Experimental Study on the Localization and Estimation of Radioactivity in Concrete Rubble Using Image Reconstruction Algorithms

To reduce a large amount of contaminated concrete rubble stored in the Fukushima Daiichi Nuclear Power Station site, recycling low-radioactivity rubble within the site is a possible remedy. To promote recycling while ensuring safety, not only the average radioactivity but also the radioactivity distribution of concrete rubble should be efficiently evaluated because the details of rubble contamination caused by the accident remain unclear and likely include hotspots. However, evaluating inhomogeneous contamination of thick and/or dense materials is difficult using previous measurement systems, such as clearance monitors. This study experimentally confirmed the potential applicability of image reconstruction algorithms for radioactivity distribution evaluation in concrete rubble filled in a chamber. Radiation was measured using plastic scintillation fiber (PSF) around the chamber (
$50\times 50\times40$
cm
³
). Localized hotspots were simulated using standard sources of
¹³⁷
Cs, which is one of the main nuclides of contaminated rubble. The radioactivity distribution was calculated for 100 or 50 voxels (voxel size: (10 cm)
³
or 10
$\times 10\times20$
cm
³
) constituting the chamber. For 100 voxels, inner hotspots were undetected, whereas for 50 voxels, both inner and surface hotspots were reconstructible. The distribution evaluated using the maximum likelihood expectation–maximization (ML-EM) algorithm was the most accurate; the average radioactivity was estimated within 70% accuracy in all seven cases.