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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: Yay!!
By ppardee on 5/13/2011 7:13:17 PM , Rating: 2
So, with a half-life of uranium at 4.47 billion years, we have roughly 13 billion years to wait before we can make jokes?

Or maybe we can start in 150 years when the Cs-137 decays to acceptable levels?

Or we can just grow more seaweed. It seems pretty good at sucking up the bad stuff. Almost like nature has a way of neutralizing bad things in the environment...


RE: Yay!!
By Ushio01 on 5/13/2011 7:41:53 PM , Rating: 1
quote:
Or maybe we can start in 150 years when the Cs-137 decays to acceptable levels?


You mean 70 days.


RE: Yay!!
By PaterPelligrino on 5/13/2011 8:21:07 PM , Rating: 2
The phrase "decays to acceptable levels" is vague enough to offer a lot of wriggle room, and I'm not sure what the answer to that is; however, the half-life of Cesium-137 is 30 years.

It is the biological half-life of Cesium-137 that is 70 days, meaning that 70 days after ingestion , it loses half of it's radioactive toxicity through elimination, etc.

The Cesium-137 released by the Chernobyl meltdown 20 years ago is still a threat to human health.


RE: Yay!!
By randomly on 5/13/2011 8:30:50 PM , Rating: 3
Cs-137 has a 30 year half life. In 150 years 97% will have decayed away.

Almost none of it will have decayed in 70 days.

You may be thinking of the iodine-131 which has an 8 day half life. Only 1 part in a thousand will be left after 80 days, in 160 days only 1 millionth of the I-131 will be left, including whatever was in the seaweed.

Although the CS-137 lasts much longer, the iodine is more of a concern because of how strongly the body absorbs and concentrates it in the thyroid. CS-137 is thousands of times less hazardous because it's not readily absorbed or concentrated in tissues to the degree that iodine is.

Fortunately there will be almost no radioactive iodine left in a few months. Only about 1/200th of the original I-131 is left currently.

on the other hand your comment about 70 days is also partly valid since radiation levels for almost all areas except the plant itself are already fairly low.


RE: Yay!!
By drewsup on 5/15/11, Rating: 0
RE: Yay!!
By Solandri on 5/13/2011 8:49:32 PM , Rating: 5
quote:
So, with a half-life of uranium at 4.47 billion years, we have roughly 13 billion years to wait before we can make jokes?

This is a common misconception among people who don't really understand radioactivity.

Stuff with a really long half-life (thousands of years or longer) is relatively safe. Yes they're radioactive, but the long half-life means it emits the radiation so slowly that your exposure from handling it even for days is limited.

Stuff with a really short half-life (a few seconds to a few months) is also relatively safe. Put it behind some shielding and wait a while, and most of it has decayed into something else which emits no radiation or a lot less radiation. If you get exposed to it when it's fresh, then you have problems. But it only stays fresh for a short period. This stuff was why radiation levels were so much higher right next to the reactors than in the surrounding area. By the time these substances got carried by the air to the surroundings, enough time had passed that they'd decayed into harmless or relatively harmless substances.

It's the stuff with intermediate half-life which is dangerous. It's not short enough to become safe relatively quickly, and it's not long enough that the decay happens slowly enough to make it safe to handle. Stuff with half-lives of a few years to a hundred years or so falls into this category.

Iodine-131 has a half-life of 8 days. So for the first few weeks it's dangerous. After that, the remaining amount is so small that it's safe. Its decay product is stable Xenon-131.

Cesium-137 is a real problem. It has a half-life of 30 years. It's a beta emitter, but decays into a form of Barium-137 which gamma decays almost immediately. The worst of both worlds - a long half-life with a double decay emission.

For this reason, Cesium-137 and Strontium-90 are the primary radioactive byproducts of concern from a nuclear accident. Due to the nature of the accident, there wasn't a lot of Strontium-90 (29-year half-life) released as was in Chernobyl. That stuff builds up in your bones, contaminating you for life if you're exposed.


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