Materials Identification System for 200 liter/55 gallon Waste Drums using Computer Tomographic Techniques - 17162
In 2014 a thermal runaway event within a drum in-situ at the Waste Isolation Pilot Plant (WIPP) facility near Carlsbad, NM, resulted in the drum being breached and activity released. This emphasized the need for a better determination of drum contents and identification of material combinations and in-drum conditions. Enhanced characterization is required to identify and quantify the presence of absorbent materials, determine whether they are organic or inorganic, locate their position in the drum with respect to other materials, and quantify how much liquid is being held in the absorbent material. As part of the waste characterization process to ensure compliance with the WIPP waste acceptance criteria, drums are currently inspected by Real Time Radioscopic (RTR) x-ray to identify prohibited items such as free liquids and un-punctured aerosol cans, but that technique cannot satisfy these newly identified requirements, and a new approach to non-destructive examination was required. This approach complements the current RTR inspection with Cone-Beam Area Detector Computed Tomography (CT) scanning of the whole drum, followed by more-quantitative CT scanning of specific areas of concern, using a highly collimated Linear Detector Array. The density of representative waste drums required x-ray energies of up to 9 MeV, which meant that sorting of materials by their photo-electric absorption is not viable as this occurs in the 80 - 180 keV range. However, a notable characteristic of higher energy imaging in this context is the general correspondence between material density and total attenuation in the 1.5-4.0 MeV range due to Compton Scatter. Development of the imaging chain, comprising the x-ray source and the detectors was critical to achieving the required system performance. Theoretical analysis of the energy spectra of the 9 MeV beam, and filtration of the beam to remove the lower energies which cause beam hardening artifacts in the image was required. Theoretical evaluation of the required efficiency and image acquisition performance of area detectors and linear array detectors, was carried out to complete the selection of equipment. A system of density calibration rods was designed and, together with the selected equipment, was configured and used to acquire cone-beam CT data and line-scan CT data by scanning a 55 gallon drum containing surrogate waste. From the acquired data, CT reconstruction was performed and the basis of algorithms to identify materials were developed. Following this, a series of blind tests were done using a range of surrogate materials the contents of which were unknown to the system operators. (authors)