http://www.bloomberg.com/news/2011-03-16...plant.html
This is a long article. For those with dial-up I excerpted parts about the spent fuel rods. Better to read it all though. I bolded the paragraph mentioning cesium.
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"Japan’s crippled nuclear power plant is releasing “extremely high” levels of radiation that could be life-threatening, the head of the U.S. Nuclear Regulatory Commission told lawmakers as hundreds of Japanese fled south of areas hit by last week’s earthquake and tsunami.
All the water in one of the Fukushima Dai-Ichi power plant’s spent-fuel cooling pools has drained, NRC Chairman Gregory Jaczko told a House Energy and Commerce Committee panel in Washington. “Radiation levels are extremely high, which could possibly impact the ability to take corrective measures,” he said.
Japanese officials denied that the water from the cooling pools was gone, the Associated Press reported.
Tokyo Electric Power Co. workers are struggling to prevent a nuclear meltdown at the complex, which has six reactors, 135 miles (217 kilometers) north of Tokyo. With his nation still reeling from strongest earthquake to hit it on record and a resulting tsunami, Japanese Prime Minister Naoto Kan said on March 15 the danger of further radiation leaks has increased.
The United Nations’ nuclear agency plans an emergency meeting on the crisis. Japan faces a “serious situation,” Yukiya Amano, head of the International Atomic Energy Agency, told reporters in Vienna before departing for talks with authorities in Tokyo today.
Amano said fuel stored in units 4, 5 and 6 at the Tepco facility is exposed and releasing radiation. Separately, Tepco official Masahisa Otsuku said the No. 2 reactor’s containment vessel may have been breached.
The failure of backup generators used to pump cooling water caused explosions in at least three of structures surrounding the station’s reactors, as well as a fire in a pond containing spent fuel rods.
Spent-Fuel Pools
Temperatures in the spent-fuel-rod cooling pools of the shuttered No. 5 and No. 6 reactors were rising to as high as 63 degrees Celsius (145 degrees Fahrenheit) at 2 p.m. yesterday, said Tsuyoshi Makigami, head of nuclear maintenance at Tepco. Water levels at spent fuel pools at the three inactive reactors, Nos. 4, 5 and 6, dropped by about 2 meters, exposing the fuel rods, Amano said.
Exposed to air, the fuel bundles could chemically react with moisture, catch fire and spread radiation into the atmosphere, said Edwin Lyman, a physicist with the Union of Concerned Scientists, based in Cambridge, Massachusetts.
“Spent fuel is pretty hot and so it is stored under water to keep it cool,” said Kelley, who worked for 30 years at the U.S. Energy Department. “If the water leaks or boils away, then the fuel is exposed,” then after burning, the uranium corrodes and releases cesium, contaminating the area, he said.
Peak-Level Threat
The NRC’s Jaczko said radiation at the Japanese site is fluctuating and at peak levels “would be lethal within a fairly short period of time.”
He told reporters later that the information came from NRC staff that were dispatched to Japan to help with the response and have been in contact with industry officials there."
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