On Wednesday, Scott Burnell, spokesman for the U.S. Nuclear Regulatory Commission said, "We now understand that conditions for the spent fuel pool in Reactor 4 are very serious." He added that "our understanding is the conditions have deteriorated."
The waste—used fuel rods from the six reactors—continues to be highly radioactive even when it is no longer useful for generating electricity and is removed from the reactor core.
To prevent the used fuel rods from heating up, sustaining damage, and then releasing radiation, they must be continually cooled by water for many years.
The Fukushima complex has seven pools serving six reactors. On Wednesday, officials acknowledged they faced serious cooling problems with at least four of the pools.
So far, at least two pools have lost enough water to expose fuel rods to the air, allowing them to overheat and release hydrogen gas. The gas is a product of the breakdown of zirconium cladding that encases fuel rods.
Nuclear physicist James Acton and WSJ's Lee Hotz discuss whether Japan can avoid nuclear disaster as workers scramble to cool the overheating spent fuel at the Fukushima Daiichi power plant.
The rods, known as spent fuel, are stored on an upper floor adjacent to, and at the same height as, the reactor. That keeps the distance the fuel rods must be moved to a minimum and was supposed to protect them from tsunamis.
However—unlike the reactor core, which is surrounded by a strong steel vessel—the only shielding between the radioactive waste and the outside air is concrete.
Increased heating and consequent damage to the fuel rods could mean "that there's a clear pathway for the release of radionuclides," or radioactive materials, said Edward Lyman, a physicist at the Union of Concerned Scientists.
As the water in the pool heats up and evaporates, the vapor carries some radioactive elements, including tritium gas. Worse, if the water level drops and exposes the spent fuel, those rods can suffer the same type of damage—a partial or full meltdown—that can occur to a reactor core.
"If the temperature continues to increase, you reach a point where the fuel pellets can melt," Dr. Lyman said. As a result, "a lot of fission products will start to bubble out."
Inside the Reactors
Reactor Monitor
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Smoke rising from the plant's No. 3 reactor indicates that a pool where spent fuel rods are stored within the building had dried out, a spokesman for utility operator Tokyo Electric Power Co. said. The water from the helicopters was aimed at this pool.
Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, said he believes there could be a crack in the spent fuel pool, increasing the risk that water drains from the pool and leads to overheating.
The pool at unit 4 is especially worrying.
Its fuel rods were removed from the reactor core in December 2010, and therefore contain a high level of radioactivity and are generating more heat than used fuel rods stored elsewhere at the Fukushima site.
The spent fuel pool at that unit has lost all or most of its water and radiation levels "are extremely high," Mr. Jaczko said.
It is unclear what the condition is of the seventh and largest pool that holds approximately 65% of used fuel rods from the six reactors, but they are likely the oldest and coolest.
