Researchers detected a type of radioactive plutonium in soil from three different locations in Fukushima Prefecture, although the amount is too tiny to affect human health, the team said in a report published in a science magazine.

According to the report in the March 8 edition of Scientific Reports, the team said they detected plutonium-241 in the amount of 34.8 becquerels per kilogram in soil from the town of Namie, 20.2 becquerels per kilogram in the village of Iitate and 4.52 becquerels per kilogram in the town of Naraha.

Because the half-life of plutonium-241 is 14 years, the substance likely came from the Fukushima No. 1 nuclear plant and not from fallout of past atmospheric nuclear experiments, the research report says.

“The amount found this time is not much more than the fallout from atmospheric nuclear experiments (conducted in the 1960s,) and therefore there is no problem,” said Kunikazu Noguchi, a radiation protection expert at Nihon University.

He was not in the researcher team.

This is the first time plutonium-241 from the Fukushima nuclear crisis has been detected. The Ministry of Education, Culture, Sports, Science and Technology announced in September that it detected plutonium-238, -239 and -240 in Fukushima Prefecture.

The amount found then was also not much more than the fallout from atmospheric nuclear experiments, Noguchi said.

Nature Publishing Group, which publishes Scientific Reports, also said on its blog Thursday that the radioactive plutonium in the new finding will not have a health impact.

“Nevertheless, the measurements are interesting,” the blog says. “The relatively low levels (around 10,000 times lower than in Chernobyl) suggest that the heavily shielded concrete casings around the reactors did offer some protection from the worst of the fallout.”

Plutonium, which is excreted from digestive organs easily but tends to stay in lungs for a long time, is heavier than iodine, cesium and other radioactive substances, and thus does not travel far from its source.

Still, it is possible that plutonium was blown to prefectures neighboring Fukushima, Noguchi said.

Plutonium-241 is the only type of radioactive plutonium that emits beta rays, which travel dozens of centimeters. Plutonium-238, -239 and -240 emit alpha rays, which travel just a few centimeters.