Time-Resolved Spectroscopy Investigates Nuclear Waste
The development of nuclear weapons and the generation of nuclear power produce large quantities of waste uranium that poses both a chemical and radiological hazard. Although the US is no longer producing these weapons, and the growth of the power plants has slowed, the safe disposal of the contaminant U(VI) nuclide remains a critical issue.
For example, the waste ponds at the US Department of Energy's Hanford Site in Washington state received more than 30,000 kg of the substance between 1943 and 1975. Because of the long-term hazards associated with this material and with that being held elsewhere, it is important to understand what happens to the U(VI) after disposal. Specifically, scientists want to know whether the uranium finds its way into stable mineral compounds or remains in a mobile form.
Low-level nuclear waste from power plants or weapons facilities was discharged into ponds such as this one at the Hanford Site in Washington, bordering the Columbia River. Uranium, in the form of uranyl ions, infiltrates into sedimentary rock. Time-resolved fluorescence microscopy provides insight into the migration and stability of such nuclear waste products.
Carbonate minerals such as calcite and aragonite are among the most common secondary minerals on Earth. Near the surface, uranium is present primarily in combination with two oxygen atoms, forming a uranyl ion. This ion can replace the carbonate groups in either calcite or aragonite. Researchers thus would like to understand whether waste uranium precipitates from waste ponds into these minerals.
Calcite and aragonite are chemically identical -- CaCO3 -- but differ in structure: essentially in the number of neighbors with which the uranyl ion can interact. This difference manifests in different uranyl fluorescence lifetimes for the minerals, which appear as a distinct time-dependence of the spectral features.
Accordingly, Zheming Wang and colleagues at Pacific Northwest National Laboratory in Richland, Wash., analyzed samples from infiltration pond 300A at Hanford. They cooled the samples to liquid-helium temperatures and illuminated them through a sapphire window with pulses of 415-nm light from a frequency-doubled MOPO-730 laser from Spectra-Physics. They recorded the emission spectra using a Princeton Instruments PI-Max time-gated, intensified-CCD camera at the exit port of an Acton Research SpectraPro 300i double-monochromator spectrograph. They also analyzed the spectrum from a 13,700-year-old, naturally occurring uranium-rich calcite sample.
The spectra of the infiltration pond samples were complex, with interference from organics and biomass, but because the fluorescence lifetimes were on the order of nanoseconds, time-gating eliminated most of the background noise. All samples displayed spectral evolution that could be explained by the superposition of two independent exponentially decaying fluorescence signals: a sharp-featured spectrum with a lifetime of about 100 µs, and a broader spectrum with a lifetime of about 400 µs.
Comparison of the spectra with other measurements led to the association of the broad spectrum with calcite and the sharp spectra with the aragonite structure. Infiltration pond samples that were gathered at a depth of 6 feet exhibited fluorescence-quenching behavior not seen in the other samples, which the researchers attributed to the presence of additional contaminants, such as copper and iron.
Wang anticipates that the technique will be used as a monitoring method as well as to qualitatively identify the processes affecting waste uranium. The first result will be the development of more accurate thermodynamic and kinetic models and more effective remediation methods.
Additional sediment and groundwater testing will help the group develop new uranium transport models, and Wang is also interested in the more fundamental aspects of the research: "the puzzles nature offers and how the latest technology can help to solve such complicated environmental problems.
- A doubly refracting mineral used to produce polarizing prisms. It is uniaxial negative and in the trigonal division of the hexagonal system of crystals. Its indices are e = 1.486, w = 1.658; its hardness is 3 Mohs; and its specific gravity is 2.711.
MORE FROM PHOTONICS MEDIA