Friday, April 01, 2011

Numbers, Please!

Something that some have been anxiously awaiting to see at Fukushima is "re-criticality." It's also referred to as "localized criticality" or "episodic fissioning." I've read descriptions that make it sound as though the fuel elements are dripping pieces with sparks of neutrons, light, and other scary things.

So far none of those proclaiming it have provided persuasive explanations, with numbers.

In order to have criticality or fission (with one exception, which I'll explain later), there must be a critical mass. The water being injected into the reactors has boron in it, and the fuel elements are spaced to allow water flow around them. The boron absorbs neutrons and prevents fission. So "re-criticality" isn't happening in fuel elements that retain their original configuration.

But the tops of the fuel elements are exposed in the reactors. So they can heat up (although the partial immersion will help to remove heat from them) and deform or break open. At worst, if they are suspended from the top, they may break off and fall to the bottom of the reactor vessel. Or small pieces may fall to the bottom of the vessel. The fuel pellets within the fuel elements are centimeter-sized. They are oxides and have very high melting points.

If fuel pellets come out of the fuel elements, each contains a small amount of uranium or plutonium and is incapable of becoming critical. Some large number would have to accumulate in the proper configuration without borated water between them to form a critical mass. And if that were the case, criticality would continue; it wouldn't be a flash.

If the fuel elements break off and fall to the bottom of the reactor, it seems unlikely that they would form critical masses. They would have to fall into a very orderly pile, and, again, criticality would continue.

It may be a lack of imagination, but I just don't see how you could get small bursts of criticality, and I haven't seen the proponents of "re-criticality" provide as much thinking-through as I've given here, let alone numbers for what the critical masses might be (they depend on configuration) or how critical configurations might occur.

The one exception is whatever plutonium-240 might be in the reactor. Plutonium-240 spontaneously fissions, but it is present in very small quantities. I'm wondering if the alleged activation products result from plutonium-240 neutrons, but, like those making that claim, I haven't looked up the numbers. The next things I want to do will require numbers, so that may take a little time. And I'll try to check this as well.


Meredith Angwin said...

Thank you for this Cheryl! I have been trying to understand about radioactive iodine and came to the conclusion (correctly, I think) that the reactors had some radioactive iodine left from when they were running, but since it has an 8 day half life, it would soon be gone, whether it was (hopefully) retained within the reactor or escaped to the environment (as some of it has). But when I began reading about "re-criticality" I was concerned that more iodine was being produced. From what I read here, the more-iodine scenario is very unlikely.

Fred Z said...

Sounds as if you're probably right. However, reading your article and relying only on what you say, I can imagine a couple of conceivable mechanisms for "rearranging" fuel elements into orderly critical masses: 1) gravity 2) water. I.e, there are vast amounts of water flowing around in a hot, turbulent environment that is full of debris. "Structures" could form around obstacles, couldn't they? Let's hope not though!

Cheryl Rofer said...

Meredith, an eight-day half-life means that it will be eighty days until the iodine is undetectable. We're not there yet.

I've used a rule of thumb there, that ten times the half-life is required to pretty much eliminate a radionuclide. Ten half-lives mean that what remains will be about 1/1000 of the initial amount.

Fred - I don't know enough about the reactors to know what the normal flow paths are and how they might be disrupted by broken fuel elements. Nor do I know that water is flowing through the reactors. That's another question that I'd like some answers to from TEPCO: are they circulating the water? If so, is it recirculation or an open loop? I can infer either from what I've heard.

Cheryl Rofer said...

And, just now, some numbers come in! But they are far from proving (or even strongly supporting) the case for "re-criticality." I'll let the authors speak for themselves:

Assuming that the TEPCO measurements are correct, this analysis seems to indicate that we cannot discount the possibility that there was another strong neutron source during the time that the workers were sending seawater into the core of reactor #1. However, without knowing the details of the configuration of the core and how the seawater came in contact with the fuel, it is difficult to be certain. Given these uncertainties it is nonetheless important for TEPCO to be aware of the possibility of transient criticalities when work is being done; otherwise workers would be in considerably greater danger than they already are when trying to working to contain the situation. A transient criticality could explain the observed 13 “neutron beams” reported by Kyodo news agency (see above). This analysis is not a definitive proof, but it does mean that we cannot rule out localized criticality and TEPCO should assure that the workers take the necessary precautions.

There are a lot of hedges there.

MT said...

"Fission" doesn't imply criticality or even chain reactivity, though, right?

Cheryl Rofer said...

Good point, MT. There are plutonium isotopes in any reactor after it's been run for a while that spontaneously fission. They don't need a critical mass or chain reaction.

What seems to be being implied by some of the commentary, and why I'm asking for numbers, is that criticality and a chain reaction is being developed in a transient way, perhaps as a result of the configuration of the fuel.

Michele Kearney said...

Do we know for certain that boron is being added to the fresh water that is being poured into reactor?

Cheryl Rofer said...

Good question, Michele. It's one of the things I've been wondering about.

Maybe I'll make a post of all the questions I have.

MT said...

The uranium undergoes fission spontaneously too though. You don't have to start an atomic bomb, right? My understanding is you assemble the critical mass and it starts itself, because at least one atom among the bazillion is always about to go off, and at the critical condition, that's all it takes. Absent that prospect of a self-perpetuating and amplifying chain of fission reactions, spontaneous fission by the uranium isotopes is not anywhere near so pernicious a decay source as the plutonium and other short-lived nasties.

Cheryl Rofer said...

Yes, the uranium undergoes spontaneous fission, too, but in much smaller numbers than plutonium-240. And none of this would account for some of the alleged effects, like the mysterious neutron beams (which I strongly suspect are errors of measurement).

Some atomic bomb designs incorporate a neutron source. On the one hand, uranium doesn't supply enough spontaneous fissions and their neutrons for a satisfying bang, and, on the other, plutonium can't be used in some of the designs uranium can be used in because even a tiny amount of plutonium-240 causes predetonation and subverts a bigger bang.