In this reactor design, the used nuclear fuel rods are stored in pools of water at the top of the reactor building. These “spent” rods are still highly radioactive: the radioactivity is so great the rods must be stored in water so they do not combust. The explosion at Fukushima Daiichi reactor unit 1 apparently destroyed at least one wall and the roof of the building: some reports stated the roof had collapsed into the building.
Two days later, the nearby building containing the plutonium-uranium (MOX) fueled Fuksuhima Daichii reactor unit 3 exploded. So why bother about the rubble of reactor No 1? The WaPo quotes a nuclear engineer who knows the answer:
Although Tokyo Electric said it also continued to deal with cooling system failures and high pressures at half a dozen of its 10 reactors in the two Fukushima complexes, fears mounted about the threat posed by the pools of water where years of spent fuel rods are stored.
At the 40-year-old Fukushima Daiichi unit 1, where an explosion Saturday destroyed a building housing the reactor, the spent fuel pool, in accordance with General Electric’s design, is placed above the reactor. Tokyo Electric said it was trying to figure out how to maintain water levels in the pools, indicating that the normal safety systems there had failed, too. Failure to keep adequate water levels in a pool would lead to a catastrophic fire, said nuclear experts, some of whom think that unit 1’s pool may now be outside.
“That would be like Chernobyl on steroids,” said Arnie Gundersen, a nuclear engineer at Fairewinds Associates and a member of the public oversight panel for the Vermont Yankee nuclear plant, which is identical to the Fukushima Daiichi unit 1.
People familiar with the plant said there are seven spent fuel pools at Fukushima Daiichi, many of them densely packed.
Gundersen said the unit 1 pool could have as much as 20 years of spent fuel rods, which are still radioactive.
We’d be lucky if we only had to worry about the spent fuel rods from a single holding pool. We’re not that lucky. The Fukushima Daiichi plant has seven pools for spent fuel rods. Six of these are (or were) located at the top of six reactor buildings. One “common pool” is at ground level in a separate building. Each “reactor top” pool holds 3450 fuel rod assemblies. The common pool holds 6291 fuel rod assemblies. [The common pool has windows on one wall which were almost certainly destroyed by the tsunami.] Each assembly holds sixty-three fuel rods. This means the Fukushima Daiichi plant may contain over 600,000 spent fuel rods.