Measurements and Calculations of Beta Dose Rates on Contaminated Ground at the Fukushima Daiichi Nuclear Power Plant Site

The accident at the Fukushima Daiichi Nuclear Power Plant resulted in a substantial release of radionuclides into the environment. Consequently, the emergency operation workers were exposed to both external gamma and beta radiations arising from the fallout; however, their beta doses were scarcely recorded at the time due to an insufficient number of dosemeters. To solve this problem, the authors performed Monte-Carlo simulations which estimated the gamma and beta dose equivalent rates in outdoor work environments during the initial weeks of the accident and allowed the personal beta exposure to be reconstructed from the individual gamma doses recorded. The computer model consisted of air-ground interface, and the person receiving the dose was assumed to be standing on uniformly contaminated ground. The source term used was based on the isotopic compositions of Te, Te-I, I, Cs, Cs, Cs-Ba, Ba-La, etc., all identified in soil by 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 mid-March. The computed beta-to-gamma dose ratio ranged from ~10 at 50 cm to ~3 at 130 cm above the ground. The maximum possible beta exposure to the unprotected skin of the worker, engaged in outdoor operations, was estimated to be ~1 Sv, with the expectation of further dose reduction reflecting the effects of ground surface roughness and protective clothes.