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Tepco engineers use concrete to seal leaks  (Source: TEPCO)
The top five feet of the core's 13 ft-long fuel rods had melted down after being exposed to the air

The Tokyo Electric Power Company (Tepco) has announced that the No. 1 reactor at the Fukushima Daiichi nuclear power plant has suffered a nuclear meltdown.

The Fukushima Daiichi nuclear power plant was first commissioned in 1971 and is located in the Futaba District of Fukushima Prefecture, Japan. On March 11, 2011, a 9.0-magnitude earthquake struck Japan, resulting in the disabling of reactor cooling systems, radiation leaks and an evacuation zone. 

Engineers from Tepco entered the No. 1 reactor for the first time at the end of last week and found that the top five feet of the core's 13 ft-long fuel rods had melted down after being exposed to the air. 

Engineers originally thought only 55 percent of the core was damaged since it was submerged in enough water to keep cool and stable, but after discovering a pool of molten fuel at the bottom of the containment vessel, they now worry that this molten fuel burned a hole at the bottom of the vessel prompting water to leak. 

Tepco recently sealed a leak at the No. 3 reactor after radioactive water had seeped into the ocean. Also, the No. 2 reactor had radioactive water flowing into the ocean in April. According to Greenpeace, "significant amounts" of radioactive material had slipped into the sea. In fact, illegal amounts of iodine and caesium were found in seaweed as far as 40 miles from the Fukushima Daiichi plant. 

In 22 samples of seaweed, ten contained five times the legal limit of iodine 131 and 20 times of caesium 137. This is an issue for several reasons, including the fact that the Japanese household consumes almost 7 lbs of seaweed annually, and fisherman are preparing to harvest this seaweed on May 20. 

Engineers have decided to quit flooding the entire reactor core with water because it might make the leak worse. Currently, there is plenty of water at the bottom of the containment vessel to keep the remaining fuel rods and the melted fuel cool. 

"We will have to revise our plans," said Junichi Matsumoto, Tepco spokesman. "We cannot deny the possibility that a hole in the pressure vessel caused water to leak."

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RE: *Partial* meltdown
By CZroe on 5/13/2011 9:17:52 PM , Rating: 2
I, too, read the headline and started thinking that a real melt down had occurred. I'm not sure, but I think a "melt down" is more than just fuel melting combined with radioactive water leaking. I believe it specifically refers to fuel melting it's way out of the containment vessel entirely, like what happened at Chernobyl after the explosion (fuel now site exposed in a frozen glass-like state below the reactor chamber's original location). It doesn't sound like that happened here at all.

RE: *Partial* meltdown
By Angstromm on 5/14/2011 3:22:18 AM , Rating: 2
Actually, "meltdown" refers to core melt, partial or otherwise. And it's not what would be called a sanctioned term with a clear cut definition used by various nuclear regulatory agencies.

RE: *Partial* meltdown
By Peter898 on 5/14/2011 3:25:49 AM , Rating: 4
No it isn't and no it doesn't, a 'meltdown' is exactly that, the melting of nuclear fuel due to insufficient cooling .
It doesn't have to leave the containment or burn it's way to China (well, in the case of Japan it obviously wouldn't be
China, ignoring the fact that 'China Syndrom' is just a buzz-word, not scientific fact)

That's the generally agreed definition, although none of the International nuclear bodies have a strict definition of the term .
You can read more on Al Gores internet-tubes, that is if you want to sound like you actually know anything about the subject .

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