Evaluating the Use of Low-cost and Lightweight Unoccupied Aerial Systems in Environmental Radiation Mapping

Abstract

The commercial emergence of unoccupied aerial systems (UAS) over the past decade has presented new opportunities for the field of airborne radiation mapping. Foremost of these is the collection of aerial gamma-ray (γ) spectrometry (AGS) measurements at low altitudes and in a more accessible manner than through using occupied aircraft. Despite rapid improvements in γ-imaging technologies and computer vision over the past decade, many operators continue to utilise a simple and inexpensive approach to mapping radiological contamination with UAS. The research presented in this thesis evaluates the use of lightweight (<20 kg) and low-cost radiation mapping UAS for a full range of radiological characterisation tasks in order to better understand the practical limits of this method. These tasks extend from monitoring for contamination leaks in small low-level waste repository (LLWR) facsimiles (≤ 1000 m²) in Fukushima to conducting long-range mapping investigations (several km²) within the Chornobyl Exclusion Zone (CEZ).

Simple detection systems, incorporating either a 32.8 cm³ CsI(Tl) scintillator (SIGMA-50) or a smaller 1 cm³ CZT semiconductor (GR1) handheld γ-spectrometer were utilised alongside conventional interpolation of γ-ray intensities as function of the 2D position below the detector. A rigorous calibration procedure quantified their response to γ-rays in 3D space, whilst the results informed the development of a finite difference method for calibrating detectors for AGS surveys over areas with complex vertical radionuclide distributions in the near-surface earth layer. A bespoke processing algorithm for AGS was developed to optimise the usefulness of the handheld γ-spectrometers.

Using this approach, the ¹³⁷Cs distribution across more than 15 km² of high value areas within the CEZ was determined in high spatial resolution (30 m pixel^-1). Furthermore, a contamination leak occurring at a Fukushima waste storage facility was identified using a combination of AGS and low-cost airborne photogrammetry. A flow accumulation study using the digital surface model (DSM) derived from photogrammetry determined that meteoric water flowing across the site was responsible for the creation of the leak. Repeat monitoring over subsequent years confirmed the remediation of the contamination plume. While relatively successful in carrying out the intended tasks, the low-altitude nature of UAS measurements can still produce worrying underestimations of localised hot spot intensities that could present hazards to responding workforces. Overall, the utilisation of low-cost simple detection systems with lightweight UAS can be effective for a variety of characterisation tasks. However, the use of modern $\gamma$-imaging solutions should be preferred whenever budgets allow.

Date of Award	24 Jun 2021
Original language	English
Awarding Institution	University of Bristol
Sponsors	National Nuclear Laboratory
Supervisor	Thomas Bligh Scott (Supervisor) & Nick Smith (Supervisor)