March 15, 2011 — A third explosion at the Japanese nuclear plant damaged by the earthquake and tsunami, as well as a fire at another reactor, has resulted in fresh release of radiation that is now considered to be harmful and has increased fears over a catastrophic meltdown.
Radiation levels around the Fukushima Daiichi plant rose to 8 times the legal limit, according to the Tokyo Electric Power Company (TEPCO) as reported by BBC News. After the third explosion (on Tuesday morning in Japan), the radiation reading rose to 8217 microsieverts an hour, having stood at 1941 just an hour earlier. The annual legal limit is 1000 microsieverts.

"Now we are talking about levels that can damage human health," said Japan's Chief Cabinet Secretary Yukio Edano. He instructed local residents to stay indoors and make their homes airtight.

The exclusion zone around the Fukushima Daiichi nuclear plant has been increased again, this time to a radius of 19 miles (up from 12 miles yesterday, after the second explosion). In addition, around 800 workers at the plant have been evacuated, with only 50 workers now remaining.
The spike in radiation released after this latest explosion, in the third reactor that was operational when the earthquake and tsunami struck on March 11, has led to fears that this time there has been damage to the reactor containment vessels that houses the nuclear fuel rods.

Unlike in the previous 2 explosions at 2 other similar reactors, this time the roof did not blow off, and it is thought that the trapped pressure cracked the containment vessel around the reactor's core, allowing radioactive material to seep out, according to an ABC News Report.

High Risk of More Radiation

The Japanese government has not commented on the state of this containment vessel, but in a nationally televised address, Prime Minister Naoto Kan warned, "There is still a very high risk of more radiation coming out."

Other reports from Japan say that radiation levels have varied wildly. A Reuters report of a briefing given by TEPCO and the Japanese government on Tuesday, after the third explosion, noted that radiation levels as high as 400,000 microsieverts (400 millisieverts) an hour were measured at some places within the plant complex.

The radiation levels of up to 400 millisieverts per hour recorded at the Fukushima plant "are levels that you have to take very seriously indeed to ensure you avoid immediate health effects," Professor Richard Wakeford, PhD, from the Dalton Nuclear Institute at the University of Manchester, United Kingdom, told the BBC World Service. He said that Japanese authorities will be imposing a ban on food and drink from the area and issuing potassium iodide tablets to block the intake of radioactive iodine to the thyroid. In fact, even before the latest explosion took place, potassium iodide was being distributed by the Japanese government.
In addition to the third explosion, there has been a fire at another reactor (reactor number 4), which was operational when the earthquake struck because it was undergoing maintenance. The fire occurred in that reactor's cooling pool, where the spent nuclear fuel is stored. TEPCO said that pool may still be boiling and the water levels may be falling, but it cannot check at the sites or determine what has burned because radioactivity at the cooling pool is high. Radiation leakage from the complex is likely to spread, the company warned.

According to a report from the Associated Press, the incidents at the nuclear plant to date have injured 15 workers and military personnel and exposed up to 190 people to elevated levels of radiation.

That report also quoted Chief Cabinet Secretary Yukio Edano as saying that fuel rods appear to be melting in all 3 of the reactors that were operational when the earthquake struck — all 3 of which have since exploded.

"Although we cannot directly check it, it's highly likely happening," he said.
This has increased fears of the nightmare scenario of a meltdown, which would trigger a massive build-up of pressure inside the containment units. If they then crack, radioactive dusts and gas would spew out into the air.
However, the latest reports from Japan (as of noon EST) are that radiation levels have fallen around the Fukushima Daiichi nuclear plant.

International Atomic Energy Agency "Remains Concerned"

The International Atomic Energy Agency says it "remains concerned" over the status of the Fukushima Daiichi nuclear power plant. Workers are continuing to inject seawater into all 3 reactors to cool them, and attempts to return power to the site are ongoing.

After the explosions at reactors 1 and 3, the primary containment vessels of both units were reported to be intact. However, the third explosion in reactor 2 may have affected the integrity of its primary containment vessels, the IAEA said in a statement.

The agency also notes that iodide tablets have been distributed to people at evacuation centers, but no decision has been taken yet about their administration.

Worst-Case Scenario

Whether radiation exposure becomes a major risk depends fundamentally on what happens to the 3 reactors at Fukushima Daiichi, commented Nathan Hultman, PhD, an energy policy expert and assistant professor at the University of Maryland School of Public Policy, College Park.

"All 3 of these reactors are suspected of having at least a partial meltdown, and therefore stabilizing them will require at minimum a venting of radioactive steam," he said. "The worse the meltdown has been the higher the radiation levels in the steam."

"This is at least moderate concern," Dr. Hultman said. "A full core meltdown would be far more worrisome, leading to molten fuel in the bottom of the containment unit and increasing the likelihood of highly radioactive substances escaping."

"The worst case scenario is if such molten material would breach the containment unit en masse," he commented. "The containment vessels are designed to withstand this scenario, and if we were to face a complete meltdown, we can only hope that they were well built and not damaged by the earthquake."

Authors and Disclosures

Journalist Zosia Chustecka

Zosia Chustecka is the News Editor for Medscape Oncology. A pharmacology graduate based in London, UK, she has edited and written extensively for publications aimed at clinician audiences. Her work has been recognized by the British Medical Journalists Association, and most recently she was the recipient of the 2010 National Press Foundation Cancer Issues Fellowship. She can be reached at zchustecka@medscape.net.

Disclosure: Zosia Chustecka has disclosed no relevant financial relationships.

Source: Medscape Network