Evaluation of ambient dose equivalent rates influenced by vertical and horizontal distribution of radioactive cesium in soil in Fukushima Prefecture

Malins, A.  ; Kurikami, Hiroshi  ; Nakama, Shigeo ; Saito, Tatsuo  ; Okumura, Masahiko   ; Machida, Masahiko  ; Kitamura, Akihiro   

The air dose rate in an environment contaminated with Cs and Cs depends on the amount, depth profile and horizontal distribution of these contaminants within the ground. This paper introduces and verifies a tool that models these variables and calculates ambient dose equivalent rates at 1 m above the ground. Good correlation is found between predicted dose rates and dose rates measured with survey meters in Fukushima Prefecture in areas contaminated with radiocesium from the Fukushima Dai-ichi Nuclear Power Plant accident. Better predictions are obtained by modeling the horizontal distribution of radioactive cesium across an area if multiple soil samples are available, as opposed to assuming a spatially homogeneous contamination distribution. Reductions seen in air dose rates above flat, undisturbed fields in Fukushima Prefecture are consistent with decrement by radioactive decay and downward migration of cesium into soil. Analysis of remediation strategies for farmland soils confirmed that topsoil removal and interchanging a topsoil layer with a subsoil layer result in similar reductions in the air dose rate.