As we reported yesterday, spent fuel rods at Fukushima Daiichi pose a risk of fire and radiation release should water drain from the pools they are stored in.

Spent fuel now appears to be a problem at Fukushima’s reactor No. 4, where there was a fire early this morning.

Japan's NHK World TV reported the problem late Monday, as did the New York Times:

But late Tuesday, Japan's nuclear watchdog said a pool storing spent fuel rods at that fourth reactor had overheated and reached boiling point and had become unapproachable by workers at the plant. The fire earlier Tuesday morning was sparked by a hydrogen explosion generated by rising temperatures at the fuel pool, which released radioactivity directly into the atmosphere.

The fourth reactor had been turned off and was under refurbishment for months before the earthquake and tsunami hit the plant on Friday. But the plant contains spent fuel rods that were removed from the reactor. If these rods had run dry, they could overheat and catch fire. That is almost as dangerous as the fuel in working reactors melting down, because the spent fuel can also spew radioactivity into the atmosphere.

Shigekatsu Oomukai, a spokesperson for the Nuclear and Industrial Safety Agency, said the substantial capacity of the pool meant that the water in the pool was unlikely to evaporate soon. But he said workers were having difficulty reaching the pool to cool it, because of the high temperature of the water.

This diagram shows where spent fuel pools are located in the boiling water reactor system at Fukushima, a 1970s design by General Electric known as the Mark 1. As the graphic shows, while the spent fuel lies under a deep pool of water, it is outside the concrete-and-steel containment designed to trap radiation leaks.

GE defended the reactor design in a report by the Washington Post:

"The BWR Mark 1 reactor is the industry's workhorse with a proven track record of safety and reliability for more than 40 years," GE said in a statement. "Today, there are 32 BWR Mark 1 reactors operating as designed worldwide. There has never been a breach of a Mark 1 containment system."

The Associated Press and other news media reported rising radiation levels from Fukushima. Reactor No. 4 had been shut for maintenance before the quake and tsunami, the AP said:

Japanese officials told the International Atomic Energy Agency that the reactor fire was in a storage pond and that "radioactivity is being released directly into the atmosphere. Long after the fire was extinguished, a Japanese official said the pool, where used nuclear fuel is kept cool, might be boiling.

"We cannot deny the possibility of water boiling" in the pool, said Hidehiko Nishiyama, an official with the economy ministry, which oversees nuclear safety.

The Times said spent fuel posed a long-term danger at Fukushima Daiichi. Should spent fuel catch fire:

"It's worse than a meltdown," said David A. Lochbaum, a nuclear engineer at the Union of Concerned Scientists who worked as an instructor on the kinds of General Electric reactors used in Japan. "The reactor is inside thick walls, and the spent fuel of Reactors 1 and 3 is out in the open."

Voice of America reported:

The International Atomic Energy Agency said it was informed by Japanese authorities that the fire took place at a storage pond for spent fuel rods at the plant's number 4 unit, and that radioactivity was released directly into the atmosphere at dose rates equivalent to 4,000 chest X-rays every hour.

The latest report from the IAEA suggests radiation levels may be falling.