a visual tour of the fuel pools of fukushima...,

tangled wreckage inside the pool

ratical | You already know that Fukushima’s fuel pool number 4 may be the single greatest threat, but that pool number 3 is very dangerous as well.

You’ve heard that unit 3’s fuel pool contains less radioactive material than unit 4 . . . but still a tremendous amount of radiation. Scientific American reported last year:

The pools at each reactor are thought to have contained the following amounts of spent fuel, according to The Mainichi Daily News:

• Reactor No. 1: 50 tons of nuclear fuel
• Reactor No. 2: 81 tons
• Reactor No. 3: 88 tons
• Reactor No. 4: 135 tons
• Reactor No. 5: 142 tons
• Reactor No. 6: 151 tons
• Also, a separate ground-level fuel pool contains 1,097 tons of fuel; and some 70 tons of nuclear materials are kept on the grounds in dry storage.

You’ve learned that unit 3’s reactor was the only one at Fukushima which burned plutonium. As Japan Times notes:

Reactor 3 . . . uses highly dangerous mixed oxide fuel, Tokyo Electric has reported.
***
No. 3 reactor is the only one at the crippled power station that was powered by the plutonium-uranium MOX

You’ve gotten the fact that – if the water drains out for any reason – it will cause a fire in the fuel rods, as the zirconium metal jacket on the outside of the fuel rods could very well catch fire within hours or days after being exposed to air. See this, this, this and this. (And that even a large solar flare could knock out the water-circulation systems for the pools.)

this and this – starting at 4 minutes into the video.

35 ton machine blocking the pool

You’ve listened to experts say that – unless the rods are removed from the fuel pools before a major earthquake strikes (using special equipment which keeps the rods submerged in water the whole time) – they will likely catch fire and release huge amounts of radioactivity. See

You’ve read that – after reviewing photos from several different angles – the Nuclear Regulatory Commission’s initial impressions were that spent fuel pool number 3 might not be there at all, and that nuclear engineer Arnie Gundersen said a couple of days ago:

Unit 3 is worse [than No. 4]. It’s mechanically, it’s rubble, the pool is rubble. It’s got less fuel in it. It faces the same problem. Structurally the pool has been dramatically weakened. And, god, nobody has even gotten near it yet.

And you may have caught the recent headline that a 35-ton machine fell into Unit Three's spent fuel pool. As Kyodo News reports:

The operator of the crippled Fukushima Daiichi nuclear power plant on Friday found that a 35-ton machine had dropped inside the spent fuel pool of the No. 3 unit, possibly because of a hydrogen explosion that occurred in the early stage of plant’s nuclear accident last year.

Tokyo Electric Power Co., commonly known as TEPCO, reported the finding after placing a camera inside the water-filled pool the same day to prepare for removing, as part of the decommissioning process, the nuclear fuel stored there.

One photo showed part of the machine, originally located above the pool and used to insert and remove fuel, appeared to have dropped onto the nuclear fuel storage racks.

But – until you see pictures – it is hard to get a sense of what all this means.