Measurements and calculations of dose rates on contaminated ground at the Fukushima Daiichi Nuclear Power Plant Site

Tsujimura, Norio   ; Yoshida, Tadayoshi ; Furuta, Sadaaki

The accident at the Fukushima Daiichi Nuclear Power Plant resulted in a substantial release of radionuclides to the environment. Consequently, the emergency operation workers were exposed to both external and radiations arising from the fallout; however, their doses were scarcely recorded at the time. To solve the problem, the authors performed Monte-Carlo simulations which estimated the and dose equivalent rates in outdoor work environments during the initial weeks of the accident. The computer model consisted of air-ground interface and a person was assumed to be standing on the uniformly contaminated ground. The source term used was based on the isotopic compositions of soil samples identified by a HPGe spectroscopy analysis. The calculated results were benchmarked by comparison with both the dose rates measured for contaminated soil samples collected from the site and those measured by stationary and portable instruments on-site in the mid March. The computed -to- dose ratio ranged from 9 at 50 cm to 3 at 130 cm above the ground. The maximum possible exposure to the unprotected skin of the worker, engaged in the outdoor operations, was estimated to be 1 Sv, with the expectation of further dose reduction reflecting the effects due to the ground surface roughness and protective clothes.