Japan struggles to cool radioactive materials, after helicopter mission ruled unsafe
Wednesday, March 16, 2011; 10:45 AM
Japanese officials scrambled Wednesday for ways to cool overheated elements at a damaged nuclear plant that can emit potentially lethal radioactive steam, after aborting a risky mission to use a helicopter to douse part of the plant with water.
As radiation levels in the air above the Fukushima Daiichi plant spiked dangerously for the second consecutive day, a skeleton crew of workers charged with cooling efforts was temporarily relocated.
Within an hour, though, the radiation levels dropped again, and the small group was permitted to return.
In order for them to resume trying to cool the damaged sectors, Japan's health and welfare minister had to waive the nation's standard of radiation exposure, increasing the level of acceptable exposure from 100 millisieverts to 250 - five times the level allowed in the United States.
The workers were focusing on the plant's unit 3 reactor building, where a white plume of smoke was spotted Wednesday morning, and on unit 4, where fires flared up Tuesday and again on Wednesday morning.
The blazes triggered fears that spent uranium fuel sitting in a pool above the reactor was burning. Such a conflagration would generate intense concentrations of cesium-137 and other dangerous radioactive isotopes. But a spokesperson for the Nuclear Energy Institute, an industry lobbying group, said Tokyo Electric Power Co. concluded that the first fire in unit 4 was not in the spent fuel pool, "but rather in a corner of the reactor building's fourth floor."
Initially, government spokesman Yukio Edano said the steam coming from the unit 3 reactor building could mean that its containment vessel had ruptured in an earlier explosion - a potentially dire development. A reactor containment vessel in the plant's unit 2 is believed to have ruptured on Tuesday.
But Edano said Wednesday afternoon that the unit 3 containment vessel was unlikely to have suffered severe damage. The Japanese news agency Kyodo quoted the country's nuclear disaster task force as saying: "The possibility of the No. 3 reactor having suffered severe damage to its containment vessel is low."
Still, Edano said officials presumed that the steam coming from unit 3 was indeed radioactive. He said emergency crews were still trying to determine its source .
The rising steam was just the latest problem for the embattled plant, which suffered heavy damage to its cooling systems after Friday's devastating earthquake and tsunami. Since then, the Japanese government and Tokyo Electric, which owns the facility, have struggled mightily to keep the plant's six reactors cool. Each day has brought new problems.
Tuesday's blast at unit 2 was not outwardly visible, but was potentially more dangerous than some of the earlier explosions, because it may have created an escape route for radioactive material bottled up inside the thick steel-and-concrete reactor vessel.
Radiation-laced steam is probably building between the reactor vessel and the building that houses it, experts said, creating pressure that could blow apart the structure, emitting radiation from the core.