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Luminescence Aids Detection of Toxic Leaks

Photonics Spectra
Dec 1999
Aaron J. Hand

As a legacy to the uranium enrichment process that has provided nuclear reactor fuel since the 1940s, the US Department of Energy stores more than 700,000 metric tons of depleted uranium hexafluoride (UF6) in about 57,700 steel cylinders in outdoor storage yards. UF6 is a radioactive, toxic and corrosive solid that is somewhat volatile, so government labs are interested in detecting leaks as they occur. Eight UF6 storage cylinders are known to have leaked so far.

Researchers in the chemistry division of the Argonne National Laboratory have found luminescence to be an effective way to detect uranyl fluoride, which forms when UF6 reacts with atmospheric water, indicating a cylinder leak. Inspectors now use visual inspection and ultrasonic measurement of wall thickness, but both methods are time-consuming, and visual inspection is prone to error, said James V. Beitz, leader of the research team at Argonne.

"Our luminescence method will provide a rapid, high-sensitivity, signature means for early detection of leaking uranium hexafluoride storage cylinders," Beitz said.

The detection methods rely primarily on imaging time- and wavelength-resolved luminescence, exciting samples with pulsed light sources. The researchers have found that low-intensity light sources are most effective because high-intensity sources cause high concentrations of electronically excited uranyl ions. This brings about exciton annihilation, which means that the excited uranyl ion is less likely to survive to luminesce, Beitz said.

Uranium can be found in most surface waters and many soils at a level of about 0.1 ppb, Beitz said. "Our luminescence detection method strongly discriminates against such environmental uranium because uranyl fluoride luminescence, under our excitation conditions, has a much longer luminescence lifetime."

Beitz has completed and is testing a prototype luminescence imaging detection system. It relies on a membrane mirror light shutter and optical filters to prevent scattered excitation light from reaching a cooled charge-coupled device photosensor array equipped with bandpass filters. When the shutter opens, the array records a wavelength-resolved digital image that shows the location of uranyl fluoride luminescence, if present, on the surface of a UF6 storage cylinder.

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