Heavy Element Photophysics and Photochemistry
Uranyl
Fluoride Luminescence and the DUF6Story
The U.S. Department of Energy currently stores ~560,000 metric tons of
uranium hexafluoride (UF6) in ~46,500 cylinders. Because the
uranium in this material has been depleted as to the fraction of it that
is the isotope uranium-235, this material is usually referred to as depleted
uranium hexafluoride (DUF6). Due to its residual ~0.2% uranium-235
content, DUF6is consider source material within the meaning
of the Atomic Energy Act and is notcurrently classified as waste. Past
DUF6 storage practices have been criticized by the Defense
Nuclear Facilities Safety Board (DNFSB) who also recommended improved
monitoring of cylinders to facilitate tracking and trending of cylinder
defects. Our work shows that uranyl fluoride
luminescence is a promising method for early detection of leaking DUF6cylinders.
The photograph on the left shows DUF6storage cylinders that
have corroded and, in consequence, are unlikely to meet their originally
projected 70-year useful life. A total of seven cylinders have leaked
to date. An example of one such cylinder is shown at the right. Present
storage monitoring practice includes visual inspection and labor-intensive
checking of cylinder wall thickness. Information is available on the DOE
Programmatic Environmental Impact Statement (PEIS) for long-term management
of DUF6.
Oak Ridge National Laboratory (ORNL) is one of three DOE sites that store
a large number of DUF6cylinders. Work at ORNL has shown that
uranyl fluoride UO2F2) is the primary uranium species
that forms when DUF6cylinders leak and atmospheric water (H2O)
reacts with the stored UF6. The overall chemical reaction can
be represented as
UF6+ 2H2O ® UO2F2+
4HF.
The resulting HF (hydrofluoric acid) and the presence of additional H2O
results in formation of solids (primarily HF adducts of hydrated UO2F2)
that create a diffusion barrier and slow the rate of reaction of stored
UF6with atmospheric water. Cylinder breach sites are reported
to exude liquid that is presumed to be acidic HF solutions of uranyl fluorides.
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