University of Tokyo researchers have revealed through observational data analysis and computer simulation that what are known as nocturnal local winds played a key role in carrying radioactive material released into the air by the Fukushima Daiichi nuclear accident, from Fukushima to the Tokyo metropolitan area.
Radioactive material was frequently detected in Japan’s capital region following the March 2011 accident at the Fukushima Daiichi nuclear power plant, about 200km away. However, the mechanism by which the air mass including radioactive materials (radioactive plumes) got transported to the country’s largest metropolitan area from Fukushima Prefecture to its northeast was not fully understood.
A research group led by Project Researcher Takao Yoshikane and Associate Professor Kei Yoshimura at the University of Tokyo Institute of Industrial Science analyzed observational data and ran computer simulations (using the semi-Lagrangian transport model) to determine whether the radioactive plumes were carried by chance haphazard activity in the air or by a regular mechanism in the atmosphere. The scientists found that the radioactive plume moves along two local wind systems that appear during the night on calm days when the impact of northwesterly seasonal winds and low-pressure systems are low, namely local wind systems comprising a northerly sea wind blowing off the Fukushima coast toward the eastern Kanto region, including Tokyo, and a northeasterly local wind associated with a meso-scale—few-hundred-kilometer—low-pressure system (meso-low), which develops around the Kanto region in the nighttime. They also discovered that the gravity current (consisting of a lighter upper layer of warm air and heavier lower layer of cold air), which appears due to difference in north-south temperatures, is primarily responsible for the formation of the nighttime local winds.
These findings indicate that should radioactive material be released over a long period of time, radioactive plumes could be frequently carried even to faraway places by nocturnal local systems, and cause serious contamination in those areas. On the other hand, it is possible to make a rough prediction, by knowing the cycle of the winds, of when, where, and how the radioactive plumes will travel. The current results could prove useful in determining when to seek shelter to avoid exposure to radiation.
“Stronger risk management strategies that allow for quick and cool-headed response to unforeseen situations are being sought,” says Yoshikane. He continues, “It is necessary to take into account local factors specific to each area, such as geographical features and traffic conditions. We hope that by expanding our study we can contribute to the development of risk management strategies through exchanges with people in other fields, government agencies, and local governments.”
This study was supported by the Strategic Basic Research Programs (CREST) of the Japan Science and Technology Agency (JST) as a project covering “A tracer simulator of fallout radionuclides for safe and sustainable water use” in the research area “Innovative technology and system for sustainable water use.”