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

Abstract

In 2011, after the Fukushima Dai-ichi Nuclear Power Plant accident, the initial decrease in the ambient dose equivalent rate (dH*(10) dt−1), an alternative quantity to the effective dose, was studied using monitoring data obtained from March 16, 2011. The dH*(10) dt−1 was normalized by the 137Cs activity per unit area (norm-dH*(10) dt−1) to analyze the data across monitoring sites with different deposition levels. The norm-dH*(10) dt−1 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−1 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−1 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.

Temporal Change in Atmospheric Radiocesium during the First Seven Years after the Fukushima Dai-ich Nuclear Power Plant Accident

“…Recent studies have also suggested that anthropogenic activities can promote reduced air dose rate (Saito et al, 2019;Nakama et al, 2019;Yoshimura et al, 2020), indicating their importance in controlling the radiocesium levels on the ground and their effect on variation and temporal change of the RF. Radiocesium activity was monitored in atmospheric aerosol samples collected at 11 sites for five months after the FDNPP accident as part of a Nuclear Regulation Authority…”

“…In particular, we found that the effective half-life outside ERZ (0.22 y) was shorter than that inside the zone (0.29 y). Moreover, based on recent studies, the air dose rate outside ERZ decreased faster than inside ERZ, whereas the reduction was considered to be facilitated by anthropogenic activities (Saito et al, 2019;Nakama et al, 2019;Yoshimura et al, 2020). Therefore, anthropogenic activities may promote environmental remediation including atmospheric radiocesium concentration, suggesting that the anthropogenic activities possibly reduce the risk of inhalation radiation in the future.…”

“…The severe accident that broke out at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011 released a large amount of radiocesium into the atmosphere. Radiocesium is considered to be an important radioactive substance deposited on the ground after the accident in terms of radiation exposure due to its high level of release and long half-life (2.06 y for 134 Cs and 30.17 y for 137 Cs) (World Health Organization, 2012;Yoshimura et al, 2020). The exposure pathway in the contaminated area has been categorized into external and internal exposure (International Atomic Energy Agency: IAEA, 2013); the internal exposure mainly involves the inhalation of resuspended atmospheric aerosol, which is crucial for the retrospective assessment and prediction of the radiation risk.…”

Eight-year variations in atmospheric radiocesium in Fukushima City

“…We conducted eight-year measurements of atmospheric 134 Cs and 137 Cs in Fukushima city after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 to understand the time variations in and emission sources of 134 Cs and 137 Cs and to propose effective ways to reduce atmospheric radioactivity. Among the various radionuclides released to the environment, radio-Cs is particularly important due to its abundance in terrestrial ecosystems (the impacts of other nuclides were negligibly small 100 days after the accident; Yoshimura et al, 2020), long half-lives (2.06 y for 134 Cs and 30.17 y for 137 Cs), and bioaccumulation (accumulation in muscle tissues, with biological half-lives of 30-150 d;WHO, 2011). Radio-Cs forms aerosols in the air and is therefore efficiently deposited onto the ground surface via precipitation in addition to via dry deposition.…”

Evacuating Orange County, California, in About Eleven (11) Seconds

“…Further study is required to determine if these numbers are significant, as thyroid cancer is a very common disease. Yoshimura et al (2020) report that there were significant differences between the types and amounts of radiation exposure at Chernobyl and Fukushima.…”