MORE than half a million drums of radioactive waste are stored at 30 Department of Energy sites across the nation, with thousands more to come as additional facilities at weapons complex sites are dismantled. All of these drums must be assayed to determine and verify their contents and levels of radioactivity before they can be transported for permanent storage or disposal.|
A system that assays containers of radioactive waste safely, accurately, and nonintrusively has garnered a prestigious R&D 100 Award for Lawrence Livermore National Laboratory and its commercial partner, Bio-Imaging Research, Inc. (BIR) of Lincolnshire, Illinois. This award is presented annually by R&D Magazine to "the 100 most technologically significant new products and processes of the year."
The award-winning Waste Inspection Tomography for Non-Destructive Assay (WIT-NDA) system was developed by a team of engineers and physicists headed by Livermore's Patrick Roberson and Harry Martz. The system combines active and passive computed tomography and nuclear spectroscopy to accurately quantify all detectable gamma rays emitted from waste containers. The WIT-NDA is part of BIR's Waste Inspection Tomography system, which provides nondestructive examination and assay of radioactive waste and has been commercially available since August 1999. "The WIT-NDA is an excellent example of successful technology transfer between a DOE national laboratory and a small private business," says Richard Bernardi, WIT program manager for BIR.
Safe and Accurate
The WIT-NDA has a unique two-step process that provides an assay of greater accuracy than previously possible. It collects two tomographic measurements—one active and one passive—using six external radioactive sources collimated to shine through the waste drum into six opposing, high-purity germanium detectors.
The first step is active computed tomography, which, like radiographic techniques that produce medical x rays, measures the attenuation of radiation intensity that travels from an external source through an object to a detector. In this active measurement, the external radiation sources are aimed at the sealed drum. The sources emit gamma rays at discrete energy levels. As the rays pass through the drum and the various densities of material within, they are attenuated to varying degrees. On the other side of the drum, the gamma-ray spectrometer measures the resulting attenuated gamma radiation. Measurements are taken for 2.25-cubic-inch volumes over the entire drum (a total of 2,304 volume elements for a standard 55-gallon drum). By detecting and measuring the attenuated gamma-ray intensity levels at specific energies, one can determine a map of the linear attenuation coefficient (a function of material density and atomic number) of the waste drum and its contents. These maps can be reconstructed to depict a drum's waste matrix attenuation per volume element and energy.
In the second step, called the passive measurement, the six transmission sources are shuttered. The six detectors measure the gamma-ray spectra emitted from inside the drum. Measurements are taken of all the volume elements. During the passive computed tomography reconstruction, the attenuation in these emission measurements, caused by material between the isotope and the detector, is corrected by using the active attenuation map. This correction leads to a far more accurate assay of radioactivity within the drum. The spectra are also used to automatically identify the isotopes within the drum, because each isotope emits a unique signature within the energy spectrum.
Other systems and techniques do not approach the accuracy of the WIT-NDA system. This was demonstrated by a DOE test that compared the performance of the WIT-NDA and fifteen other NDA systems. It was a blind test using a drum containing a simulated sludge with low levels of transuranic waste, one of the most challenging things to assay but typical of the types of wastes and drums the DOE must assay and dispose of. The WIT-NDA performed the best of all the systems by detecting the radioactivity within the sludge drum to within 1 percent of the known value. The nearest competing system detected only about 80 percent of the known radioactivity.
In the combined series of required DOE-sponsored tests for all NDA techniques that are proposed for certifying waste for disposal at WIPP, the WIT-NDA had a mean accuracy of 97.6 percent, with a precision within 4.1 percent. As Steven Cooke of DOE's Federal Energy Technology Center (now the National Energy Technology Laboratory in Morgantown, West Virginia) notes, "These results are truly exceptional in a difficult arena where par for the course is often plus or minus 50-percent accuracy."
Key Words: active and passive computed tomography, nondestructive assay (NDA), nondestructive evaluation, R&D 100 Awards, waste disposal, waste inspection tomography, Waste Isolation Pilot Plant (WIPP).
For more information contact Patrick Roberson (925) 422-8693 (firstname.lastname@example.org).