Checks of rice grown in 16 prefectures show that more than 90 percent of the grain is free of radioactive cesium.
Checks of rice grown in 16 prefectures show that more than 90 percent of the grain is free of radioactive cesium.
So far, 2,500 rice paddies have been inspected at the behest of the central government in the aftermath of the disaster at the Fukushima No. 1 nuclear power plant.
Nationwide radiation checks which started in early August are continuing.
Initially, the central government requested 17 prefectures to check for levels of radiation. This will eventually cover between 3,500 and 5,000 rice paddies, according to farm ministry estimates.
The 17 prefectures include those where crop shipments were prohibited after the earthquake, or which adjoin these municipal areas.
Ninety-six percent of all rice paddies, or 2,429 fields under cultivation in the 16 prefectures, were found to be cesium-free.
Even among the remaining 98 rice paddies, where varying radiation levels were found, the highest measurement was only 136 becquerels of radioactive cesium per kilogram. This was in Fukushima city.
However, the figure is well below the government's safety standard of 500 becquerels per kilogram.
Three other rice paddies--in Fukushima and Miyagi prefectures--had levels exceeding 100 becquerels.
In Fukushima Prefecture, site of an unprecedented nuclear crisis that is still unfolding, no radioactive cesium was detected in 453 of 531 rice paddies, or 85 percent of all the fields under cultivation there.
When hydrogen explosions tore through reactors at the Fukushima plant and released radioactive materials following the March 11 earthquake, rice planting had not yet started.
The contamination levels of rice were determined by the degree to which the plants absorbed radioactive substances from the soil.
Before rice planting got under way in May, the central government estimated the coefficient rate of radioactive cesium transferring to rice from soil at 0.1. In Fukushima Prefecture, rice planting was prohibited in fields with more than 5,000 becquerels of radioactive cesium per kilogram of soil.
At the time, it was estimated there were 39,000 hectares of rice paddies with contamination levels ranging from 1,000 to 5,000 becquerels.
Concerns remained over the possible detection of rice with radiation levels above the government standard.
But that proved to be wrong. A farm ministry official in charge of the matter let out a sigh of relief, saying, "That's what we had expected."
When calculating the coefficient rate of radioactive cesium transferring to rice from soil, the ministry drew upon 40 years of research on the effects of overseas nuclear experiments.
The coefficient rates for 564 experiments the ministry researched vary between 0.00035 and 0.64, which average out at 0.012. The ministry adopted the rate of 0.1, nearly 10 times the average rate.
Characteristics of the soil in Fukushima Prefecture, which was initially believed to have higher contamination levels than other prefectures, explains the zero or low radioactive cesium levels found in rice grown there.
The Fukushima Agricultural Technology Center experimented with growing rice in soil contaminated with radiation levels at which rice planting would be prohibited.
When testing unprocessed rice for the amount of radioactive cesium transferred from soil, the figure came to a maximum of 80 becquerels per kilogram.
Many rice paddies in Fukushima Prefecture are on clay-rich gray lowland soil.
Yasuyuki Muramatsu, a professor of radiochemistry at Gakushuin University who specializes in the effects of radiation on animal life, said: "This type of soil absorbs cesium in large amounts and does not release it easily. This may explain the finding that so little cesium transferred to unprocessed rice from the soil."
There is another possible reason. The prefectural government instructed farmers to spread 6-8 kilograms of potassium per 1,000 square meters of rice paddies before planting and another 2 kilograms before the ears formed. Potassium's makeup is somewhat similar to cesium. Researchers believe that cesium levels will be constrained when rice absorbs potassium first.
The government also recommended that farmers dig more irrigation ditches to improve drainage efficiency and drain their paddies while the rice was growing to prevent rice ears from collapsing and coming into contact with contaminated soil.
Farmers were also advised to cut rice ears higher up when harvesting so soil would not come into contact with the grain.
Ikuro Anzai, professor emeritus specializing in radiation protection at Kyoto's Ritsumeikan University, said he doubted that rice grown in Fukushima Prefecture would be found to exceed radiation levels above safety standards.
"Given that cesium bonds firmly with the soil and is rarely absorbed by rice, it stays there," Anzai said. "The problem comes after harvesting the rice. The top soil needs to be removed and disposed of. We will have to closely monitor the situation for many years to come."
(The article was written by Keiichiro Inoue, Ryo Inoue and Shunsuke Kimura.)