How Dangerous Is the Plutonium Leaking from the Japanese Nuclear Reactor?

On March 29, Japanese officials announced that toxic plutonium had been detected in the soil surrounding the earthquake- and tsunami-damaged Fukushima Daiichi nuclear plant. The plutonium is thought to come from partially melted fuel rods in one of the plant’s reactors. Tokyo Electric Power Co., which runs the stricken plant, maintains that the plutonium doesn’t pose any threat to human health, but given the number of times the company has been caught downplaying the crisis over the last few weeks, it’s understandable that the public would be skeptical. Isn’t even a tiny bit of plutonium extremely dangerous?

Yes, but it’s far from the biggest problem at Fukushima right now. We don’t yet know exactly how much plutonium was detected in the soil near the plant, but it’s unlikely to pose a serious health threat, particularly for those beyond the immediate vicinity of the plant. According to the International Atomic Energy Agency, the amount of plutonium detected does not exceed the levels normally tracked by Japanese authorities. Traces of plutonium are often found in soil around the world — an unfortunate consequence of decades of nuclear testing — and it’s only because of the isotopic composition of this sample that authorities can say for certain it came from the damaged reactor.

Plutonium is scary stuff, largely because of how long it stays radioactive: The plutonium-239 isotope, among those used in one of the Fukushima reactors — has a half-life of 24,000 years. However, the immediate dangers posed by plutonium exposure are often exaggerated. According to a 1995 report from the Lawrence Livermore National Laboratory, you would have to ingest about .5 grams of plutonium to die immediately, compared to about .1 grams of cyanide. The plutonium at Fukushima isn’t in the air, but inhaling about 20 milligrams of plutonium would probably kill you within a few months. External exposure carries almost no risk.

It is possible that, down the road, plutonium inhalation could be a concern, but it’s still a relatively small one. Inhaling 0.0001 milligrams of plutonium would increase the risk of cancer mortality from about 200 in 1,000 to 201.2 in 1,000.

But at the amounts and levels currently detected, it’s unlikely that anyone will be inhaling even that much plutonium from the Fukushima leak. What about plutonium in the drinking water? It, too, is a relatively minor threat: Plutonium is a heavy element that does not dissolve easily in water. If 10 ounces of it were introduced into a reservoir, only about 3 milligrams (one part in 100,000) would be dissolved; the rest would settle into sediment. If, somehow, the entire 3 milligrams were ingested by a population, it would, in theory, only cause about 0.6 additional cancer deaths.

Plutonium may be grabbing the headlines right now, but it’s not the most dangerous substance being emitted from Fukushima. The steam intentionally vented from the plant contains iodine and cesium, both of which have a far shorter half-life than plutonium, but are being released in much higher amounts and, being airborne, can travel much farther. Radioactive iodine-131 has been detected off the coast of Fukushima at levels 1,150 times higher than normal. These elements may not be quite as radioactive as plutonium, but if ingested or inhaled, they also pose a risk of causing cancer.

The severity of Japan’s crisis shouldn’t be downplayed, but plutonium is not the element that should be keeping people up at night.

Thanks to John Lee, professor of nuclear engineering and radiological sciences at the University of Michigan.