Initial decrease in the ambient dose equivalent rate after the Fukushima accident and its difference from Chernobyl.

In 2011, after the Fukushima Dai-ichi Nuclear Power Plant accident, the initial decrease in the ambient dose equivalent rate (dH*(10) dt), an alternative quantity to the effective dose, was studied using monitoring data obtained from March 16, 2011. The dH*(10) dt was normalized by the Cs activity per unit area (norm-dH*(10) dt) to analyze the data across monitoring sites with different deposition levels. The norm-dH*(10) dt showed a rapid decrease during the first 60 days, followed by slow decrease and was modeled using two exponential functions. The norm-dH*(10) dt obtained in areas dominated by paved surfaces and buildings showed a faster decrease than the unpaved-dominant field, and this decrease was facilitated in residential areas compared with the evacuation zone. The decrease in norm-dH*(10) dt was compared with simulation results using parameters obtained in Europe after the Chernobyl Nuclear Power Plant accident that represent a decrease due to radionuclide migration (e.g., soil penetration and horizontal wash-off). The simulation results showed a faster decrease than our results, implying that there was less radiocesium migration in Fukushima than in Europe. The results also suggested that the regional variation in the decrease rate led to uncertainty regarding the external dose estimation.