DOE OSTI2016
Measures required to assure long term safe storage of Low-level and intermediate-level short-lived (LIL/SL) waste generated by the nuclear-power industry in France include the characterization of the waste and identification of materials within the waste. Digital Radiography (DR) techniques using x-ray at energies in the 6 MeV range have been shown to be effective for validation of the integrity of the concrete containers, and it was considered that the use of this technique together with the addition of Computed Tomography (CT) techniques could effectively identify materials within the waste. This paper describes the initial evaluation of two DR imaging techniques and two CT techniques using x-ray energy spectra up to 9 MeV to inspect a surrogate waste package, which comprised an outer concrete cylindrical container of 1400 mm diameter with 148 mm wall thickness, and an overall volume of 2 meters cubed, within which a 200 liter drum containing representative heterogeneous inactive waste, was placed in the center and the annulus between the 200 liter drum and the outer cylindrical package was filled with cement grout. The surrogate waste within the 200 liter drum comprised three separate layers of high density, medium density and low density mixed materials. The equipment used was a linear accelerator producing high energy spectra at either 6 MeV or 9 MeV and a high-precision CT manipulator with X, Y, Z and rotation motions, which were common to all techniques investigated. Two different digital detectors were used in the DR and CT performance evaluations, a Digital Detector Array (DDA) comprising a columnar Thallium-doped Cesium-Iodide scintillator screen in intimate contact with a 2 k by 2 k array of amorphous silicon detectors, 200 μm x 200 μm in area, and a Linear Diode Array (LDA) comprising a line of Cadmium-Tungstate columns, 1 cm thick and 400 μm x 400 μm in area. The work comprised evaluations of linear accelerator spectra and the attenuation effects of the waste container and contents, image acquisition performance of each detector, and CT techniques using area detector and linear detector. The results presented in this paper include mathematical evaluation of the two CT techniques, and analysis of measurement data, digital images, CT reconstruction images, and estimated material densities. The data was compared with recorded details and photographs of the actual configuration of materials within the surrogate waste. The Digital Detector Array can generate CT inspection volumes with considerable resolution and detail for the low-density and medium-density sections of the surrogate waste, however, the high-density sections were not imaged with sufficient detail and the images contained a large number of artifacts mainly due to the detectors lack of high energy photon stopping power. Materials identification with the Digital Detector Array, while possible for certain materials, for materials in the steel to plastic density range the attenuation values of the voxels in the reconstruction are too similar and difficult to resolve from the artifacts present. The Linear Diode Array can generate CT inspection volumes of regions of interest of the low-density and medium-density sections with considerable resolution and contrast, and potentially acceptable levels of resolution and contrast of the high-density section of the waste. While a few artifacts were present in the reconstructions, the majority are not due to the fundamental properties of the detector and are considered to be correctable. Materials identification with the LDA inspection data appear to be viable, with steel, copper, and higher-density metals being readily be distinguished from other materials on the basis of voxel value. Future work is needed to obtain a better definition of optimum spatial resolution and inspection speed, and to develop a system of 'calibration rods' which would consist of a series of 50 mm diameter cylinders of 7 materials; Plastic, Teflon, Concrete, Aluminum, Copper, Tungsten and Lead. The cylinders will be distributed at a certain radius in the surrogate waste and will be used as reference points in the evaluation and demonstration of the contrastive performance for the low, medium and high-density cases. The ability to identify the materials within a sealed waste container, by nondestructive examination, enables the verification of waste acceptance criteria without the need to open the waste containers. This eliminates the need for complex and expensive equipment to open and remotely handle the intermediate level waste, avoids potential hazards and risk of personnel dose uptake, and does not increase the volume of waste caused by repackaging of materials after examination and the decontamination or disposal of equipment used.
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗