It has been nearly a month since the earthquake and tsunami devastated northeastern Japan, but the situation surrounding the reactors at Tokyo Electric Power Co.'s Fukushima No. 1 nuclear power plant remains perilous.
TEPCO is considering installing additional cooling systems as part of a new plan to stabilize the reactors damaged by the March 11 disaster.
According to estimates released by the utility Wednesday, 70 percent of fuel rods at the No. 1 reactor have been damaged. At the Nos. 2 and 3 reactors, 25 percent to 30 percent of fuel rods have been damaged, the utility said.
The company based the damage assessment on levels of radioactive xenon and krypton detected in gas near the reactors' containment vessels. These substances are supposed to be contained within nuclear fuel and should not be released.
TEPCO said it gauged the extent of the xenon and krypton leaks by considering factors such as radiation levels and the length of time the fuel rods had been exposed.
Damage to fuel rods could result in a hole in their zircalloy cladding, or a meltdown of the fuel rods themselves. TEPCO said details of the damage to the reactors' fuel rods remain unknown.
In the reactor cores, temperatures at the midpoints--where fuel rods are supposed to be located--are higher than at the base of the reactors. This suggests that meltdown of the fuel rods, which would see them accumulate at the bottom, has not occurred, according to TEPCO.
On March 14, radiation levels at the containment vessel of the No. 3 reactor, where a large portion of the fuel rods was believed to have been exposed, were measured at 167 sieverts per hour. This figure is roughly equivalent to radiation levels 400 meters from ground zero after the 1945 U.S. atomic bombing of Hiroshima.
Experts have expressed alarm at the current situation at the Fukushima plant.
"If fuel rods melt, they might accumulate in a shape like a soft-boiled egg about four meters in diameter and two meters thick," said Michio Ishikawa, top adviser to the Japan Nuclear Technology Institute. "It might look like steaming magma, with uranium fuel and zirconium pipes melted together."
If fuel rods melt and form a mass, the exposed surface area will be reduced. Cooling the reactors would therefore take even longer because the mass would retain more heat and pumping water on it would have less effect.
Radiation levels at the No. 1 reactor peaked at 162 sieverts per hour on March 14, and the No. 2 reactor saw radiation levels peak at 138 sieverts per hour on March 15.
Those highs were measured after those reactors lost significant volumes of cooling water around their fuel rods, leaving large parts of them exposed to the air. This has also happened at the No. 3 reactor.
Injection of water has caused radiation levels at all the three reactors to fall. Current levels are between 20 and 31 sieverts, or just 11 percent to 22 percent of their peaks.
However, the current radiation levels are still far from safe. Two workers were killed in a 1999 nuclear accident in Tokaimura, Ibaraki Prefecture, after being exposed radiation levels of between six and 18 sieverts.
TEPCO is aware that working near the reactor containment vessels is dangerous.
"Radiation levels are extremely high, so we can't do any work there right now," said one official of the utility.
Osaka University Prof. Keiji Miyazaki, an expert in nuclear engineering, said, "Radioactive substances probably leaked from damaged fuel rods and reached the reactor containment vessels through valves or some other route."
"The radiation levels are way too high for workers to get close [to the reactors]," he said. "It'll certainly take a very long time to inspect the condition of the damaged reactors."
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http://www.yomiuri.co.jp/dy/national/T110407006089.htm
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