18 Janvier 2016
January 18, 2016
THE ASAHI SHIMBUN
MINAMI-SOMA, Fukushima Prefecture--Radioactive substances that contaminated rice paddies here in 2013 came from the crippled Fukushima No. 1 nuclear plant, an international group of researchers said, rejecting a denial issued by Japan’s nuclear safety authority.
The researchers, led by Akio Koizumi, a professor at Kyoto University’s Graduate School of Medicine, reached the conclusion after analyzing radioactive substances and taking spot readings of radioactivity levels around Minami-Soma.
Koizumi presented the final report of the group, consisting of 11 researchers from Japan, Europe and the United States, to local farmers and other parties at a community center in Minami-Soma on Jan. 17.
“The cause of further contamination was the radioactive particles dispersed from contaminated rubble during the cleanup effort at the Fukushima No. 1 nuclear plant,” Koizumi concluded in the report.
Earlier, the agriculture ministry and the Nuclear Regulatory Authority (NRA) gave different views on the source of the contaminated rice.
In 2013, rice crops from areas of Minami-Soma were found with unexpectedly high radioactivity levels more than two years after the triple meltdown at the nuclear plant located 20 kilometers south of the city.
One theory was that highly radioactive substances were dispersed when workers were lifting and removing contaminated rubble at the Fukushima plant on Aug. 19 that year. Two workers at the plant were exposed to high doses of radiation during the cleanup process.
The Ministry of Agriculture, Forestry and Fisheries said the cause of the contaminated rice was “unknown” although it acknowledged “the possibility of the dispersal of contaminated dust.” The farm ministry discontinued its investigation without specifying the source of the contamination.
The NRA, however, said the contaminated rice was not related to the cleanup work at the nuclear plant.
The Minami-Soma city assembly expressed outrage over the NRA’s stance. Some in the city suspected the NRA of a cover-up.
Koizumi and the other researchers digitally recreated an accidental dispersal of contaminated dust from the plant in August 2013.
They used a new analysis system to estimate the amount of radioactive cesium that spread toward Minami-Soma based on radioactivity readings around the city and other factors.
The group’s cesium estimate was more than 3.6 times the amount initially estimated by the NRA.
The research group in September 2014 also collected soil samples from 10 locations around the contaminated rice paddies to determine the amount of strontium 90 in the area.
They confirmed that the ratio of strontium 90 to radioactive cesium in the soil samples was similar to the ratio that would be found near the Fukushima nuclear plant.
Beta-ray emitting strontium 90 is less airborne and tends to remain within close proximity of nuclear weapon testing sites or nuclear accidents. Radioactive cesium is more volatile and can easily adhere to fine dust spread by the wind.
In general, the amount of strontium 90 decreases the farther it gets from a nuclear plant, compared with radioactive cesium. In fact, hardly any strontium 90 has been detected far away from the Fukushima plant.
Based on the amounts of radioactive particles recorded around Minami-Soma, the researchers concluded that a highly irregular plume of radioactive cesium reached Minami-Soma on the third week of August 2013.
“Every single piece of data in the paper supports the fact that contamination by radioactive dust came from the debris at the nuclear plant,” Koizumi said.
Asked about the NRA’s conclusion, Koizumi said: “It seems they were blinded by their estimated amount of dispersed particles, and their choice for the analysis system was misguided. This kind of attitude would only increase the anxiety of residents in the affected areas.”
The group’s findings were published in the international academic journal Environmental Science & Technology last month after a peer review.
(This article was written by Masakazu Honda and Miki Aoki.)