Interrogation of Active Drum Waste

Igwesi, David Ikenna
(2021) Interrogation of Active Drum Waste. PhD thesis, University of Liverpool.

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The characterisation and evaluation of a mechanically cooled, broad energy germanium (BEGe) detector has been investigated, with potential application in spectrometric interrogation of a 500 litre standard radioactive waste drum. The BEGe detector is of cylindrical shape with a detection area of 6500 mm2 and utilises a 0.6 mm thin carbon epoxy window to protect the crystal as well as limiting the attenuation of low energy gamma rays. The detector was first calibrated using standard calibration sources of various energies at a standoff of 25 cm and measurements were taken for 600 seconds each to estimate the efficiency and Full Width at Half Maximum (FWHM being the resolution) of the detector. The detector is thereafter slit-collimated with lead slabs of 10 mm apertures to narrow the angle of acceptance from the position of emittance of the gamma ray hitting the detector and essentially gives spatial location of the emitting radionuclide. The measurement procedure for the interrogation of the waste drum involves segmenting the waste drum axis into 24 vertical segments of 5 cm step. The collimated detector then applied to scan the waste drum from top to bottom on each of the segments for each of the point sources used. Thereafter, radial and angular scanning of the waste drum were performed at the segment with the highest count rates. Image reconstruction for the localisation of the hotspots was performed by combining the vertical count rate obtained with either the angular or radial count rates, and the combined data were implemented using a filtered back projection (FBP) algorithm developed using a MATLAB code. The total activity of the sources within the waste was calculated by taking the average of all the activities from each radial segment of the drum. The measurement procedures were repeated for an extended Cs-137 source 15 cm long and 1.5 cm diameter. The results indicated that a well collimated BEGe detector is suitable for the localisation of hotspots and quantification of activity of the source inside the drum. Remarkably, a reconstructed image of length 14.9 ± 1.7 cm for the extended source shows the effectiveness of the BEGe detector for imaging of the different forms of hotspot. Furthermore, Monte Carlo simulations performed to validate the activity of the sources and estimate the attenuation effects of some attenuating materials on the activity of the sources clearly showed that an accurate utilisation of the MCNP simulations technique can obviously be applied to improve the quantification of the activity of the source, and estimation of the attenuation effects of matrices on the activity of the radioactive waste drum. Additionally, the feasibility of improving the localisation of hotspots with Compton camera is presented. The Gamma-Ray Imager plus (GRI+), a three-tiered Compton camera imaging system, designed at the Precisions Radiometrics Instrumentation Development and Education (PRIDE) Laboratory comprised of a circular orthogonal-strip lithium-drifted silicon Si(li) detector as scatterer, a cuboid orthogonal-strip HPGe detector as absorber and a coaxial HPGe detector. Data were acquired for two Cs-137 point sources positioned 30 cm apart inside the waste drum. Using the analytical image reconstruction algorithm developed at the University of Liverpool, an intensity location on the surface of the cone, known as a Compton cone, was back projected into an imaging slice for each event. A multiple number of reconstructed cone projections onto a two-dimensional imaging slice produces an intensity map which represents the location of the hotspot. FWHM of 69.47 ± 1.50 mm and 74.98 ± 2.77 mm were obtained for the two Cs-137 point sources. The results from this novel technique that has not been used to image 500 litre standard waste drum show that the FWHM values for the GRI+ are lower than those obtained from the BEGe detector system indicating a good image resolution. The Compton camera is a reliable alternative for hotspot localisation as it distinctively resolved two Cs-137 point sources of different activity and source positions. This technique is therefore recommended for radiological characterisation of waste drum used for low and intermediate level waste (LILW).

Item Type: Thesis (PhD)
Divisions: Faculty of Science and Engineering > School of Physical Sciences
Depositing User: Symplectic Admin
Date Deposited: 04 Jun 2021 10:48
Last Modified: 18 Jan 2023 22:46
DOI: 10.17638/03123270