日本开展土地放射性指数调查

New research has found that radioactive material in parts of north-eastern Japan exceeds levels considered safe for farming.

研究发现，日本东北部部分地区放射性物质超出了农业生产的安全水准。

The findings provide the first comprehensive estimates of contamination across Japan following the nuclear accident in 2011.

Food production is likely to be affected, the researchers suggest.

The results are reported in the Proceedings of National Academy of Sciences (PNAS) journal.

In the wake of the accident at Japan's Fukushima nuclear power plant, radioactive isotopes（同位素） were blown over Japan and its coastal waters.

Fears that agricultural land would be contaminated spurred research into whether Japanese vegetables and meat were safe to eat.

An early study suggested that harvests contained levels of radiation well under the safety limit for human consumption.

Contaminated crops

Now, an international team of researchers suggests this result deserves a second look.

To estimate contamination（污染） levels, Teppei Yasunari, from the Universities Space Research Association in the US state of Maryland, and his colleagues, took measurements of the radioactive element caesium-137 in soil and grass from all but one of Japan's 47 regions and combined these results with simulations based on weather patterns following the meltdown.

Caesium-137 lingers（徘徊） in the environment for decades, and so is more of a concern than other radioactive elements released in the cloud of steam when the reactors' cooling systems failed, leading to explosions.

The team found that the area of eastern Fukushima had levels of the radioactive element that exceeded official government limits for arable land（耕地） .

Under Japanese Food Sanitation Law, 5,000 becquerel per kg (Bq/kg) of caesium is considered the safe limit in soil (caesium-137 makes up about half of total radioactive caesium).

The researchers estimate that caesium-137 levels close to the nuclear plant were eight times the safety limit, while neighbouring regions were just under this limit.

The study showed that most of Japan was well below (average about 25 Bq/kg) the safety limit. Relatively low contamination levels in western Japan could be explained by mountain ranges sheltering those regions from the dispersal（分散，传播） of radioactive material, the authors said.