What the Spent Fuel Pools Might Mean (It’s Probably Not What You Think)

by on March 17th, 2011 16 comments

Screencap from the entry movie from pro-nuclear activist William Tucker's site TerrestrialEnergy.org.

You are going to have to forgive me when I hurl a few rejoinders in the coming paragraphs. I imagine my gallows humor may strike many of my readers as just about as dry at the spent fuel pools ABOVE Fukushima I’s reactors #3 and #4 — yes, that’s right, TWO spent fuel pools, not just one, in which the “risk of re-criticality is not zero.” And they’re above the reactors.

Above.

First, Richard Black of the BBC, in a sidebar to this article:

The targets were cooling ponds situated above the reactors, which store fuel rods. The ponds in buildings 3 and 4 – and possibly more – are certainly short of water, possibly completely dry.

[Link.]

I’m sorry, did you miss it? Black said:

The targets were cooling ponds situated !!!!!!!!!!!!ABOVE!!!!!!!!!!!!!!!!!! the reactors, which store fuel rods.

…Emphasis mine.

I”m just curious. Does that sound like an awesome idea to anyone other than me? Or did nobody else get their feelings hurt by being ridiculed publicly by nuke-jocks for asking whether it was true that the China syndrome was possible. In the so-called China syndrome, an out-of-control nuclear core burns through the floor of the containment vessel and the floor of the containment building and into the basement. It’s a theoretical construct that doesn’t happen, won’t happen, and can’t happen. “Three Mile Island proved” it can’t happen, because the melted slag is not hot enough to burn through the bottom of the containment vessel.

But, for what it’s worth, in the meltdown of a core in which the containment vessel is breached (that’s different than the containment building), the melted core ends up in the basement. It did at Chernobyl. It formed an “elephant’s foot” in the basement under the reactor. It didn’t at Three Mile Island, because the containment vessel wasn’t breached. Nuke pundits say that TMI was “proof” that a meltdown can’t glorp out the bottom of the containment vessel, which I strenuously disagree with. It’s not “proof,” it’s a very strong indication. But that’s the only model we have for the meltdown of a large civilian water-moderated reactor, so it’ll have to do. Fukushima is not another Chernobyl because its ten reactors are water moderated and have containment buildings (which Chernobyl did not, and which the spent fuel pools do not).

At Fukushima, by the way, they did do something else the Soviets didn’t. They placed the spent fuel pools above the reactors. Which is 142 tons of spent fuel rods in the case of #4 reactor. As best I can tell (and I’m guessing), that’s something like ten-ish times the size of an active core.

Did I mention spent fuel pools don’t have a containment vessel, as such. I mean…sure, they’re contained. But they’re not supposed to ever go critical, so they aren’t designed to withstand criticality. The floors of the containment structures of the spent fuel pools aren’t designed to sustain any fuel rods at all undergoing active criticality, let alone meltdown.

I’m not saying a meltdown could happen with spent fuel — I don’t know. But I certainly don’t see how it could. But then, I don’t really see how it could go critical. This stuff is spent. It has 1% the fertility of new fuel. Right?

New Scientist confirms the fact that the spent fuel pools are above the #3 and #4 reactors, and both of them are undercooled — possibly completely dry.

Okay, that’s bad design, but back to why the spent fuel might enter re-criticality, which is something nuclear engineers know just doesn’t happen, which is why it’s so strange that Tepco, who’s been talking bullshit about “don’t worry” this and “no big deal” that from the start, now can’t shut up about it.

Again, how’s this happen? It doesn’t. It’s supposed to be impossible. Anyone with even the most rudimentary knowledge of nuclear engineering knows that re-criticality in a spent fuel pool just doesn’t happen. It just doesn’t. So why is Tepco saying it might, or more accurately saying the probablility of it “is not zero,” which from my experience of Tepco over the last week tells me it either probably has, or is probably going to, or they haven’t got the foggiest idea how likely it is?

Incidentally, the first spent fuel pool reported to be in trouble was Reactor #4. Reports from NHK and Nikkei led me to believe there were two blasts, as there were said to be two holes in the #4 building. That’s probably important, so if anybody out there has the reference, please let me know.

Here Is What’s Happening In The Spent Fuel Pools At #3 and #4 Reactor

  • The spent fuel rods are heating up. Enough to be a problem.
  • Since they put out something like 1% of the heat of active fuel rods, or less, maybe way less, that is kind of bizarre.
  • The structures that keep the spent fuel rods away from other spent fuel rods were apparently damaged in the explosions at #4 (and now #3).
  • That means the spent fuel rods are closer together than they ought to be.
  • That the means the chance of criticality is increased (the closer fissionable material gets to other fissionable material, the closer the chance of criticality).
  • The rods must be cooled.
  • The spent fuel pools are sufficiently “hot” (radioactive) that doing that’s difficult because of the risk to workers.
  • So they’re spraying it from helicopters, which isn’t working. It didn’t work at Chernobyl either, incidentally.
  • Fissionable nucleii put out neutrons, which results in other fissionable nucleii grabbing them, becoming unstable, and fissioning. That puts out heat, creates other neutrons, and you have a chain reaction, which is called a criticality.
  • Neutrons cannot be captured by fissionable nucleii if they are moving too fast.
  • Water is the best way to cool the rods.
  • Water slows neutrons.
  • Water therefore makes it more likely that a given neutron emitted by one atom will be captured by another fissionable nucleii, and fission.
  • Tepco is adding boric acid to the water to keep that from happening.
  • Tepco executives appear to be a bunch of third-graders high on bath salts.
  • No offense is intended to any third-graders reading this. And, incidentally, would you like to take over crisis management in the nuclear industry?

Here’s What I Don’t Know

  • I have no idea if the fuel rods “must be cooled” because they might otherwise reach meltdown temperatures. I don’t know the melting point of MOX fuel, but it’s lower than uranium fuel.
  • That would be really hot.
  • That seems like it would have to be completely impossible.
  • Remember, spent fuel rods, depending entirely on how old they are, put out something like 1% or less (maybe way less) of the heat that active fuel rods do.
  • So their melting under their own heat would be completely impossible.
  • Unless they reach criticality.
  • If the spent fuel rods achieve re-criticality, then they will be generating their own heat.
  • Except. Um…
  • To my knowledge, this time there will be no control rods. At all.
  • There will also be no containment structure.

There would be no control rods because spent fuel pools are not designed to need them. They don’t ever go re-critical, remember? There’s no containment structure because spent fuel pools don’t need them. Remember?

Here’s What Happens If That Happens:

  • I don’t know, and I don’t know the likelihood of it happening. I also don’t know the magnitude of the problem if it does. However:
  • The size of the spent fuel pool at reactor #4 is 142 tons; I don’t know about the one at #3.
  • I am presuming from news reports that this mass estimate is the total weight of the fuel rods, which includes the whole assembly; it’s not just the fuel; it’s also cladding, which is zirconium alloy, and other stuff that keeps the fuel rods together.
  • I don’t know if a given spent fuel pool is uranium or MOX. Various reactors at Fukushima were loaded with either uranium or MOX — and I have no guarantee that the pools at #3 and #4 only include spent fuel from those reactors, so there’s no way (from here) to find out if we’re talking uranium or MOX in the #3 and #4 spent fuel pools.
  • On short notice, I can’t find a good reference for what the mass is for the MOX fuel in active core of a Fukushima-sized reactor.
  • The best I can do is to guess, based on megawatt output and my meager reading in uranium metallurgy, that IF all the reactors at Fukushima were uranium-fueled reactors, we would be talking about something between 5 and 10 kilograms for a single active core.
  • That’s 5 to 10 kilograms.
  • They’re not uranium-fueled reactors; some are mixed uranium and plutonium (MOX). (Plutonium is far more toxic, by the way, though some of its fission products are ultimately the really dangerous stuff). Knowing little about metallurgy, I can guess at the uranium because uranium’s a whole lot simpler to understand. In any event, plutonium is denser than uranium. It is machined differently. MOX fuel is a different thing entirely. Etc. etc. etc.
  • So it’s a complete stab in the dark when I say that the “nightmare scenario” of a single core melting down completely was a scenario involving something like one-tenth to one-fifth the amount of nuclear material in the #4 spent fuel pool.
  • Again, these are not active uranium or MOX fuel rods. They are spent. They put out 1% of the heat of active rods. They have a lot less propensity to fission.
  • That said, Tepco sure does seem worried.

Again, this is not supposed to happen, which is why the fact that’s happening is a far bigger concern than if, say, the core just melted down — which everyone has known was a (remote) possibility in any reactor since the 1940s.

What’s so important about any of this? The magnitude of the potential problem, and the dishonesty Tepco has shown in the way it’s dealing with us. Depending on what caused the Reactor #4 explosions, it may also represent a gross malfeasance on Tepco’s part well beyond what we know about so far.

Here’s why.

How The Hydrogen Was Produced

Reactor #4 was shut down at the time the earthquake and tsunami happened. Three of the other five reactors at Fukushima I (Fukushima Daichi) were active. Those three all experienced hydrogen explosions. Though I first heard about it from anti-nuke activist Karl Grossman, who appears to be either a lying wing-nut or a confused wing-nut, he was right on one thing — the hydrogen appears to have come from the interaction of zirconium alloy (which is used in the fuel rod cladding, or shell) with very hot water, wherein zirconium oxide, hydrogen and oxygen are produced.

I read someone on a wing-nut board — who seemed to not be quite as nutty as the rest — suggest that those hydrogen explosions couldn’t happen in the case of simple decay heat if the reactors had been properly shut down. I don’t know; I’m not a nuclear engineer. But I did hear an engineer say something very interesting — again, uncited, unquoted, wing nut, take it as a friend-of-a-friend story and PLEASE DEBUNK IT if you’re a nuclear engineer.

The speculation is this: Was the rapidity with which such a large quantity of hydrogen was generated de facto proof of meltdown. That would mean all three active reactors at Fukushima Daichi were in meltdown very early in this process. Tepco then either didn’t tell us, or didn’t know.

Again, there’s probably a reason why the production of hydrogen is not de facto proof of meltdown. Nuclear advocates, and I count myself among you, God help me, I encourage your rude, hateful, trolling, aggressive, blatantly ridiculing opinions in the comments, correcting me, as I’m not a chemist.

For all I know, the hydrogen could have been produced by another process. Both radiolysis and thermolysis have been mentioned, though they both seem unlikely. Radiolysis would be where neutron flux broke the bonds between the hydrogen and oxygen in water, creating gaseous hydrogen and gaseous oxygen. Thermolysis would be the same thing caused strictly by very high temperatures. The molecular bonds in water would only break down at something like 2,000 degrees Fahrenheit and above, I believe. Therefore, the zirconium reaction is a much simpler explanation. I believe that would also happen at something like 2000 Fahrenheit.

Which is almost exactly the melting point of metallic uranium. That’s 2075 degrees Fahrenheit.

Uranium oxide, however, the form used in MOX fuel, melts at a much higher temperature. But wait! Mix them together and you get something that melts at a lower temperature, at least according to one guy, the Japanese reactor designer who freaked everyone out a few days back:

At a press conference in Tokyo, Masashi Goto, who worked for Toshiba as a reactor researcher and designer, said the mixed oxide (MOX) fuel used in unit 3 of the Fukushima Daiichi nuclear plant contains plutonium, which is much more toxic than the fuel used in the other reactors.

MOX fuel is a mixture of uranium and plutonium reprocessed from spent uranium, and is sometimes involved in the disposal of weapons-grade plutonium.

[Masashi Goto] added that the MOX also has a lower melting point than the other fuels. The Fukushima facility began using MOX fuel last September, becoming the third plant in Japan to do so.

[Link.]

I don’t know that temperature, but I’m sure one of my intellectually brutal nuclear-advocate readers does. Sock it to me! For now, all I know is that it’s lower than the uranium fuel used in the reactors other than #3, but there’s no guarantee that the fuel in the spent fuel pool at #4 is ONLY fuel from #4. I don’t know how this crap works…for all I know, the spent fuel pool could be nothing but MOX.

Is that a suggestion — though very, very far from proof or even a scientifically-worthy indication — that if the spent fuel in the #4 pool is MOX, it melts at a temperature low enough that even spent fuel rods, if they were somehow experiencing re-criticality — as Tepco has admitted they might — could have generated it?

Because if it was, and if a lot of other things are true that I, as a non-scientist, non-nuclear engineer, can neither investigate nor rule out, then that wouldn’t just mean the #4 explosions could have been hydrogen.

It would mean that, at some point, the spent MOX fuel rods in Reactor #4′s spent fuel pool, said to be physically and scientifically unable to experience criticality, were damaged by the explosion in #3, forced into proximity — and experienced criticality, boiling off the water from the spent fuel pool garden hose, and generating hydrogen.

Which would mean the spent fuel pool experienced re-criticality a couple days ago.

Were the Reactor #4 Explosions Hydrogen or Machine Oil?

The structure that holds the spent fuel rods in the spent fuel pools at #4 may have been compromised by the explosion at reactor #4 and the explosion at reactor #3 — which is close by. However, the explosion at #3 preceded the explosions at #4. Therefore, the hydrogen reaction could have created the disruption of the containment structures at the spent fuel pool at #4 that would have created the possibility of criticality. If that happened, and for some reason (extremely unlikely in the annals of engineering) the spent fuel started melting early, as far as I can tell it wouldn’t have generated hydrogen because it wouldn’t have been hot enough. But as far as I can tell, spent fuel pools don’t go critical in the first place.

The initial explosion at reactor #4 was said by Tepco, the company that operates the plant, to be caused by machine oil, not hydrogen.

However, if it turns out that #4 was not machine oil, they were hydrogen, that’s a whole ‘nother ball game, indicating criticality may have been reached in the spent fuel pool without Tepco even knowing it.

Is Tepco Lying About The Cause of the Explosions at Reactor #4?

If the initial reactor #4 explosion WAS a hydrogen explosion, and originated with the spent fuel pool there rather than from the reactor itself, then that would (as I understand it) indicate — though absolutely not prove in any way, shape or form — that the spent fuel in the #4 reactor’s spent fuel pool was heating up to the point where the zirconium alloy fuel cladding in the fuel rods was reacting with water to form hydrogen.

That would be hot. Really hot.

A Last Word About How Speculative This All Is

Again, I’m still shocked that anyone’s talking about re-criticality. This still seems really unlikely, borderline impossible. That’s why it’s freaking everyone out that Tepco, which has so thoroughly covered its ass all along, is suggesting not just this extreme scenario, but this way-beyond-extreme scenario.

But then, this crap was in the BBC. This crap was in New Scientist. Tepco has been playing down the crisis from the start. They appear to have vastly underreported the events from early in the process. Now they’re talking about re-criticality in a spent fuel pool. They. Tepco. The nuclear industry.

My speculations sound crazy to many of you, I’m sure. But Tepco’s already blown the roof off of all projections — and everything any of the pro-nuke trolls have said — by saying that the spent fuel can reach re-criticality to begin with. Which, in case you forgot, they already said.

I gather that, to a nuclear engineer, this is a bit like if the Air Force sat all its pilots down one day and said, “We were just messing with you. Those weather balloons really are alien spacecraft.”

Which, quite frankly, I’m half-expecting Tepco to do, any minute.

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  1. misery
    March 17th, 2011 at 19:57 | #1

    I read through everything you had to say 3 times and was quite impressed. Both with what knowledge you have and added when relevent, but also with how you presented unknown or unsure information by not only adding your best educated guess but asking to be corrected or proven wrong.
    “If we always succeed then we never learn. For it is in defeat that we learn how to succeed”-unknown
    You are very knowledgable and far better then most when it comes to that which you do not know. You share what you can, you welcome the knowledge of the unknown, and instead of saying something that you aren’t sure about you say you don’t know and share an idea. I have personally viewed since the beginning of the nuclear facility’s crisis’ 6 different local news channels who during a broadcasts ask and seem to believe without question when their meteorologist tries to explain nuclear fallout dispersal, the means at which radiation can and will only travel, or what if any amount of radioactive particles are present at or escaping from the reactors. None of them was as tactful as you and 2 of them boldly and frankly stated that no plume, or mushroom cloud or any harmful radiation was venting from the reactors and that even if all 3 facilities (9 reactors) completely meltdown no amount of radiation could reach the US. Now I understand not creating panic when there’s no reason to panic yet but to instead remove any chance or even thought of a possable panic is worse. Because you never panic more than when you’re faced with what they said you would never face or need to prepare for. If you have any questions regarding anything from the X-class solar flare shockwave that hit earth 2 hours before the first quake hit japan and any as far ahead as what pattern

  2. misery
    March 17th, 2011 at 20:20 | #2

    >correction
    before the first quake hit japan and anything in between as far ahead as where all this techtonic energy release is heading next and why or what may be happening beneath our crust…no in my own opinion we need to look to space because there is something cosmicly huge and possably wrong happening and our species may be in its way but that would mean our earth too, I realize how much our poles protect us from more deadly cosmic radiation then our atmosphere could handle but what if they are weakening or worse another polarity shift.

  3. John
    March 18th, 2011 at 22:04 | #3

    FWIW the odds or a re-criticality in a spent fuel pond are extremely low for several reasons. 1) the ponds have a lot of boron shielding between cages of fuel rods – boron soaks up neutrons. 2) you really need slow neutrons for a criticality, when the water boils off the neutron are no slowed down as much so they cause less splits. 3) the fuel in the ponds is spent, meaning the density of uranium is low. that’s not to say that a dry fuel pond is not a very bad thing, 1% of the heat from a 700Mw reactor is still a lot of energy (7megawatts – enough to run thousands of homes) and if the rods get hot they will spew all sorts of nasty stuff into the atmosphere. Check out http://mitnse.com for some sane, independent commentary on this stuff from MIT Nuclear Systems Engineering dpt.

  4. John
    March 18th, 2011 at 22:09 | #4

    Also the plant is 40 years old and they started using mox in September for the vast majority (all?) of the spent fuel is likely to be uranium.

  5. John
    March 18th, 2011 at 22:11 | #5

    oh and: Radiation readings at my house (CA94965) are up to 0.40 uSv/hr on the wet path outside (cf 0.22 uSv/hr baseline for the same stone not exposed to the rain and 0.06 indoors)

  6. john
    March 18th, 2011 at 22:18 | #6

    Great article. I am not a scientist so I have no idea. I believe:
    Radioactive material has been leaking into the atmosphere for a week. The wind has carried it and they are picking up trace amounts on the west coast of the US one week after the meltdown started. With little or no cooling for the better part of a week now the plant has basically been abandoned due to lethal levels of radioactivity and they are hoping for a miracle. It is really hard to tell without live coverage from the site. They wouldn’t have to tell us anything, we could just see it for ourselves. Are the pools still even there in 3 and 4? I can’t tell from all the jumpy, blurry, infrequent and far off in the distant pictures they have provided so far. I noticed last weekend when 3 blew there were 3 explosions and there seems to be a big round hold in the adjacent turbine building on the sea side of the reactor. I wonder what that hole was made from?

  7. Zardoz
    March 18th, 2011 at 22:23 | #7

    There might be a reason why Tepco is uncharacteristically, talking about the impossible disaster with the pools. Maybe they’re trying to conceal the worse disaster that’s really happening for a little while longer. That is, if some uncharacteristic gamma rays and neutrons are detected, they want to say that its the spent fuel rods, diverting our attention from what’s really in trouble. The reactor cores, maybe?

    Why would they do this? BP did something like this with the gulf. I remember how they said, hey, there’s a little leak. Then days later you hear, hey, that leak was was a tinny bit bigger than we thought. Then you hear, well, it’s really a little bigger than we thought the second time . . .

    With BP, it worked. They bled the outrage out of the public gradually. That’s what Tepco might be trying. Diverting our attention to the smaller part of the problem so they could only admit the scale of the real problem gradually. If it worked with BP, why wouldn’t Tepco be doing it now?

    Their statement might not have any accuracy, it might be all manipulation.

  8. Joe Helfrich
    March 18th, 2011 at 22:33 | #8

    I did see one report fairly early on, that indicated that the still active (as in not depleted) fuel for the cores that were already shut down for inspection at the time of the tsunami was being stored in the spent fuel pools.

    I haven’t seen anything else about that since then, so it could have been retracted/corrected….or it could explain an awful lot about why Reactor 4 is such a source of trouble, and they’re so eager to get things running again at the shut down cores that aren’t currently exploding.

  9. March 18th, 2011 at 23:54 | #9

    Joe — I confirmed the freshly-decommissioned core for #4 was stored in the spent fuel pool — I meant to post on this today but haven’t had a chance. I’ll do an update on the Nuclear Energy Institute report and the IAEA info. It’s not quite as nefarious as I thought it might be, but I think there are some really important lessons here about corporate communication and accountability.

  10. March 18th, 2011 at 23:55 | #10

    Zardoz — that’s a really interesting way to look at it. I think there’s some truth in what you mention.

  11. March 18th, 2011 at 23:59 | #11

    John – THANKS so much for the info, that is really helpful. And the readings are invaluable. What state or county are you in?

    The readings are also good to have from what I can understand of what we would expect. As you know, I don’t know much about nuclear science but it’s nice have the numbers for what it’s worth. Also utterly fascinating to me, though not surprising, that there is a demonstrable increase. The world is so big and yet so small. And science is awesome!

  12. eckhart
    March 20th, 2011 at 01:29 | #12

    It is very simple – no need to dissect Fukushima !

    How likely is it, that a pool will always be filled with water during 1000, 2000, 3000 … years ? Yes, you are right: = 0 %.

    It follows therefor, that with 100 % certainty spent fuel will be left without cooling at a given time in the future. It will radiate and contaminate.

    Logic dictates, it is 100 % certain that 100 % of the spent fuel will, at a given time in the future, radiate and contaminate.

  13. eckhart
    March 20th, 2011 at 01:31 | #13

    The use of nuclear energy can only be justified once the spent fuel can be recycled. Regardless of Chernobyl or Fukushima.

  14. eckhart
    March 20th, 2011 at 01:54 | #14

    Common sense Fukushima:

    spent fuel pools do not exist anymore; they exploded. The fuel rod content is scattered around.

    spent fuel as well as active fuel have been in meltdown since after failure of cooling

    meltdown continues and cannot be stopped as cooling is not possible anymore

    all or most original pump systems have been destroyed beyond repair.

    electric cables have been destroyed beyond repair and have to be installed fresh.

    Reactor 1 – 4 are toast and at least at Chernobyl level, 5 and 6 are at least at Three Mile Isl level

    Fukushima – each reactor 1000 times as potent as Chernobyl !

    many people have died during the explosions; many other experience radiation effects

    The Fukushima 50 hand-pumped water, unrolled cables and mostly waited for new orders

    Radiation leads to contamination. Contamination accumulates. Radioisotopes are here to stay.

    Radiation enters the entire life cycle by necessity and thus impacts the entire life cycle, also by necessity.

  15. eckhart
    March 20th, 2011 at 01:56 | #15

    Common sense fear:

    the melting nuclear fuel is so hot, that it will not be possible to bury the site for a long time to come.

  16. May 18th, 2011 at 15:52 | #16

    If reactor four was shut down why is it a mess. I think they forgot about the pools of fuel rods while fighting the problems of other reactors and let the water get low. Also if 14 reactors of the same design were effected by the same earthquake and tsunami why is it that only four are effected. I think that they restarted these reactors after the earthquake and before the flood. There were also early reports on the internet only ,that other reactors north of fuckinchina were in trouble and now we hear nothing about them. I like to see how they entomb these in a earthquake prone area.