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updated 3/15/2011 9:06:52 AM ET 2011-03-15T13:06:52

Fears about health risks rose dramatically in Japan Tuesday with news of a greater radiation release and renewed warnings to remaining residents within 30 kilometers (19 miles) to stay indoors.

Japanese officials said that more radiation was released at a nuclear plant disastrously damaged by last week's tsunami. Prime Minister Naoto Kan said radiation had spread from four reactors.

"The level seems very high, and there is still a very high risk of more radiation coming out," he said.

Thyroid cancer is the most immediate risk, and the Japanese government made plans to distribute potassium iodide pills to prevent it. Worse case scenarios — lots of radioactive fallout — can lead to other cancers years later.

Even a meltdown would not necessarily mean medical doom, experts said. It depends on the amount and type of radioactive materials.

Donald Olander, professor emeritus of nuclear engineering at the University of California at Berkeley, said even the much higher levels of radiation are "not a health hazard."

The world has seen two big nuclear reactor scares — in 1986 at the Chernobyl plant in Ukraine, and in 1979 at the Three Mile Island plant in Pennsylvania.

At Three Mile Island, even though a quarter of the reactor core melted, the steel containment structure held. The radiation released was so minuscule that it did not threaten health — the equivalent of a chest X-ray to local residents.

At Chernobyl, where there was no containment vessel, far more radioactive material was released, and of a more dangerous type than at Three Mile Island. It stayed in soil and got into plants in Ukraine, contaminating milk and meat for decades. Thousands of children developed thyroid cancer from radiation exposure, and scientists are still working to document other possible health problems.

The lessons have not been lost on the Japanese as they grapple with the Fukushima Dai-ichi power plant, whose cooling systems failed after a power outage from the massive earthquake last week.

They have evacuated 180,000 people from areas near the troubled reactors, where relatively minimal fallout was mostly confined at first. They've told people still in the area to wear masks, which can keep radioactive particles from being inhaled.

Most importantly, they have stockpiled and are making plans to give out potassium iodide — pills that can keep radioactive iodine from being taken up by the thyroid gland and causing cancer.

"Those are all preventable cancers" if the protective pills are taken right after exposure, said University of New Mexico radiologist Dr. Fred Mettler. He led an international group that studied health effects of the Chernobyl disaster and is a U.S. representative to the United Nations on radiation safety.

At Chernobyl "they had millions of square kilometers to cover and it was all rural areas and they didn't really have anything stockpiled," he said.

The Russian reactor also lacked a containment vessel like those in Japan and the United States to prevent or minimize release of the more dangerous types of radioactive materials, Mettler said.

"Right now it's worse than Three Mile Island," Olander said, but isn't near the Chernobyl situation. Some radioactive iodine was released before the latest crisis Tuesday. Iodine is relatively short-lived, and potassium iodide pills can be used to block its uptake.

Of greater concern is the release of cesium, which officials had said was released in small amounts earlier. Cesium is absorbed throughout the body — not just by the thyroid — and stays in organs, tissue and the environment much longer, Mettler explained.

Cesium particles are relatively large and heavy, so they would not likely travel far in a plume. Much of it would drop near the reactor site, and officials hope, may be carried by winds east over the Pacific Ocean where it would fall harmlessly, Mettler said.

Any release of cesium is a concern environmentally and for health, said Jacqueline Williams, a radiation biologist and safety expert at the University of Rochester Medical Center in upstate New York.

"Prior to Chernobyl, we believed that the cesium would be diluted out, that once the cloud went through and it rained, the cesium would be washed out. What we found out was there was an accumulation of cesium in certain types of vegetation, and it accumulated rather than diluted," she said.

Animals fed on the vegetation and became contaminated, and meat and milk were affected.

"You can't be quite so blase about the fallout," Williams said.

At Three Mile Island, however, "the public health risk was close to zero because the radiation was contained within the site itself," Williams said.

Mettler agreed. The research he led documented 6,000 to 7,000 additional cases of thyroid cancer in people who were children and teens when Chernobyl occurred, "and there are questionable increases of leukemia in the cleanup workers but it's not certain."

And were there long-lasting problems from Three Mile Island?

"Not that most of the scientific community believes," Mettler said.

Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Video: Radiation leaks from Japan nuke plant

  1. Closed captioning of: Radiation leaks from Japan nuke plant

    >>> good morning. breaking news. japan 's nuclear crisis takes a dire turn. high levels of radiation spewing from the damaged nuclear plant following an explosion at a third reactor and a fire in a fourth. an official is calling it, quote, a very bad scenario as officials work to contain the risk today, tuesday, march 15, 2011 .

    >>> welcome to "today" on this tuesday morning. i'm meredith vieira .

    >> officials in japan are saying the radiation leaking from the crippled nuclear power plant is enough to impact human health .

    >> the big concern is the number 2 reactor which exploded on monday sending more radiation into the air and then a fire at reactor 4 broke out. that one had been shut down for maintenance before the quake. all but 50 employees of the plant have been evacuated. in a nationally televised address japan 's prime minister urged anyone living near the plant who had not already evacuated to seal themselves indoors and warned of the very high risk of more leaks.

    >> you hear the story and i think about the bravery of the 50 people who remain at the plant trying to cool the reactors at great personal risk.

    >> you wonder if they will have to leave as well and then what's going to happen?

    >> what's the outcome of that?

    >> in tokyo , slightly elevated radiation levels have been detected. we'll get the latest on an extremely tense situation straight ahead.

    >> there is a rare piece of good news. rescuers found a 70-year-old woman alive in her home four days after it was swept away by the tsunami. ann curry has details and a closer look at the toll this crisis is taking on people there. let's begin on this tuesday morning with the nuclear crisis. robert bazell , nbc's chief science correspondent, is in tokyo . good morning to you.

    >> reporter: good morning, matt. the nuclear crisis in japan has taken a turn for the worse. many people are saying it's ominous. that explosion in reactor number 4 blew a hole in the cement containment facility allowing radiation to leak. the fire in the other reactor is allowing some radiation to get out. there are several reactors at the fukushima site. one of them in the cement dome. so far the metal cylinder has not broken but radiation is escaping.

    >> the danger seems to have significantly increased over the past couple of days.

    >> reporter: the japanese prime minister naoto kan told people the radiation wasn't a widespread hazard and there was no need to evacuate beyond the 12 miles already established. those leaving have been checked for radiation. all the but the most essential personal nel have been told to leave the site. they offered public apologies.

    >> translator: this is a very poor, very bad scenario.

    >> reporter: satellite photos before and after show the damage. the fire that broke out today is at a reactor that was shut down for maintenance before the earthquake and was not releasing large quantities of radiation. experts agree the total amount of radiation isn't a significant health hazard beyond the area of the reactor.

    >> the amount of radiation that's likely to be released is going to be relatively small compared to an accident like chernobyl.

    >> reporter: it's not chernobyl for sure, but it's worse than mile islan d.

    >> bob, we mentioned that tokyo is 170 miles south of the power plant . i understand there is -- maybe chaos is too strong a word but there are a lot of people who are uneasy in tokyo trying to get out of the country any way they can. is that correc is that correct? are you seeing that?

    >> reporter: mostly foreigners are trying to get out. people aren't leaving tokyo who live here. no one is saying the radiation levels are posing a threat at this time. a lot of foreigners who have the option of getting out are looking at this and saying, well, it's not so bad right now. there has not been this catastrophic leak of radiation everybody fears but let's not wait around for that to happen. if it did nobody's getting out of anywhere. that's the mentality driving a lot of people to book flights and trains to get as far as from here as they can.

    >> robert bazell in tokyo , thank you very much.

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