Contaminated rice bags are displayed in a warehouse in Fukushima city on November 17.

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Tests found higher-than-allowed cesium levels in rice grown northwest of Fukushima Daiichi

It's the latest of several bans on food shipments since the March disaster

A recent study warned farming could be "severely impaired" by cesium fallout

Tokyo CNN  — 

Japanese authorities have halted the shipment of rice from some farms northwest of the Fukushima Daiichi nuclear power plant after finding higher-than-allowed levels of radioactive cesium, local authorities said Thursday.

The rice was grown in the Ohnami district of Fukushima Prefecture, about 60 kilometers (36 miles) northwest of the plant. The prefectural government banned rice shipments from the district after tests found a sample of brown rice from one farm contained radioactive cesium at a level about 25 percent higher than government regulations allow – a level that experts say would likely pose no immediate threat to human health and only a slight long-term risk.

It’s the latest of many limitations on livestock and produce imposed after the March disaster at Fukushima Daiichi, the worst nuclear accident since Chernobyl. The plant spewed vast quantities of radioactive particles into the environment following the historic earthquake and tsunami that triggered meltdowns in its three operating reactors.

The sample that triggered Thursday’s action was taken from about 840 kg (1,850 pounds) of rice grown in the same field, well outside the 20-kilometer exclusion zone drawn around the plant after the worst nuclear accident since Chernobyl.

About 154 Ohnami farms have produced nearly 200 tons of rice so far this year, prefectural officials estimated. But after interviews with 86 farmers, they said only 1 ton of rice has been sold to local shops, with the rest kept in storage or consumed within farms.

Fukushima Prefecture ranks among Japan’s top producers of fruits, vegetables and rice, and Japan’s effort to keep radioactive contaminants out of food “appears to be in very good order,” according to a report on cleanup efforts released Tuesday by the International Atomic Energy Agency.

But a separate study published this week by researchers in the United States, Japan and Norway warned that the spread of cesium-137 could leave agriculture “severely impaired” in several Japanese prefectures. The isotope has a radioactive half-life of 30 years, making it one of the longer-lived nuclear wastes spread by the Fukushima Daiichi disaster.

Japanese authorities have so far estimated about 8,300 hectares (20,000 acres) of paddies and fields are contaminated beyond acceptable limits for farming, the IAEA report found. Most decontamination work so far has involved scraping off up to 5 centimeters (2 inches) of topsoil, but that could leave Japan to deal with an amount of irradiated dirt and debris comparable to a year’s worth of municipal garbage collection nationwide, the IAEA noted.

CNN’s Yoko Wakatsuki and Matt Smith contributed to this report.