SEOUL, June 14 (AJP) -Researchers have for the first time reconstructed how highly radioactive cesium-rich microparticles released during the Fukushima nuclear disaster spread across northeastern Japan, uncovering a previously hidden contamination pathway that may reshape how future nuclear accidents are assessed.
The findings, published in the Journal of Hazardous Materials, come from an international research team led by scientists from National Taiwan University and the University of Tsukuba.
The study focuses on cesium-rich microparticles, or CsMPs — microscopic glass-like particles formed inside the Fukushima Daiichi reactors during the 2011 core meltdowns. Measuring only a few micrometers in diameter, the particles contain highly concentrated radioactive cesium and are largely insoluble in water.
Scientists have long considered CsMPs a unique concern because they can remain intact in the environment and potentially become lodged in human lungs if inhaled.
While radioactive contamination from Fukushima has been extensively mapped over the past 15 years, the distribution and release mechanism of CsMPs remained poorly understood.
To address the gap, researchers analyzed soil samples collected from 100 locations across Fukushima Prefecture before July 2011. Using a quantitative cesium particle method originally developed in 2018, the team measured both the abundance of CsMPs and their contribution to total radioactive contamination.
The results revealed substantial regional differences. Researchers found between zero and 52.3 cesium microparticles per gram of soil, while in some locations CsMPs accounted for as much as 61.9 percent of total radioactive cesium contamination.
The study showed that contamination patterns from CsMPs differed significantly from those of conventional soluble radioactive cesium.
By combining field measurements with atmospheric dispersion simulations generated by Japan's WSPEEDI emergency radiation forecasting system, researchers reconstructed the particles' release and transport history.
The team concluded that large quantities of CsMPs were emitted between approximately 2 a.m. and 3 a.m. on March 15, 2011, during one of the most critical phases of the Fukushima accident.
A radioactive plume carrying the particles moved in a broad clockwise arc from south to southwest and then northwest across Fukushima Prefecture, dispersing contamination over a far wider area than previously recognized.
At peak concentrations, researchers estimated that the plume contained up to 4,700 cesium microparticles per cubic meter of air near the plant, with roughly 2,070 particles per cubic meter transported southwestward.
The study also found that radioactive materials released after midnight on March 16 contained virtually no cesium microparticles and instead consisted primarily of water-soluble forms of cesium.
Rainfall played a crucial role in determining where the particles ultimately accumulated. Areas experiencing precipitation while contaminated plumes passed overhead recorded significantly greater concentrations of CsMPs than would have been predicted from total radiation levels alone.
"The findings provide important insights into how radioactive microparticles released during nuclear accidents disperse in the environment," said Satoshi Utsunomiya, a professor at National Taiwan University and the study's corresponding author.
He said the research would contribute to future environmental risk assessments and help establish what he described as a new scientific field, "Advanced Radioactive Particle Science."
The findings arrive as scrutiny of Fukushima remains intense more than 15 years after the disaster.
On Friday, South Korea's government said Tokyo Electric Power Company Holdings (TEPCO) had temporarily halted the 20th discharge of ALPS-treated water from Fukushima Daiichi for the second time this week after an alarm was triggered during the water transfer process.
According to Kim Young Soo, First Vice Minister for Government Policy Coordination, the discharge automatically stopped at approximately 5:04 p.m. Friday while TEPCO investigated the cause.
The utility had already suspended the same discharge operation on June 10 following a separate alarm linked to a faulty valve between a treated-water storage tank and a transfer pump. Operations resumed on June 11 after repairs were completed, only to be interrupted again two days later.
TEPCO later said no abnormalities were found in the discharge facilities and attributed the latest alarm to a temporary reduction in the volume of seawater used to dilute the treated water before release.
The company subsequently resumed operations after confirming the system's safety.
Japan began releasing ALPS-treated water from Fukushima into the Pacific Ocean in August 2023 as part of the decades-long decommissioning process.
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