RADIATION MONITORING & ALARA |
Robots for NDA of radwaste
A prototype of a robotic waste assay system has been in operation at Bohunice in Slovakia since 2019. It is a dramatic departure from the way radioactive waste characterisation was previously performed. Dusan Lexa explains
NON-DESTRUCTIVE ASSAY (NDA) TECHNIQUES are routinely used by radioactive waste producers, intermediate and final storage facilities, and regulatory authorities. The term refers to a series of techniques that measure
radiation induced or emitted spontaneously from nuclear material. In some cases, the emitted radiation is unique to the isotope(s) of interest and the radiation intensity can often be related to the mass of the isotopes. Other techniques to measure nuclear material involve
CEO, DuAl GmbH Dusan Lexa
sampling the material and analysing the sample with destructive chemical procedures. NDA obviates the need for sampling and it reduces
operator exposure. It is much faster, but usually less accurate, than chemical assay. The accuracy of destructive assay is critically dependent
on assayed material sample representativeness. In contrast NDA accuracy hinges on knowledge of activity and density distribution of the assayed material. It is usually easier to assure the former than gain the latter.
There are three broad categories of NDA methods —
gamma spectrometry, neutron counting, and calorimetry. The focus here is on the high purity germanium (HPGe) gamma spectrometry of material contained in standard waste packages such as drums, pallets, etc. This accounts for the vast majority of NDA systems currently in use. The simplest form of NDA is what has become known as an open geometry measurement where the waste package is entirely in the field of view of the detector. The accuracy of the activity determination depends on the degree to which the content conforms to the assumption that the matrix density and activity distribution are homogeneous. Severe errors result if either is grossly inhomogeneous. More advanced NDA methods aim to improve the
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potentially disastrous lack of accuracy of the one-shot open geometry approach. Some irregularities, if known in advance, can be easily accounted for in the calibration, for example varying fill height. However, the density and activity variations in the matrix itself are generally unknown and so are impossible to take into account in a mathematical or empirical efficiency calibration. The best one can do is try to mitigate or quantify their effect. The development of the segmented gamma scanner (SGS) in the 1970s falls into the mitigation category. The idea behind the SGS is simple yet powerful. The drum turns around its axis during measurement and a vertically collimated detector moves up and down alongside. (Sometimes the detector is stationary and the drum moves up and down.) SGS averages out radial and vertical inhomogeneities in activity and matrix density, but the evaluation is still predicated on homogeneity and the same average density, obtained from the known net drum weight and fill height, is assumed for every vertical segment. Even so, SGS has become almost synonymous with NDA. There is a large installed base and a number of instruments are commercially available. A simpler variation of the SGS, known as integral gamma scanning (IGS), is sometimes used. In this variant the drum rotates but the open geometry detector is stationary. The lack of accounting for matrix density variations has
5 6 Above: RoboCount™ 2020
1 Industrial robot 2 Spectrometer module 3 Control unit 4 Pedestal 5 Waste package rotator 6 Collimator tray (liquid nitrogen docking station not shown)
34 | November 2021 |
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led to the development of SGS with transmission correction (SGS-TC). This method seeks to quantify the vertical matrix density inhomogeneities by employing a tightly collimated radioactive source, most frequently Eu-152. The source moves in unison with the detector during a transmission scan, yielding a mean matrix density value for each segment. A normal SGS emission scan follows. Vertical density variations are common within the matrix
and effectively addressed by SGS-TC, but radial density variations in each segment remain inscrutable. In addition,
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