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Radioactive cesium from 2011 Fukushima nuclear disaster spread over wide area: study

June 26, 2026 (Mainichi Japan), https://mainichi.jp/english/articles/20260626/p2a/00m/0na/004000c

TOKYO — A research team including members of the University of Tsukuba and National Taiwan University has clarified the dispersal routes of highly radioactive “cesium-rich microparticles” (CsMPs) released in the Fukushima nuclear disaster.

In the accident at Tokyo Electric Power Company Holdings Inc. (TEPCO)’s Fukushima Daiichi Nuclear Power Station in 2011, radioactive materials mainly contaminated areas to the plant’s northwest, but CsMPs were carried across a wide area of Fukushima Prefecture. The team also found that they were generated in large quantities four days after the March 11 Great East Japan Earthquake that year.

Actual extent of dispersal had remained unclear

CsMPs are spherical microparticles several micrometers in diameter. In the Fukushima Daiichi accident, they were formed when high-temperature nuclear fuel melted through to the floor and components of the melted concrete turned glasslike, encasing radioactive materials. The microparticles do not dissolve easily in water, and there are concerns that if inhaled they can remain lodged in the lungs, but the reality of how they spread had not been well understood.

The research team developed a method to examine the number of CsMPs contained in soil and analyzed soil samples taken from 100 locations in Fukushima Prefecture immediately after the accident. As a result, large numbers of CsMPs were found to the northwest and southwest of the plant, with as many as 52 particles per gram of soil. At some locations, 60% of the radioactivity in the soil was due to CsMPs.

The team then examined the dispersal process together with simulations of radioactive plumes, or air flows, and found that large-scale releases had begun in the early hours of March 15, 2011. A radioactive plume containing as many as 4,700 CsMPs per cubic meter was carried clockwise over a wide area of the prefecture, starting at the plant and moving from south to southwest and then northwest. It also reached Tokyo, the team said.

On the other hand, radioactive plumes released from 12 a.m. on March 16 onward contained no CsMPs. Instead, they are believed to have contained cesium in a form that easily dissolves in water.

‘Highly significant’ for future responses

Satoshi Utsunomiya, a National Taiwan University professor of environmental science on the team, said, “It is highly significant that we have clarified the process of when CsMPs were generated inside the plant and when their formation ceased.” The findings are expected to lead to decontamination efforts that better reflect actual conditions and to guidelines for responding to nuclear disasters.

Shinya Yamasaki, an associate professor of analytical chemistry at the University of Tsukuba, commented, “It has been shown that radiation maps and the distribution of CsMPs are different.”

The findings were published in the Journal of Hazardous Materials.

(Japanese original by Yurika Tarumi, Lifestyle, Science & Environment News Department)

June 30, 2026 - Posted by Christina Macpherson | Fukushima continuing, Japan, radiation