THIS STORY HAS BEEN FORMATTED FOR EASY PRINTING
DEVASTATION IN JAPAN

So far, little fear of a new Chernobyl

Residents evacuated from areas around the Fukushima nuclear plant were checked for radiation yesterday in Koriyama. Residents evacuated from areas around the Fukushima nuclear plant were checked for radiation yesterday in Koriyama. (Wally Santana/ Associated Press)
By Carolyn Y. Johnson
Globe Staff / March 14, 2011

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Scientists and radiation specialists monitoring the unfolding nuclear crisis in Japan say it is the worst since Chernobyl and also strikingly different from that disaster because it may not, in the end, pose a major risk to human health or the environment.

But the extent of infrastructure failure in Japan and the spreading number of reactors involved are worrisome, they said yesterday, complicating efforts to prevent meltdowns that could release harmful radiation.

With the information coming out of Japan spotty and fast-changing, and with the situation not fully under control, the scientists said there is uncertainty about the outcome and the extent of the threat to the public from a meltdown, which occurs when the hot fuel — uranium pellets held together in a tube-like sheath called a fuel rod — begins to degrade and melt, leading to the possible release of radiation.

Specialists described various scenarios that could develop in the coming days:

■ The efforts to cool down the nuclear fuel will be successful, with little or no release of harmful radiation.

■ The fuel will not cool down quickly enough, resulting in a partial or complete meltdown. The molten fuel could collect in the bottom of the reactor vessel and breach its wall. If that happens and the fuel is released into the containment structure, it is possible that explosions could occur and that radiation could be released.

But Richard Lester, head of the department of nuclear science and engineering at MIT, said, “The most likely scenario is that there will be partial melting of the fuel, in one or more of these reactors, which may have already occurred — we don’t know. But the most likely scenario, as far as I can tell, is the fuel that has melted will remain contained inside the reactor vessel where the fuel was initially.’’

An explosion at the Fukushima Daiichi No. 3 reactor was reported last night, but officials said the inner reactor container remained intact. The Pentagon said yesterday that small amounts of radioactive particulates were registered by helicopters flying 60 miles from the plant, signaling that some environmental contamination had spread, according to The New York Times.

The public health impact of the unfolding crisis is difficult to predict. If radiation stays within the reactor and the containment structure, and because the immediate area around the plant has been evacuated, exposures would likely be avoided.

Much also depends on the winds, which tend to blow toward the sea. If the situation gets worse, more evacuations will probably take place. Iodine tablets are reportedly being distributed by the Japanese government, which can block the uptake of cancer-causing radioactive iodine in the thyroid. Uptake in people exposed was one of the major consequences of the Chernobyl disaster.

Dr. John Boice, a professor at Vanderbilt University School of Medicine, said there was no risk of people in the United States being exposed to harmful amounts of radiation, even if the situation were to get drastically worse in Japan. The Nuclear Regulatory Commission yesterday said that the small radiation releases that have occurred thus far were carried out to sea, away from people, and are not expected to bring harmful levels to the West Coast of the United States, Hawaii, or Alaska.

Two reactors in Japan were said yesterday to be having partial meltdowns, with concerns about at least two others, according to news reports. The problems are all seemingly related to the inability to cool off the reactor fuel due to power outages, back-up generator failures, and cooling system problems.

“In a situation like this, one always has to be very concerned,’’ said Lester. “But a Chernobyl-like situation is not on the horizon, mainly because of the differences in the design of the reactor.’’

In the 1986 Chernobyl disaster, the Ukrainian plant did not have a containment structure. That, along with the reactor’s explosion and a subsequent fire, allowed radioactive material to disperse. The troubled Japanese reactors are surrounded by containment walls, thick metal, and reinforced concrete that should better contain radiation if the reactor is breached, according to nuclear specialists.

While it is difficult to compare nuclear disasters, especially because the Japanese reactor problems are caused by a major national disaster that has wreaked havoc widely, experts said that what is happening in Japan is closer to — though more serious than — what happened at Three Mile Island in Pennsylvania in 1979, when fuel in a reactor partially melted. In that accident, a small amount of radiation was released, but no impact on health was detected.

Edwin Lyman, a senior scientist with the Union of Concerned Scientists, said that it is clear that the measure Japanese officials have taken, cooling the fuel in reactors using sea water, certainly delayed damage to the fuel. But he cautioned that the situation is still evolving.

“The measures have delayed the onset of total core damage; the question is are they delaying the inevitable or not,’’ Lyman said.

Olli Heinonen, a senior fellow at the Belfer Center for Science and International Affairs at the Harvard Kennedy School, said that a nuclear plant is designed to withstand a meltdown, but that the earthquake adds unknowns.

Even in the worst-case scenario “you lose the coolant and everything goes from under control and then you have the meltdown of the core. It’s basically designed to withstand it,’’ Heinonen said. “But first of all, there has been an earthquake now — what’s the consequence of that? You have uncertainties there.’’

As the situation develops, public health and environmental consequences should become more clear. So far, Japanese officials have reported that as many as 160 people have been exposed to radiation. The greatest risk is to workers who are working close to the source of radiation.

Lester noted that from his knowledge of reports of the highest radiation exposure, the dose is not high enough to cause acute radiation sickness. Those doses could increase the lifetime risk of cancer somewhat, he said.

Lyman estimated that the radiation experienced by workers would increase their risk of cancer by about 1 percentage point.

“The concern is completely understandable — especially in Japan, where there are particular sensitivities to the release of radiation, for obvious reasons,’’ Lester said, referring to the atomic bomb blasts that ended World War II. “But the health consequences of this situation — the nuclear situation — are just very, very negligible so far compared with everything else going on there.’’

Carolyn Y. Johnson can be reached at cjohnson@globe.com.

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