Nuclear Meltdown in Japan: What's the Risk of Radiation?

FAQ on Radiation Risk From Tsunami-Damaged Nuclear Plants

From the WebMD Archives

March 18, 2011 -- Experts now say it will be weeks until the emergency situation at the earthquake/tsunami-damaged nuclear power plants in Japan are brought under control.

Damage and failed cooling systems have raised the fears of a total meltdown in at least one of the six nuclear reactors at the Fukushima Daiichi facility. At least one reactor appears to have suffered a partial meltdown. Moreover, a protective pool in which spent nuclear fuel is stored has been leaking water, resulting in the release of radioactivity.

There's hope that a major nuclear disaster can be averted. But what are the risks from the radiation that already has been released -- and from the radiation that could be released if containment efforts fail?

Here's WebMD's FAQ.

What does it mean for a nuclear energy plant to melt down?

"Meltdown" is not a technical term, but it vividly describes the worst-case scenario for a nuclear reactor.

Nuclear reactors generate power via controlled nuclear fission, which occurs when enough radioactive material is gathered into a critical mass. Control rods can separate the radioactive material, thus ending the nuclear reaction.

That's exactly what happened when the March 11 earthquake hit Japan. Control rods effectively halted the nuclear reactions.

This means there is no danger of a nuclear blast from Japan's damaged nuclear plants, even if fuel rods melt inside one of the containment vessels and pool into critical mass.

But the nuclear materials inside the reactors remain radioactive, which means they give off a great deal of heat. It takes a long time to cool these materials down, and spent fuel rods must be kept submerged in a cooling bath until their radioactivity decays and their intense temperature goes down.

If not cooled, these materials will melt. In a worst-case scenario, they could possibly melt right through the thick metal shield that contains the reactor, spilling highly radioactive materials into the environment. That could happen if the melted fuel pools to critical mass and restarts a fission reaction. This would not mean a nuclear explosion, but it would generate a great deal more radioactivity and heat.

When the tsunami hit Japan's Fukushima Daiichi nuclear plant, three active reactors lost power. That meant serious trouble, as the pumps needed to cool the overheated reactors with water stopped running when their battery backups were exhausted. While power has been restored, workers are still fighting to keep the fuel rods submerged.

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Nuclear reactors have two main containers that keep radioactive materials from spreading outside the plant. One container is the thick-walled building surrounding each reactor. The other is a thick metal vessel that makes up the outer wall of the reactor itself.

As water cools the fuel rods in the innermost container, steam is created. The intense heat also releases hydrogen from the water. If the pressure inside the container gets too high, the steam has to be vented. This releases some radioactivity. It also releases hydrogen, which can build up inside the building and explode.

Three of the six buildings housing Fukushima Daiichi's six reactors had hydrogen explosions. One, on March 14, injured 11 workers and could be felt for miles. However, the internal containment walls appear to remain intact. The third blast at unit 2 may have caused a small breach in the inner containment vessel, as radioactivity around the plant shot up to dangerous levels before going down.

All this water being pumped through the reactors becomes radioactive. Some of it is extremely radioactive, and has badly injured two workers. As of March 28, there were fears that the accumulating water would escape tunnels below the facility and pour into the ocean. While this would be an ecological disaster for the local area, the radiation would rapidly be diluted by the Pacific Ocean. However, seafood from the area would be unsafe to eat.

How much radiation has escaped Japan's damaged nuclear plants?

One of the emergency measures being taken to prevent a meltdown is the release of steam from the reactors. This means some radiation is released into the environment with each release of the high-pressure steam.

Japan's nuclear energy agency reported on March 14 that measures of radiation outside the plant are higher than legal limits, but that they were not dangerously high. That changed on the morning of March 15, when radiation levels shot to 400 millisieverts per hour -- well above the danger zone. Radiation levels then dropped to about 0.6 millisieverts per hour.

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By comparison, a chest X-ray is 0.02 to 0.67 millisieverts. In a year, the typical U.S. resident is exposed to 3 millisieverts. A person who receives a short-term dose of 1,000 millisieverts will experience radiation sickness but probably will survive. Short-term doses of 2,000 to 10,000 millisieverts have an increasing probability of causing a fatal cancer.

Radiation levels in Tokyo are reported to be 20 times above normal but still not in the danger zone. However, a no-fly zone has been imposed for an 18-mile radius around the nuclear plant.

On March 14, officials ordered the evacuation of all residents living within 20 kilometers (about 12.5 miles) of the plant. On March 15 they advised people living between 20 kilometers and 30 kilometers (about 18.6 miles) to remain indoors. News reports indicate many of these residents are fleeing the area instead. Some 100,000 people are reported to be in the area.

Another radiation issue is in reactor 4, which had been shut down before the earthquake. Spent fuel rods still were cooling in the plant's rooftop pool. That cooling system appears to be in trouble, as a fire was reported in the plant. Should these fuel rods become exposed, a large amount of radiation will be released. As of March 18, plant officials had used helicopter drops and water cannons to try refill the pool, which may be leaking.

For now, the chief radiation danger is to plant workers desperately trying to mitigate the disaster. Only a skeleton crew remains on site, threatened by radiation, fire, and hydrogen explosions.

One of the reactors, reactor 3, is said to be running on mixed-oxide (MOX) fuel. MOX fuel contains plutonium in far greater amounts than regular nuclear fuel and would be more toxic if released into the environment.

How dangerous is the radiation from the damaged Japanese nuclear reactors?

So far, the only radiation injuries from Japan's nuclear disaster have been to workers trying to end the situation.

Some of the workers trying to prevent a meltdown have already suffered radiation sickness and injury from explosions related to hydrogen buildup outside the reactor core. At least two others had their feet exposed to dangerously radioactive water.

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"The scariest outcome from such releases of radiation are the immediate effects, which are going to be only felt by personnel who have to go into the building and shut it down," radiation biologist Jacqueline P. Williams, PhD, tells WebMD. Williams is a researcher in the department of radiation oncology at the University of Rochester, N.Y.

The long-term effects of radiation exposure, Williams says, are the various cancers that can occur.

The most cancers most commonly associated with radiation are leukemia and cancers of the thyroid, lung, and breast.

But it's easy to overestimate the danger, says radiation authority Henry D. Royal, MD, co-director of the Mallinckrodt Institute of Radiology at Washington University, St. Louis. Royal was co-leader of the international team that studied the health effects of the Chernobyl nuclear disaster.

"In Chernobyl, people living within 1 to 2 kilometers of the plant, who stayed indoors, received about 50 millisieverts of radiation -- equivalent to about five CT scans," Royal tells WebMD. "And in Fukushima, we are not talking about Chernobyl. This may be worse than Three Mile Island, but nothing like Chernobyl."

The worst nuclear disaster ever was in 1986 at the Chernobyl nuclear plant in northern Ukraine. This wasn't the same kind of disaster as in Japan, as it was caused by a series of human and mechanical failures. The result was a series of explosions that shot a plume of radioactive materials into the air.

Fallout from Chernobyl fell heavily on Belarus, Russia, Ukraine, Finland, Norway, Sweden, Austria, and Bulgaria.

"The Chernobyl cloud technically covered a huge area. It did follow the jet stream and everything else," Williams says. "But radioactive particles have substance, they have weight. The bigger the particle, the quicker it falls out of the cloud. So the contamination area where you have risks from contamination are relatively close to the disaster site. As far as I am aware, the cancers from Chernobyl radiation occurred in and around Chernobyl itself."

However, radioactive iodine from the Chernobyl cloud fell on fields where it was absorbed by grass, eaten by cows, and drunk as milk by children. Until 1998, there was a significant increase in thyroid cancer among children in the affected areas. Despite over 5,000 cases of thyroid cancer, there have been fewer than 20 deaths to date.

A meltdown in Japan would be devastating to the local environment. Should there be a release of radiation, and should winds blow in the wrong direction, residents of Japan would be affected to some degree. But the effects almost certainly will not go far beyond the borders of Japan.

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Could radiation from Japan's nuclear plants affect the U.S.?

No significant radiation from the Japan disaster is expected to hit the U.S., say experts from the Oregon State University department of radiation health physics.

"Any radioactive contaminants released will end up raining out of the atmosphere into the Pacific Ocean, where they will be diluted and absorbed, or in the very near vicinity of the plants," Kathryn Higley, PhD, says in a news release. "This is not Chernobyl."

Royal says there is no danger of radiation affecting any of the United States -- not even Alaska or Hawaii, which are much closer to Japan than is California. Yet he says he's already received panicked calls from pregnant women in California, asking whether they should have a therapeutic abortion to avoid giving birth to radiation-damaged children.

State radiation detectors in the U.S. have picked up signals that appear to be from the Fukushima plant. But the U.S. Environmental Protection Agency (EPA) notes that the amount of radiation is far from harmful.

"The levels we're seeing coming from Japan are 100,000 times lower than what you get from taking a round-trip international flight," the EPA said in a March 22 news release.

The EPA maintains the RadNet system, made up of more than 100 monitors that give continuous, real-time radiation readings. The system also has 40 deployable monitors capable of reading the body dose of radiation a person would receive in the vicinity of the monitor. Just to be on the safe side, the EPA has sent extra monitors to Guam, Hawaii, and Alaska.

The EPA noted on March 28 that levels of radioactive iodine found in Massachusetts rain water exceed the maximum contamination limit (MCL) for drinking water. However, the MCL is calculated on the effects of drinking contaminated water for 70 years. The radioactive iodine in Massachusetts rainwater likely did come from Japan -- but it won't be there for long, as it has a half life of days rather than years.

Because there's no risk to the U.S., Royal scoffs at people who are stockpiling potassium iodide pills. These pills can prevent radioactive iodine from collecting in the thyroid gland and causing thyroid cancer -- but Royal says there's no U.S. risk of radioactive iodine from the Japan nuclear emergency. Fortunately, if people panic and take the pills, they won't do any harm as long as they are taken as directed.

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Royal notes that even if a person received enough radiation to cause radiation sickness -- 1,000 milisieverts -- the dose would increase their risk of cancer by 40%. To put this in perspective, smoking cigarettes increases cancer risk by 1,000% to 2,000%.

Here's some more perspective. From normal background radiation, a person who lives in Denver is exposed to 1 millisievert more radiation per year than is a person living in St. Louis. Royal says that the people living in the 18-mile zone around Chernobyl were exposed to 3 millisieverts of radiation in the years following the disaster. "That is less than the difference from living a few years in Denver rather than St. Louis," he says.

Are radiation-contaminated foods being imported from Japan?

Because of the extensive damage from the earthquake and tsunami, the FDA says food exports from Japan are "severely limited."

Nevertheless, the FDA says it "is collecting information on all FDA-regulated food products exported to the U.S. from Japan, including where they are grown, harvested, or manufactured."

The agency says it will evaluate whether there is any risk to U.S. consumers. New monitoring efforts "may include increased and targeted product sampling at the border."

Fish and seafood will not be affected by the radiation, as the Pacific Ocean quickly dilutes radioactive material.

Could the nuclear disaster in Japan happen in the U.S.?

The United States has 23 nuclear reactors, at 16 different nuclear power plants, that are designed exactly like the reactors leaking radiation in Japan.

In an ominous 1972 memo, a safety official with the Atomic Energy Commission warned that the "safety disadvantages" of these reactors outweighed their advantages.

However, the nuclear plants in Japan were hit by an earthquake and tsunami of record proportions. This doesn't mean the aging U.S. plants are safe -- but neither does it mean they are an imminent danger.

Who is most at risk from radiation exposure?

Radiation risks are different for people at different stages of life:

  • Radiation has harmful effects on child development.
  • Radiation can induce cancers that appear years after an adult is exposed.
  • Elderly people's cells may have reduced ability to repair damage from radiation.

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"Radiation research worldwide has been very much in decline, and has only received a little boost since 9/11," Williams says. "In the past 10 to 20 years research has focused on medical radiation therapy and not on accidental or incidental exposure."

According to Lisandro Irizarry, MD, chair of emergency medicine at The Brooklyn Hospital Center in New York, symptoms of acute radiation poisoning are nausea and vomiting. Other symptoms include fever, dizziness, disorientation, and bloody diarrhea. Symptom onset is quickest with the greatest radiation exposure.

Radiation received for medical reasons can also cause serious side effects.

What is the best way to prevent radiation exposure?

In the event of a nuclear accident, people living near nuclear power plants generally are provided with potassium iodide pills. That's because radiation leaks tend to carry radioactive iodine. The pills load the thyroid gland with iodine and prevent uptake of radioactive molecules.

But the best way to prevent radiation exposure is to stay indoors, close the windows, and turn off external sources of air, such as air-conditioning, until the all-clear is given or until you can safely be evacuated from a contaminated area.

"Contamination from fallout comes from touching a contaminated surface, from it falling, from inhaling it, or ingesting it," Williams says. "So in case of an event, be sure to drink bottled water and eat only sealed food that has not been outside."

WebMD Health News Reviewed by Laura J. Martin, MD on March 14, 2011

Sources

SOURCES:

Jacqueline P. Williams, PhD, research professor, department of radiation oncology, University of Rochester Medical Center, N.Y. 

Henry Duval Royal, MD, professor of radiology and co-director, Mallinckrodt Institute of Radiology, Washington University, St. Louis; co-team leader of the health effects section of the International Atomic Energy Agency’s International Chernobyl Project; member, Presidential Advisory Committee on Human Radiation.

News release, Brooklyn Hospital Center.

New York Times web site, accessed March 18, 2011.

News release, Oregon State University.

News releases, Japan Nuclear and Industrial Safety Agency.

News releases, International Atomic Energy Agency.

News releases, U.S. Nuclear Regulatory Commission.

Moysich, K.B. The Lancet Oncology, May 2002; vol 3: pp 269-279.

Auvinen, A. BMJ, July 16, 1994; vol 309: pp 151-154.

Hjalmars, U. BMJ, July 16, 1994; vol 309: pp 154-157.

News release, EPA, March 28, 2011.

News release, EPA. March 22, 2011.

Peplow, M. Nature, March 31, 2011, vol: 471 pp: 562-565.

The New York Times web site, accessed March 28, 2011.

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