Typhoons that hit Japan each year are contributing to the spread of radioactive material from the Fukushima nuclear disaster into the country's waterways, researchers say.

A joint study by France's Climate and Environmental Science laboratory (LSCE) and Tsukuba University in Japan shows contaminated soil gets washed away by the high winds and rain and deposited in streams and rivers.

"There is a definite dispersal towards the ocean," LSCE researcher Olivier Evrard said Wednesday.

The typhoons "strongly contribute" to soil dispersal, he said, though it can be months later, after the winter snow melts, that contamination actually passes into rivers.

An earthquake-sparked tsunami slammed into the Fukushima plant in March 2011, sending reactors into meltdown and sparking the worst atomic accident in a generation.

After the accident, a large number of radioactive particles were flung into the atmosphere, dispersing caesium particles which typically cling to soils and sediment.

Studies have shown that soil erosion can move the radioactive varieties of cesium-134 and 137 from the northern mountains near Fukushima into rivers, and then out into the Pacific Ocean.

Last year, the radioactive content of Japan's rivers dropped due to fairly moderate typhoons.

However, more frequent and fierce storms in 2013 have brought a new flood of caesium particles.

This is "proof that the source of the radioactivity has not diminished upstream" said Mr Evrard.

Tsukuba University has completed a number of studies on Fukushima since November 2011.

Scientists "concentrated mostly on the direct fallout from Fukushima yet this is another source of radioactive deposits" that must be taken into account, he warned.

Coastal areas home to fishermen or where people bathe in particular face a potential risk.

Tens of thousands of people were evacuated from around the Fukushima plant following the disaster and nearby villages and towns remain largely empty as residents fear the risks of radiation.

The delicate process of decommissioning the site is expected to take decades.

AFP