[Martina]

OK this sorta alarmed me although life has taught me not to worry too much about worst case scenarios. Still.

From a New York Times article:

The 1,479 spent fuel rod assemblies there include 548 that were removed from the reactor only in November and December to prepare the reactor for maintenance, and these may be emitting more heat than the older assemblies in other storage pools.

Even without recirculating water, it should take many days for the water in a storage pool to evaporate, nuclear engineers said. So the rapid evaporation and even boiling of water in the storage pools now is a mystery, raising the question of whether the pools may also be leaking.

Michael Friedlander, a former senior nuclear power plant operator who worked 13 years at three American reactors, said that storage pools typically had a liner of stainless steel three-eighths of an inch thick, and that they rested on reinforced concrete bases. So even if the liner ruptured, “unless the concrete was torn apart, there’s no place for the water to go,” he said.

Mr. Lahey said that much of the water may have sloshed out during the earthquake. Much smaller earthquakes in California have produced heavy water losses from sloshing at storage pools there, partly because the pools are located high in reactor buildings.

“It’s like being at the top of a flagpole, and once you start ground motion, you can easily slosh it,” he said.

When the water in a storage pool disappears, the fuel rods’ uranium continues to heat the rods’ zirconium cladding. This causes the zirconium to oxidize, or rust, and even catch fire. The spent fuel rods have little radioactive iodine, which has a half-life of eight days and has mostly disappeared through radioactive decay once fission stopped when the rods left the reactor cores. But the spent fuel rods are still loaded with cesium and strontium that can start to escape if the fuel rods burn.

One factor that might determine how serious the situation becomes is whether the uranium oxide pellets in the rods stay vertical even if the cladding burns off. This is possible because pellets sometimes become fused together while in the reactor. If the pellets stay standing up, then even with the water and zirconium gone, nuclear fission will not take place, Mr. Albrecht said.

But Tokyo Electric said this week that there was a chance of “recriticality” in the storage pools — that is, the uranium in the fuel rods could resume the fission that previously took place inside the reactor, spewing out radioactive byproducts.

Mr. Albrecht said this was very unlikely, but could happen if the stacks of pellets slumped over and became jumbled together on the floor of the storage pool.

Plant workers would then need to add water with lots of boron because the boron absorbs neutrons and interrupts nuclear chain reactions.

If a lot of fission occurs, which may happen only in an extreme case, the uranium would melt through anything underneath it. If it encounters water as it descends, a steam explosion could then scatter the molten uranium.

At Daiichi, each assembly has either 64 large fuel rods or 81 slightly smaller fuel rods. A typical fuel rod assembly has roughly 380 pounds of uranium.

One big worry for Japanese officials is that Reactor No. 3, the main target of the helicopters and water cannons on Thursday, uses a new and different fuel. It uses mixed oxides, or mox, which contains a mixture of uranium and plutonium, and can produce a more dangerous radioactive plume if scattered by fire or explosions. According to Tokyo Electric, 32 of the 514 fuel rod assemblies in the storage pond at Reactor No. 3 contain mox.