. In Cameron, Arizona, they occur in near-surface sandstone lenses and around Sho-
shoni, Wyoming. They are mined from bentonite pits where they form in the desiccation
cracks of the clay. At Ningyo Prefecture in Japan they are found in sandstone, where they
were mined extensively until the primary ningyoite zone was encountered. Some very noted
specimen localities include the Daybreak Mine in Washington, and Cornwall, England. They
are also common alteration products in uranium-bearing pegmatites. In all these localities
they have proven to be very stable. The leaching characteristics under various conditions
still must be tested.
Among the other uranyl phosphates several other candidates are also evident as possible
repositories. In particular we should consider the phosphyranylite Ca(UO 2) 4(PO 4) 2 (OH) 4.7H 20.
It is a much rarer mineral than autunite but has a higher loading factor because the U:P
ratio is 3:2. Considerably less is known about the stability of this phase. Its conditions
of formation and synthesis are less well known but it occurs similarly to autunite.
One must not overlook the vanadates as potential repository minerals, in particular
carnotite, K 2 (U0 20 2V 2 ) 8-3-5H 20; tyuyamunite, Ca(UO 2 ) 2V 20 8 .5-8H 20, and metatyuyamunite,
Ca(U0 2) 2V 208.3H20. These three minerals occur extensively throughout the Colorado Plateau
and have been mined for uranium. They usually occur in sandstone lensesand are found in
intersticed among the sand grains. Once formed, they appear to resist weathering and alter-
ation even at surface conditions. Strontium analogs might easily be made. Ion exchange,
common in the autunites, does not seem to occur in the vanadates.
P.2.7 Technetium Hosts
Since the element technetium is not known in nature, it follow that no minerals exist
with technetium as an essential element. Technetium exists mainly in valence states Tc 4 +
and Tc 7 + with the latter forming the very soluble pertechnatate ion. Technetium 4 + forms
stable solid oxide phases and, because of a similar ionic radius, behaves much like Ti 4+
Many titanium analogs have been synthesized (Muller et al. 1964) including spinels, pyro-
chlores, perovskites, and a stable solid solution between TiO 2 and TcO 2 . Titanium minerals
may be the best hosts for technetium if reducing conditions are maintained in the
repository.