Revelations that low amounts of plutonium, a component of nuclear bombs, were detected in soil near the Fukushima No. 1 plant sent shock waves across the nation Tuesday.

But experts say despite plutonium’s dangers and the mounting fears, there is little risk of the deadly radioactive particles spreading to a wider area.

The plutonium leak “will have no impact on the surrounding residential areas,” Hironobu Unesaki, a professor at the Kyoto University Research Reactor Institute, told The Japan Times. The nuclear engineering expert added the continued dispersal of radioactive iodine and cesium is much more worrisome than the plutonium leaks.

Tokyo Electric Power Co. announced the plutonium find late Monday during a hastily arranged news conference. Officials said they found plutonium-238, -239 and -240 during a study conducted a week ago in and around the power plant.

A few hundred grams of soil were taken from five locations between 500 meters and 1 km from reactors No. 1 and No. 2, they said. Although traces of plutonium were found, Tepco stressed the contamination levels pose no health hazard.

Detected so far are levels of radioactive decay ranging between 0.18 and 0.54 becquerel per kilogram of soil — about the same amount observed in Japan after the nuclear tests carried out in the Pacific in the 1950s and 1960s.

“This does not pose any (threat) to human health,” an official of the Nuclear Industrial Safety Agency in Tokyo said Monday. But he also acknowledged that because plutonium is produced within the central parts of reactors, the five levels of the containment mechanism — designed to be airtight — have been breached since the tsunami knocked out the power plant’s electricity supply.

Tepco officials claim the plutonium leak is small, but people were still alarmed.

This is because plutonium-239 has a half-life of 24,000 years, unlike the eight days for iodine-131 and 30 years for cesium-137, the two major radioactive substances that have contaminated vegetables and tap water as far away as Tokyo.

In addition to the long-lasting risks plutonium poses, the radioactivity it emits is in alpha particles, compared with the beta particles from iodine-131. Alpha particles are known to pose a greater risk to health.

For example, in data compiled by the Japan Atomic Energy Agency, 21 mg of potassium cyanide is considered a lethal dose to an average male weighing 70 kg. When it comes to plutonium, 13 mg is enough to kill.

The U.S. Nuclear Regulatory Commission explains on its website that if one drinks or eats plutonium oxide, most of it will pass through without the body absorbing it. But if inhaled, usually between 20 and 60 percent is retained in the lungs. Plutonium entering from an open wound may also move directly into body parts and organs, the commission said.

Even Tepco is aware of the dangers and talked of the hazards related to plutonium in its 2010 nuclear power pamphlet, noting the substance “can cause cancer once retained in liver and bones.”

But one crucial characteristic limiting the spread of plutonium is that it weighs 20 times more than water and 2.5 times more than iron.

Based on such data, Chief Cabinet Secretary Yukio Edano assured reporters at a news conference Tuesday that plutonium will not become airborne and therefore spread to a wider region.

While Kyoto University’s Unesaki said the area found to have traces of plutonium will require appropriate management, he agreed that the heavyweight particles will not spread over a wide area.

“Even in the case of Chernobyl, in which traces of iodine and cesium were found thousands of kilometers away and all over Europe, plutonium was found only within a radius of 30 km from the nuclear power plant,” Unesaki explained.

The spread of plutonium at Chernobyl was due in part to the reactors’ use of solid graphite as neutron moderators, which started a ferocious fire and a strong updraft. The reactors in Fukushima use light-water as moderators.

For such reasons, even if the reactors in Fukushima were to experience a hydrogen explosion that completely obliterates all safety measures — which is impossible from an engineering point of view, according to Unesaki — the spread of iodine and cesium will do much more damage than plutonium.

So far all parties, including Tepco, the government and the nuclear safety agency, have been unable to find the plutonium leak. Possibilities include reactors No. 1, 2 and 3, which were in operation when the earthquake hit, and any of the fuel rod pools adjacent to all six reactors.

The leak may also have come from reactor No. 3, which uses MOX fuel, which is known to contain weapons-grade plutonium. Thirty-two of the 548 fuel elements in reactor No. 3 use the mixed plutonium and uranium oxide fuel, according to Tepco.

Some people expressed strong concern over using such highly toxic fuel at the aging Fukushima plant. But Tepco opted to go forward, concluding it was a more efficient way of using limited resources. Tepco began producing energy from MOX fuel last October.

But Kyoto University’s Unesaki advised that instead of fearing an unlikely catastrophe, Tepco and others should keep their focus on the task at hand.

“It’s hard to believe that conditions of the nuclear reactors will abruptly deteriorate at this point. Instead of fearing an explosive outbreak of radioactive particles, there should be more focus on not allowing the ongoing leaks to continue for an extended period of time,” he said.

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