Fig. 1　Relative inventory of components in urban areas (2015)

Those figures present the percentages of inventories relative to the values of the inventories on planar permeable fields measured in 2015 and converted to those immediately after the accident. The relative inventories four years after the accident were significantly low on paved ground and on building surface materials in the evacuation zone in which decontamination had not been conducted. These results indicate that large amounts of radioactive Cs deposited in residential areas were removed by the initial runoff and subsequent wash-off effects of rainfall, even without decontamination.

Fig. 2　Relative inventory of surface materials of roofs and rooftops

Porous materials such as mortar showed significantly higher relative ¹³⁷Cs inventories than others; smooth-surface materials such as glazed tile and metallic slate showed obviously smaller values. This indicates that ¹³⁷Cs deposited on smooth-surface materials are easily and quickly removed from the surface.

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Reference

1. Yoshimura, K. et al., Distribution of 137Cs on Components in Urban Area Four Years after the Fukushima Dai–Ichi Nuclear Power Plant Accident, Journal of Environmental Radioactivity, vols.178–179, 2017, p.48–54.