The chief of the U.S. Nuclear Regulatory Commission said all the water is gone from one of the spent fuel pools at Japan's most troubled nuclear plant, raising the possibility of widespread nuclear fallout. But Japanese officials denied the pool was dry.
"There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures," NRC Chairman Gregory Jaczko said Wednesday at a U.S. House Energy and Commerce subcommittee hearing.
If Jaczko was correct, this would mean there was nothing to stop the fuel rods from heating and ultimately melting down. The outer shell of the rods could also ignite with enough force to propel the radioactive fuel inside over a wide area.
That country's nuclear safety agency and Tokyo Electric Power Co., which operates the six-unit Fukushima Dai-ichi complex, denied Jaczko's statements that the water is gone from the pool.
Utility spokesman Hajime Motojuku said the "condition is stable" at Unit 4, which was shut when the earthquake and tsunami hit last week.
After the hearing, Jaczko left some wiggle room. If he is wrong, it would represent a very embarrassing moment for the U.S. government.
"My understanding is there is no water in the spent fuel pool," he said. "I hope my information is wrong. It's a terrible tragedy for Japan."
Jaczko said the information came from NRC staff and experts in Tokyo who are working with the utility in Japan. He said NRC staffers continue to believe the spent fuel pool is dry. "They believe the information they have is reliable," he said.
U.S officials recommended the evacuation of any American within 50 miles (80 kilometers) of the Fukushima site. Japanese officials have recommended that people within 20 miles (30 kilometers) stay indoors, and evacuate within 12 miles (20 kilometers.)
In another troubling development, the International Atomic Energy Agency in Vienna said temperatures in units 4, 5 and 6 have been rising. The temperature in Unit 4 on Monday and Tuesday was given as 183 degrees Fahrenheit (84 Celsius). For Wednesday, the IAEA report for Unit 4 stated, "no data."
Also alarming was the information at units 5 and 6, which were in cold shutdown when the earthquake hit and had not been known to be of any concern. Also, the fuel rods in use when those two reactors were shut for maintenance remain inside their reactor vessels.
In Unit 5, the pool temperature was 139 degrees Fahrenheit (59.7 Celsius) on Monday, 141 degrees Fahrenheit (60.4 Celsius) on Tuesday and 145 degrees Fahrenheit (62.7 Celsius) on Wednesday.
Temperatures at Unit 6 also rose Tuesday and Wednesday. The latest temperature was 140 degrees Fahrenheit (60 Celsius).
Even before the latest developments, the issue of spent fuel was a very worrisome one, with one Tokyo Electric spokesman saying spent fuel pools represented the greatest concern because they lack the protective shells that reactors have.
"We haven't been able to get any of the latest data at any spent fuel pools. We don't have the latest water levels, temperatures, none of the latest information for any of the four reactors," Masahisa Otsuki said.
The pool's water can evaporate when power problems rob it of circulating water. As the water level drops, the remaining water is heated by the radioactivity of the spent fuel. It can get so hot that the rods' casing catches fire.
This would spew long-lived radioactive material into the air and onto surrounding areas. A fire at this reactor was previously reported, but it's not clear if it involved the spent fuel pool or another area of the unit.
The radioactive spent fuel rods at the center of the Unit 4 crisis are every bit as dangerous as the fuel rods inside the reactor vessels of three other damaged plants _ and possibly more so.
Unlike fuel rods in reactor cores protected by 6-inch-thick steel walls, the spent fuel pools are considerably more vulnerable _ located on the top floor of each reactor's containment building, without any extraordinary protection. Plus, the containment building of Unit 4 has a big hole in a wall.
But a key question is just how much of the radioactive material decayed inside the rods before they were transferred to the storage pool.
"For the time being, the greatest concern is the spent fuel pools because there is a clear pathway for release of radioactivity from the pools into the environment," said Ed Lyman, a physicist at the Union of Concerned Scientists, an activist group.
Fuel rods consist of a zirconium outer casing and highly radioactive uranium pellets inside. Experts say the casings of the spent fuel rods in Unit 4's pool have likely already overheated and cracked, allowing radioactive gases to vent into the air. That radiation is hindering the efforts to control the developing catastrophe at the reactor complex.
Officials have acknowledged that the cores of units 1, 2, and 3 have begun to melt down, but no one has said the walls of the reactor vessels have been breached. Radiation has been leaking from the reactor structures, though.
Even at those units, the spent fuel pools could spell an even more severe problem. Explosions at units 1 and 3, which were operating at the time of the earthquake and tsunami, have left those spent fuel pools exposed to the open air.
The pools and those at units 5 and 6 _ which also were shut at the time the quake hit _ are also likely to be heating up.
The problems at Unit 4, which has been shut for months, began earlier this week with a fire sparked by its spent fuel rods. Its spent fuel is hotter and more radioactive than the fuel in the other reactors' pools because it had been removed from operations so recently.
The spent fuel in the pools contains more total radioactive material than the rods in the reactor, including a radioactive form of cesium that can contaminate a region for decades. This type of cesium, known as cesium-137, emits what is known as a hard gamma ray which is strong enough to penetrate human skin.
"You don't have to breathe it in to be affected," Lyman said. The release of cesium could render the area around a plant uninhabitable. The size of the area would depend on the amount of material released and the direction and strength of the wind at the time of release.
Cesium was the largest source of radiation around the Chernobyl reactor in the Ukraine, which suffered a catastrophic meltdown in 1986.
David Lochbaum, nuclear safety chief at the Union of Concerned Scientists, said a fire would spew "a whole gamut of nasty materials." In addition to cesium, he cited krypton, strontium and ruthenium.