Radiation exposure: How big is the threat in Japan?

Radiation exposure: Adding to the monumental losses after a Japanese earthquake and tsunami, problems at four nuclear reactors have residents near and far concerned about radiation exposure.

[Editors note: This story was updated 5.p.m EST March 16]

A new power line could restore cooling systems in Japan's tsunami-crippled nuclear power plant, said officials Thursday morning Japan time. This raises hopes of easing a crisis that has spawned dangerous radiation surges at the 40-year-old General Electric model Fukushima I Nuclear Power Station in Japan.

Increased levels of radiation in the atmosphere has prompted the US government to urge Americans living within a 50 mile radius to stay evacuate.

Gregory Jaczko, chairman of the Nuclear Regulatory Commission told the House Energy and Commerce Committee Wednesday that water used to cool spent fuel at the plant had evaporated and that radiation levels there are thought to be “extremely high.”

With increased radiation being measured as far as 175 miles away in Tokyo, and some experts calling this the "worst nuclear accident since Chernobyl," residents and officials want to know about the risk of radiation exposure.

NUCLEAR REACTOR TIMELINE

The disaster started after a magnitude 9.0 earthquake rocked northeastern Japan, Friday. Eleven nuclear reactors responded to the earthquakes by shutting down automatically. This began a slow cool-down process and ensured the reactors' cooling rods were in place. However, of the six reactors at Fukushima, three lost power when their backup generators failed. This kept the reactors from circulating their coolant.

By Saturday, the cooling systems in Fukushima reactors 1 and 3 had failed and the coolant began to evaporate from heat in all three reactors. To reduce pressure, the steam was vented, which released the first trace amounts of radiation. That same day, reactor 1 experienced a hydrogen explosion which released more radiation, but did not crack its containment vessel. Caesium-137 and iodine-131 were detected near reactor 1.

On Sunday, Fukushima reactor 3 was vented again at which point there is believed to have been a partial meltdown in the reactor. A company spokesman stated that the radiation released thus far did not pose a health risk to humans. Later that day the cooling system in reactor 2 failed, and more radioactive steam was control-released.

The concern is that that radioactive steam will eventually fall back to earth and contaminate food and water supplies.

On Monday, reactor 3 experienced a hydrogen explosion that blew the roof off its containment building. On Tuesday, a fire broke out at the fourth reactor and there were reports that the storage pond, holding spent fuel rods, had boiled over. The fire was extinguished, but Chief Cabinet Secretary Yukio Edano said that more radiation was released as a result and that "now we are talking about levels that can damage human health."

Edano went on to warn, "Please do not go outside. Please stay indoors. Please close windows and make your homes airtight. Don't turn on ventilators. Please hang your laundry indoors."

With radiation close to the reactors measuring four times higher than the maximum allowable limit of 100,000 microsieverts per hour, according to the International Atomic Energy Agency, residents are asking: How dangerous it is around Fukushima?

RADIATION DANGERS

As of Monday, the plant was emitting as much radiation in one hour as it normally would in six months, but government spokesman, Yukio Edano, said "The possibility that a large amount of radiation has been released is low."

Matthew Bunn, co-principal investigator of the Managing the Atom Project at Harvard’s Belfer Center for Science and International Affairs noted that one risk is the release of "a very modest amount of cesium and other fission products" from partial meltdowns, such as those we have seen in reactor 1 and 3. Mr. Bunn went on to say another possibility is a complete nuclear meltdown. If the fuel in the reactors overheats and melts through the bottom of the steel containment vessel, it may reach an underground water source and create much more dangerous radioactive steam.

One of the spent-fuel pools where the used radioactive rods are normally cooled has already released steam. The condition of the pools and their cooling systems after the hydrogen explosions are unknown and experts say that if the pools have been compromised, they could release more radiation than a reactor meltdown. The pools have far more radioactivity than the reactors, and do not have the protection of a containment building. Arjun Makhijani, a nuclear engineer and president of the anti-nuclear power Institute for Energy and Environmental Research, says "damaged reactors are less likely to spread the same vast amounts of radiation that Chernobyl did, but a spent-fuel pool fire could very well produce damage similar to or even greater than Chernobyl."

Another radiation concern is the dangerous amount of plutonium-based mixed-oxide fuel (MOX) that reactor 3 has. Dr. Edwin Lyman, a physicist with UCS, adds that the plutonium particles found in MOX are far more harmful than other emitted elements. With a reported three yards of a MOX fuel rod exposed in one of the reactors, these plutonium particles may be mixing with escaping gasses.

But some experts dispute the likelihood of a Chernobyl repeat. And Japan's nuclear safety agency refutes any Chernobyl comparison by saying there is "absolutely no possibility of a Chernobyl" meltdown event at Fukushima.

On Sunday, the World Health Organization said the risk of a radiation leak from Fukushima, and thus radiation exposure, is "probably quite low." That same day, Robert Engel, former IAEA inspector and Swiss nuclear engineer, said he doubted a complete meltdown would happen and that a partial meltdown, such as those that have happened already, "is not a disaster."

RESIDENTS AND WORKERS STILL AT RISK

But given the potential risks, authorities had evacuated 210,000 residents within a 13-mile radius of the Fukushima I Nuclear Power Station Tuesday. After the fire broke out in the fourth reactor, Prime Minister Naoto Kan asked people within a 19-mile radius to stay indoors, adding "These are figures that potentially affect health, there is no mistake about that." The US announcement Thursday morning Japan time urging Americans living within 50 miles of the plant is a first.

The weather has compounded threats. The radiation emitted into the atmosphere around Fukushima may have been carried by winds that were blowing south toward Tokyo.

Officials at evacuation centers around Fukushima say radiation levels are too low to cause harm, but have distributed 230,000 iodine tablets. The tablets provide iodine to the body and help prevent the body from absorbing radioactive iodine from the air.

Workers in the Fukushima plant are being exposed to more radiation than those in surrounding areas. At least one worker has taken ill from radiation exposure, while others are being monitored for signs of radiation sickness.

The World Health Organization commended Japan Tuesday for taking the right steps to protect people from radiation exposure.