Drum role for waste inspection system29 January 2001
Drums containing radioactive waste must be assayed to determine and verify their contents and levels of radioactivity before disposal. One system has been developed that does this effectively.
In the USA alone, there are half a million drums of radioactive waste stored at 30 sites. All of these drums must be assayed to determine their contents and levels of radioactivity before they can be transported for permanent storage or disposal.
The Waste Inspection Tomography for Non-Destructive Assay (WIT-NDA) system was developed by Lawrence Livermore National Laboratory and Bio-Imaging Research. 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 has provided nondestructive examination and assay of radioactive waste since 1999.
As recently as ten years ago, the contents of a waste drum could only be accurately tested by sampling, which required the drum to be opened, breaking the container seals.
In 1990, Livermore scientists began to research ways to estimate radioactivity of drum contents from the outside. The team developed a process that pinpoints where radioactive materials rest inside the drum and accurately identifies these isotopes, whether they are plutonium, uranium, or other gamma-emitting waste. “This system is unique as we don’t need prior knowledge of the radioactive waste inside, and we don’t have to calibrate the system to a specific waste stream,” said Patrick Roberson from the Livermore National Laboratory.
Determining the exact amount of radioactivity in each drum is essential to DoE’s waste disposal efforts. DoE needs to characterise its radioactive wastes to verify that the waste drums meet criticality constraints and differentiate transuranic from low-level wastes.
Each class of waste is sent to a different disposal site. For example, the DoE Waste Isolation Pilot Plant (WIPP) in New Mexico accepts only transuranic wastes and has a limit on the total amount of radioactivity that can be placed within its underground repository. So waste drums must be non-destructively assayed to determine if they contain transuranic waste, and all radioisotopes in each drum must be inventoried to ensure that the WIPP limit is not exceeded. With non-destructive analysis systems of lesser accuracy, regulators must err on the side of safety and designate waste disposal based on higher estimates of radioactivity.
WIT-NDA has a unique two-step process that provides a more accurate assay. 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, measures the attenuation of radiation intensity that travels from an external source through an object to a detector. In this active measurement, 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 2.25in3 volumes over the drum (a total of 2,304 volume elements for a standard 55 gallon drum). By measuring the attenuated gamma-ray intensity levels at specific energies, one can determine a map of the linear attenuation co-efficient (a function of material density and atomic number) of the waste drum and contents. These maps can be reconstructed to predict 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 the active attenuation map. This leads to a more accurate assay of radioactivity. The spectra are also used to automatically identify the isotopes within the drum, because each isotope emits a unique signature in the energy spectrum.
The WIT-NDA system is very accurate. This was demonstrated by a DoE test comparing its performance with 15 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 wastes and drums the DoE must assay and dispose of. WIT-NDA performed the best of all the systems by detecting the radioactivity in the sludge drum to within 1%. The nearest competing system detected only about 80% of the known radioactivity.
In the combined series of required DoE-sponsored tests for all NDA techniques proposed for certifying waste for disposal at WIPP, the WIT-NDA had a mean accuracy of 97.6%, with a precision within 4.1%.
Since WIT-NDA operates independently of whatever wastes are in the drum, it could also be used to measure radioactivity in waste products from nuclear plants or from industries that use radioactive tracers, such as the medical industry.