Abstract
Over ten years have passed since the accident at the TEPCO’s Fukushima Daiichi Nuclear Power Plant (FDNPP), and the urgent tasks of extracting fuel debris for decommissioning are underway. To facilitate efficient work planning and to reduce the radiation exposure of workers, it is essential to understand the three-dimensional distribution of radioactive material contamination in the buildings. Therefore, Torii et al. have developed a small and lightweight of omnidirectional radiation detector (Fractal Radiation Imaging Element: FRIE), which simulates fractal shape1). This study focuses on the development and evaluation of a high-dose FRIE suitable for use at FDNPP. The developed FRIE has 16 Gd3Ga3Al2O12 crystals, arranged in a Sierpinski’s tetrahedron pattern, with copper tungsten filling the spaces between the crystals to provide directional sensitivity. To assess its use in high-radiation environments, the measurable dose limit was examined using the
Monte Carlo simulation. Field tests were also conducted to obtain three-dimensional contamination distribution by integrating the radiation images with a three-dimensional point cloud model of the measurement area. The maximum likelihood expectation maximization method was used to estimate contamination distribution. The field tests confirmed that the source location could be identified within a few minutes of measurement, and the approximate radioactivity intensity of the source could also be estimated. The authors plan to mount this detector on a quadruped robot and conduct measurements in FDNPP.
