AbstractThe catalogue of events at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011, following the magnitude 9.0 Great Tōhoku-Oki Earthquake and ensuing tsunami have left a legacy ofheightened local and widespread global concern. Radiological mapping of contaminated as well as remediated localities surrounding the plant is presented utilising a remotely-operated unmannedaerial vehicle (UAV) equipped with a lightweight and high sensitivity gamma-spectrometer in addition to a height-normalising LiDAR. Over traditional foot—based methods, the use of UAV’s not onlyreduces the time required to acquire significant data, but also the radiation exposure to the operator. Aerial systems also enable the investigation of localities not possible with foot mapping. Followingmapping surveys; samples were taken of areas identified as having high levels of radiological contamination, and are currently being subjected to detailed analysis to determine their exact physical andchemical properties/composition.

Investigating FDNPP fallout from 101 to 10-7m scale for catchment models of radionuclide transport

P.G. Martin1, T.B. Scott1, D.A. Richards2, Y. Yamashiki3, Y. Onda4

1 Interface Analysis Centre, University of Bristol, School of Physics, HH Wills Physics Laboratory, Tyndall Avenue 2 School of Geographical Sciences, University of Bristol, University Road3 Graduate School of Advanced Integrated Studies in Human Survivability, Kyoto University, Japan 4 Centre for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Japan

Conclusions & Future Work

The use of low altitude unmanned aerial vehicles provides a safe method with increased spatial resolution, over existing survey methods, to accurately map radiological contamination. Initial ex-situ analysisof particles ejected from the nuclear accident collected from identified “hot” sites has been successfully demonstrated using a micromanipulator coupled with a dual SEM-FIB instrument – a novel approachwhereby individual particles are identified and subsequently isolated for analysis. Future work is planned utilising micro x-rays at Diamond Light Source, laser Raman, fluorescence imaging and HR-TEM. Anunderstanding of the dispersion and behavior in the environment is crucial to the long-term remediation of vast areas contaminated by the accident at Fukushima Daiichi.

Introduction

Unprecedented effort is currently being devoted to the remediation of significant areas of eastern Japan affected by radionuclide fallout. Critical to future plans are the accurate determination of thechanging distribution of radionuclides and pathways in the environment. Despite their accuracy, ground based radiological surveys have inherent limitations. The use of remotely operated UAV’s not onlyreduces exposure but greatly enhances the rate of data collection. Initial results show the impact of remediation on a variety of sites within Kawamata Town, Fukushima. In spite of a remarkable effort inclean-up, less attention has been given to the more challenging aspects of the longer-lived or “forgotten radionuclides” such as 235U and 239Pu.

Fig. 1. Unmanned aerial vehicle (UAV) used for the mapping of contaminatedsites.

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(a) Yamakiya Junior High School, Kawamata Town

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(b) Kawamata Town bail storage site

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Fig. 2. Radiation intensity maps detailing hotspots at (a) Yamakiya Junior High School, Kawamata Town, Fukushima Prefecture; location of enhanced clean-upand (b) a former paddy field, now site for the storage of bailed top-soil; Kawamata Town, Fukushima Prefecture [inset: aerial photograph of storage site].

Experimental

An unmanned aerial vehicle (UAV) equipped with a lightweight Kromek™ cadmium zinctelluride (CZT) gamma-spectrometer was used to map regions of interest, approximately500m² - at greater than 1m resolution. Height normalisation reflecting the inverse squarerelationship away from a point source was performed using a single-point LiDAR rangingsystem. The aerial system was flown at an altitude of between 1 and 15m, where thedecrease in radiation intensity with height can be seen to follow an inverse square law ofradiation dispersion originating from a point source.

Soil and grass samples collected from areas of elevated activity were imaged using a CarlZeiss™ Field Emission Scanning Electron Microscope (FESEM) with backscattered electrondetector (BEI). Particles of elevated z-number were seen with greater brightness over thesurrounding material. Energy Dispersive Spectroscopy (EDS) spot analysis of potential falloutparticles was carried out within the SEM to provide information on elemental composition.

To remove particles from the surrounding material for further analysis, a dual Focused IonBeam (FIB) and SEM instrument was used. After locating, a Kleindiek™ micromanipulator wasplaced in contact with the particle and attached using either platinum (trimethylcyclopentyldienyl platinum) deposition or an electron beam polymerising adhesive(SEMGlu™). Following deposition, a series of perpendicular FIB cuts were made to assist inremoving the particle from the substrate mass. Subsequently, the particle was deposited ontoa Si-wafer or Transmission Electron Microscopy (TEM) grid, using further FIB cutting to sectionthe particle and reveal its internal structure.

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Fig. 4. Scanning Electron Microscope (SEM) images of fallout particles taken underlow-vacuum conditions using a Variable Pressure detector (VPSE-G3), at 1.0mbar,120μm beam current and 20kV accelerating voltage. Corresponding EDS spectra areshown in Fig. 5.

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Fig. 5. Energy Dispersive Spectroscopy (EDS) spectra of four particles (a) – (d) shown in Fig. 4. Each spectra was obtained for 500 seconds on an areacomprising at least 2/3 of the particle using identical conditions as Fig. 4.

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Fig. 3. (right) Steps detailing the removal of a micron-scale fallout particle from a soil sample taken fromIitate Village, 30km NW of FDNPP and subsequent sectioning using FIB. (a) and (b) location of particle forremoval, (c) insertion of Si needle and GIS attachment with Pt, (d) FIB milling to free particle (e) placement ofparticle onto Si-wafer, (f) (g) and (h) sectioning of particle and (i) cut surface at centre.

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FeCe

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The authors wish to thank Sellafield Ltd, The University of Kyoto and Kromek Ltd for their support.