Removing the #Fukushima Spent Fuel Rods. Or not.

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TEPCO was going to start removing the fuel rods from the less-damaged reactor building Numnber 4 over the next few days. Today, it was announced that damage to the fuel rod assemblies, some or most of which predated the tsunami and earthquake, this could not be done. There is now uncertainty as to what is going to happen.

Here is a video by Fairewinds about this operation, which I believe was made before TEPCO decided to not continue with the removal at this time:

As you can see, there are several possible problems. Most of these problems are not related to the reasons TEPCO has given to halt the operation at this time; they are additional .

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15 thoughts on “Removing the #Fukushima Spent Fuel Rods. Or not.

  1. I think the problem is the problem of all top-down power hierarchies. The figurehead at the top derives power from the entire bottom, but those at the bottom have different and mutually incompatible goals that are sometimes at cross purposes. In this case doing the job “right” and meeting time and cost constraints.

    It is pretty obvious that if the fuel assemblies are damaged, there need to be techniques to remove those damaged assemblies. That (very likely) means cutting them into pieces. Since one cannot order “damaged fuel assembly cutters” from Acme, that means the equipment to do so must be developed; that is designed, built, tested, and practiced with, before using it on “real” fuel assemblies that have been damaged.

    Why haven’t they built devices to cut damaged fuel assemblies into pieces while capturing any and all fission products that might be released? Because no one at the top told or authorized anyone at the bottom to do so.

    The engineering tradeoffs are always the same; good, fast, cheap, pick any two. What Tepco is picking is fast and cheap.

  2. I also wonder about how they suddenly know that there was damage years ago. Did they notice the damage and then decided to check old records? Did somebody suddenly remember? Did they see damage and one of the people at the middle/bottom figured out a way to blame somebody that no longer works there?

  3. Daedelus2U: “Why haven’t they built devices to cut damaged fuel assemblies into pieces while capturing any and all fission products that might be released? Because no one at the top told or authorized anyone at the bottom to do so. ”

    Or because no such technology exists, and may not exist in our lifetime.

    To paraphrase a bit from the X-Files, in which a lizard has become merged with a rock due to a space-time warp:

    Mulder: “So, how do we get the lizard out of the rock?”

    Anonymous Govt. Agent: ‘Who says we can?”

  4. If the problem is never worked on, or thought about, there will never be a solution.

    Technology doesn’t just appear by magic, scientists and engineers have to think about stuff, build stuff and test it to make sure it works.

    If you never start the process, the technology will never be developed.

    The materials that comprise fuel rods can be cut with suitable tools. Solid, liquid and gaseous fission products can be captured with suitable equipment.

    I think the reason no one is thinking or acting on getting the right equipment is because TEPCO is an operating company, not a company that designs or builds stuff.

  5. So far they’ve been doing a good job with Fukushima. Arnie was saying things were happening during the initial disaster that everyone else was denying and in the main he was right and they were wrong, for example. He openly talks about worst case scenarios but he does not say that they are inevitable. Comparing the rhetoric from fairewinds with the rhetoric from the nuke apologists or Tepco, fairewinds is more accurate.

  6. They provide glib worst-case scenarios but in looking over my notes from 2011, I don’t see any real news broken by Arne. I completely agree that he’s a good antidote to TEPCO, but …


  7. The risk of criticality is small and is pretty easy to control with neutron absorbers such as boron and non-moderators such as carbon.

    If rods containing boron and not containing hydrogen were put in and among the assemblies, that would put a neutron absorber and remove H2O from the vicinity. The risk of criticality is greatest with the fuel that has not been irradiated. It is small with the spent fuel because the U235 has been consumed and neutron absorbing fission products have accumulated.

    Boron can also be dissolved in the water (and very likely is). It is poor practice to rely on boron in the water because if the fuel breaks up, the pieces can fall to the bottom and there is less water in the pile, then there is also less boron. If you mechanically mix boron containing stuff (like pyrex glass or boron carbide), then even if stuff breaks and sinks there is still plenty of boron around.

    If there is a criticality event, what that means is that the power released by the critical mass starts increasing exponentially. The size of the exponent depends on how many neutrons get generated and how many get absorbed or lost before they cause fission. Criticality is when for every neutrons that causes fission, more than one neutron causes another fission. The average number of neutrons produced per fission is ~2.4 or so. If 1.4 of those neutrons are lost or absorbed, then the system is critical. If 1.5 are lost then the system is subcritical. If 1.3 are lost, then 1.1 cause fission and the system is supercritical. Criticality is at exactly 1.00000

    The neutrons produced during fission are high energy (MeV). These are so fast that they diffuse out of the critical mass quickly. Hydrogen slows them down by the scattering. Hydrogen is the best moderator because hydrogen and a neutron have the same mass (momentum and energy are conserved, so high Z elements don’t moderate very well).

    Neutron production is not completely instantaneous. There are some “delayed” neutrons, but these are relatively few. It is these delayed neutrons that allow reactors to be controlled. If you maintain the number of neutrons produced to be greater than 1.000 but only by the number of delayed neutrons, then the power of the reactor will increase, but only due to the delayed neutrons. You can make this time constant as slow as you want because the delayed neutrons have a very long tail. If you insert a neutron absorber the number of neutrons drops immediately so you can stop things quite fast.

    In the event of criticality due to moderated neutrons, if the moderator is lost, then the average velocity of neutrons goes up and more are lost. This is the logic behind the “negative void coefficient”, where a loss of coolant causes a decrease in reactivity, reactor power and a loss of criticality. This is typical behavior for light water reactors. The light water is the moderator, if a bubble forms there is less moderator, the velocity of neutrons goes up and more are lost so less are produced.

    This is the type of criticality most likely to occur. There could be a boiling of the water, a void would form, the reaction would be quenched, the bubble would collapse, criticality would be restored and this could go on for a long time. This would generate very substantial circulation of water. Unless that flow of water damaged the fuel assemblies, that wouldn’t lead to the much more dangerous type of criticality, criticality due to fast neutrons. This type of criticality can occur in melted fuel. Melted fuel doesn’t have hydrogen in it because it is so hot. There is no limit to how hot a reactor can get other than the boiling point of the melted fuel. Once the fuel starts to boil, then its density goes down and more neutrons are lost from its surface and it becomes subcritical.

    If the system is subcritical immersed in water, it will not become critical if the water is removed. Water lowers the amount of fissile material required for criticality by acting as a moderator.

    Water does keep the fuel cool, and if the fuel gets too hot, then it can melt, collapse or break apart. How quickly that would happen depends on how much decay heat the fuel is producing. The fuel elements are in zirconium tubes. Zirconium can take pretty high temperatures, but it is susceptible to oxidation by air, steam, or even nitrogen.

  8. Corrector, I’m restricting your obnoxious comments to just one or two threads, so don’t bother commenting beyond that. (GTL)

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