"I was told there wouldn't be maaaaath...."

Okay, if you’re one of those people, just skip this one.

Probably the most troubling news yesterday was that there had been a “fuel rod fire” and a resulting localized spike of radiation to 400 milliSieverts/hr — which is 40 rem/hr, or enough to make you actually no-kidding sick in an hour and a half to two hours.

Now, since then we’ve had word that this wasn’t a fuel rod fire, but just a conventional fire — in fact, the fuel rods won’t burn, although they can cause a hydrogen fire — but that still left that radiation spike.  How can that be explained?

Now, this is purely mathematical speculation, but it does explain the spike.  According to MIT, the fuel rod cooling ponds are about 5 meters deep (they say “16 feet”, which is 4.9 meters, which is close enough.)  I saw elsewhere that dose rate right on top of the pool is supposed to be 4 microSieverts/hour (4 μSv/h).  From that, we can calculate how radioactive the fuel rods themselves are.

Shielding is generally measured in terms of the “halving distance” — how much material it takes to cut the radiation in half.  Lead is high, naturally — 1 cm of lead (about 2/5th of an inch) to cut gamma radiation in half.  Water is about 18 cm (when I wrote this in a comment, I used 29, that was a late night error reading a table.) 5 meters divided by 18 cm is about 28, so that means the radiation on top is reduced 2^-28 times (½×½…×½ 28 times), or going the other way, the dose rate on top of the rods with no shielding would be 2²⁸ times more powerful.

2²⁸ times 4 μSv is about 1074 Sv or about 107,400 rem/hr.

Which is… real hot. You don’t want this thing in your sock drawer.

But work it backwards and we find out that it takes about 11.4×18 cm of water to reduce that to .4 Sv — or 400 milliSv (mSv) which is what was reported.

That’s a hair over 2 meters (about 7 feet). So if about 3 of the 5 meters of water were lost — and we know water was lost from the pool — that alone would account for the radiation hot spot.

People keep accusing me of not taking this seriously enough, but this time it looks like I was a little over-excited about the “fuel rod fire.”