How the Spent Fuel Pools Might Achieve, Or Have Achived, Re-Criticality

by on March 17th, 2011 3 comments

Earthquake and Tsunami damage, Japan-March 16, 2011: This is a satellite image of Japan showing damage after an Earthquake and Tsunami at the Dai Ichi Power Plant. (credit: DigitalGlobe) www.digitalglobe.com

I saw an interesting possibility mentioned in a forum I won’t disclose because I don’t want to risk calling its reputation into question. I’m not a scientist, I don’t understand this shit, and if I am misconstruing scientific statements I do not want to call a real, and apparently responsible, nuclear scientist’s reputation into question. So just pretend I saw this opinion in an opium dream, for the moment.

Meanwhile, if you want the largely non-speculative version, check out the very comprehensible report on the spent fuel pools from the Nuclear Energy Institute, available in PDF form here. It’s from yesterday. I’m going to comment on it tomorrow because even glowing mutants need to sleep.

My Opium Dream: The Freshly Decommissioned Fuel Rods From Reactor #4

I saw the opinion from a cautious and clearly knowledgeable poster (in my opium dream) who expressed the view that one of the potentially problems with the #4 fuel pool is, ironically, the fact that #4 reactor was not operating at the time of the earthquake. Its fuel assembly may have just been removed for maintenance (presumably replacement with a fresh one). Were that true it might mean the #4 assembly (may) have been stored in the spent fuel pool at #4. This is speculative, and I did not check the credentials of this poster (in my opium dream), but I got the distinct sense there was significant expertise behind the comment. Still, I cannot vouch for it.

Oscar Wilde and a very large pink bunny said the guy’s cool, though. They were wearing gold lame tuxedos at the time; it was a hell of a dream.

What This Might Mean

If it is true that the very-recently-decommissioned fuel assembly for reactor 4 was/is in that the spent fuel pool, and there is absolutely no confirmation it is, how important would it be that is in operational terms? Most likely, not all that significant. However, it could be very significant, because were it true, that would mean the #4 fuel assembly could have been “live,” ie, operational, in criticality, in the functioning reactor, not long before it was transferred to the spent fuel pool. As long as it was stored properly and there were no earthquakes or explosions, no problem. Spent fuel becomes less “hot” (radioactively and thermally) with time, so in the case of compromised containment structures in the spent fuel pool at #4, “fresh” decommissioned fuel would pose a much greater risk for entering re-criticality than older fuel. In short, because those fuel rods would have been operational not long before, they therefore (I gather from my reading) would be more fertile, more easily fissionable.

The greater heat of freshly-decommissioned Reactor #4 fuel assembly in the spent fuel pool in #4 would also pose a greater risk of raising the temperature of the pool, which would potentially compromise the other fuel rods and, if the explosions damaged the neutron-absorbent material between the rods and/or pushed the rods closer together, some of those rods being more freshly-decommissioned ones, that could cause a problem in terms of re-entering criticality.

Meanwhile, something immersed in water really has a heck of a time getting hotter than 212 degrees Fahrenheit without being in a pressurized environment, which this is not. Therefore, the heat only gets important if the water evaporates off. I’m not going to say “boils off” as many news agencies have been saying, because I don’t believe (though I could be wrong) that it’s been confirmed the water in the spent fuel pool at#4 ever boiled. It doesn’t have to boil to evaporate; hot water still evaporates faster than cold. Remember, the “fresher,” more recently decommissioned fuel, is going to be hotter in both radiation and thermal terms than the long-ago-spent fuel, depending on how old it is. It is therefore more prone to potential thermal compromise of the zirconium fuel rod cladding, which would increase the risk of re-criticality.

The claim that a spent fuel pool cannot enter re-criticality is based on the fact that you don’t have brand-spanking-new freshly-decommissioned fuel in it. Also that the neutron-absorbent material like boron between the sheets is not compromised by an explosion or earthquake and tsunami, and that one or several of those events don’t knock the rods closer together.

Incidentally, while there is again no confirmation and even no concrete indication that the freshly-decommissioned fuel assembly from Reactor #4 was placed in the spent fuel pool at #4, to my knowledge that would NOT have been a violation of any specific safety procedures on Tepco’s part.

There may have been other violations — that’s totally unknown right now — but I don’t see any reason a freshly removed fuel assembly that has already gone through its shutdown procedure wouldn’t be put in the spent fuel pool — where the hell else are you supposed to put it?

Again, I’m not a nuclear engineer or a scientist — I got a “C” in chemistry and can’t even change my own Volkswagen’s oil.

But this ain’t rocket surgery…right?

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  1. Rob
    March 18th, 2011 at 05:47 | #1

    According to the IAEA, the November 30 shutdown of Unit #4 did involve storing the fuel in the spent fuel pool. See the March 15 18:00 UTC update here: http://www.iaea.org/newscenter/news/tsunamiupdate01.html

  2. Rob
    March 18th, 2011 at 05:50 | #2

    And actually, they reconfirm that fact in their latest update (18 March 12:25 UTC)

  3. March 18th, 2011 at 10:13 | #3

    Rob — Thank you! I also found it in an NEI ref.