A paper out today in the journal Scientific Reports shows evidence that radioactive plutonium spread tens of kilometres from the Fukushima Daiichi nuclear plant. The new work could lead people to believe that there is a health risk, but that is not the case.

Plutonium is a radioactive element that is made inside nuclear reactors. Unlike some of the other contaminants to come from Fukushima, it is not volatile, and it is much harder for plutonium to escape from a nuclear reactor during a meltdown. That doesn’t mean it can’t happen: when Chernobyl’s Unit 4 reactor exploded in 1984, it released a large quantity of plutonium into the surrounding environment.

Plutonium can be dangerous. When it decays, it usually releases heavy particles such as electrons and helium nuclei. These particles aren’t particularly dangerous outside the body, but if plutonium is ingested they can cause genetic damage.

The new paper shows that minute quantities of plutonium from Fukushima have spread far from the plant. In samples taken to the northwest and in the J-Village, where workers live, the authors found trace amounts of plutonium in the surface soil (see map). Looking at the ratio of plutonium-241 to plutonium-239, they were able to conclude that the plutonium came from Fukushima rather than other sources, such as old nuclear-weapon tests.

The additional exposure from inhaling this loose plutonium at the S2 site is around 0.5 millisieverts (mSv) over 50 years.

This dose — 0.5 mSv over half a century — is five times higher than the government’s current estimate for plutonium exposure from the accident, but it doesn’t mean there’s a health risk. Over the same period, the average person on Earth would receive 120 mSv from natural sources of radiation. Even for those who worry about low-dose radiation, it’s safe to say that this additional plutonium exposure won’t have an impact.

Nevertheless, the measurements are interesting. The distances at which the team finds the material imply that plutonium was ejected during the hydrogen explosions in the first days of the crisis. And the relatively low levels (around 10,000 times lower than Chernobyl) suggest that the heavily shielded concrete casings around the reactors did offer some protection from the worst of the fallout.

There’s another reason this work is important. As we report this week, mistrust is running high among residents in Japan. Independent measurements such as these are extremely important in providing residents and evacuees with the information they need to get on with their lives. In this case, the measurements show little additional risk. But news of plutonium, no matter how small, will no doubt be dispiriting to the residents of Fukushima.