How Much Spent Fuel Is At Fukushima?

From the Union of Concerned Scientists.

With the spent fuel pools reportedly heating up again, the claims are flying fast and furious (again) about how much spent fuel is at Fukushima. It’s deja vu all over again.

At online forums like, where they never met a conspiracy theory they didn’t feel like massaging into a panic, the claim is that we’re talking about a few thousand tons. That, of course, is complete and utter conspiracy-theory nonsense, unless you plan to believe those whacked-out hollow-earth weirdos at fringe journals like Scientific American or the “experts” at the Union of Concerned Scientists, who say it’s…a few thousand tons.

So let’s do some math, shall we? This is not my subject, but then, neither is nuclear engineering. Or chemistry. Or physics. Or graphs. I do sometimes read XKCD, however, so I feel sort of like a qualified mathematician when I chuckle knowingly at math jokes.

The screenshot above is from the All Things Nuclear site from the Union of Concerned Scientists, which is strongly anti-nuclear but as far as I can tell, generally not prone to pseudo-science.

Importantly, the figures quoted in the graph above represent fuel assemblies, not kilograms. Each assembly is about 170 kilograms, not all of which is nuclear fuel but all of which is highly radioactive (though it depends on how you define “highly.”

For you non-metric types, a kilogram is 2.2 pounds. For you non-British-system types, when Americans talk “tons,” they mean 2,000 pounds.

Therefore, if you believe these figures, and if I am understanding these figures, the amount of spent fuel stored at Fukushima I, in the pools, as of March 2011, is about 2,000 tons. These assemblies are not stored in one single pool, so a single compromise of cooling (or, say, a re-criticality) does not compromise all of the spent fuel. From these figures, the single pool at Reactor #4 that was having the biggest problems last week has about 248 tons of spent fuel, not the 172 tons that I originally reported.

The Reactor #4 pool includes, apparently, the spent fuel assembly from Reactor #4, which was shut down prior to the earthquake. That is more recently decommissioned fuel, more fissionable, and more likely to enter re-criticality (also much more likely to heat up). In fact, it is the presence of the recently decommissioned fuel from #4 that made re-criticality any sort of risk, apparently.

The site referenced above says the recent fuel from Reactor #4 was 548 assemblies (about 102 American tons), which was added to 783 assemblies (about 148 tons) already in the pool. As the Union of Concerned Scientists observes, the Kcal calculations mean the heat put out by the 783 older assemblies must be negligible.

For the purposes of this information, I have omitted the dry cask storage, since it does not require cooling. It’s older fuel and less fissionable. I’m not so sure that makes it any less hazardous in long-terms, but that’s a matter of getting real specific about what you mean by “hazardous.” In terms of the cooling problems, only the pools are directly relevant.

Unfortunately, the information is out there to look at, and it all seems to more or less agree. Whether you want to believe Scientific American, the Powerpoint over at the anti-nuke Nuclear Information and Resource Service, or the Union of Concerned Scientists, it looks like we’re talking about several thousand tons of spent fuel, and around 250 tons in the Reactor #4 pool they’re still having trouble keeping cool.

Possibly related posts: