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Fuel tube thermal runaway

  1. Jul 22, 2015 #1
    Why doesn't the ceramic pellets react with each other in the fuel tube when it is being fabricated?
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  3. Jul 22, 2015 #2


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    In order for reactor fuel to undergo a chain reaction, it requires 1) a sufficient mass of fuel in close proximity 2) in the proper geometry 3) with a neutron moderating medium. Lacking these three conditions the fuel cannot react and therefore is safe to handle/fabricate.
  4. Jul 22, 2015 #3


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    Welcome to the forum.

    A little context might be helpful. :wink:

    I presume you are referring to Uranium fuel pellets.

    Are you referring to chemical reactions? If so, then the usual methods of preventing this are such things as controlling temperature, and in special conditions, controlling the gases around the pellets. So to prevent oxidation you keep the temperature low enough to avoid oxidation reactions. And if you have to raise the temperature to perform a production step, you fill the volume around the pellets with nitrogen or some other non-reactive gas.

    Typically a fuel tube will get filled at room temperature. Then the end will get welded in place. Welding zirc is a fairly special process done with a lot of care so as to prevent damage to the tube or the fuel. They control the temperature and gas present to a quite wonderful degree. The weld has to be pressure tight and strong, and no oxidation of the tube is allowed.

    Are you referring to nuclear reactions? Individual fuel rods, even fuel assemblies, don't have enough uranium 235 to react without some moderator. You have to work fairly hard to get a fuel assembly to react. You need to get the geometry and the moderator quite well arranged for it to occur. So when filling a single fuel rod you are safe by a large margin as long as you keep things like large quantities of water away. Also graphite. Even with such material present you would need a very specific geometry to get it to react.
  5. Jul 22, 2015 #4


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    During manufacture, solid enriched UF6 (in cylinders) is vaporized and feed as a gas into a chemical process stream where it is hydrolyzed in a wet process, which produces ammonium diuranate or ammonium uranyl carbonate, or it is reacted directly with steam in a direct or dry conversion process to UO2. In the wet process, the precipitate is collected and dried to powder, and in the dry process, it is collected as a powder. Eventually the powder is collected in buckets or barrels, which can be on the order of tens to hundreds of kg. The powder is collected and processed with additives and then pressed into 'green' pellets, which are about 50 to 60% of the theoretical density of stoichiometric UO2. The 'green' pellets are sintered to final density and collected on trays or in boats. The pellets are then ground to final dimension.

    As long as the material is maintained at relatively low density or in a particular geometry and away from a moderator, e.g., water, there will not be a nuclear reaction.

    Criticality analysis is part of the design of a facility in which the various stages of the manufacturing process are simulated to ensure that the fissile material will remain subcritical.
  6. Jul 22, 2015 #5
    Thank you all very much, I was just curious about this, and couldn't find the answer on my own.
  7. Jul 23, 2015 #6
    The fuel is specifically designed that a single fuel bundle can not go critical on its own under it's design conditions. If you were to overmoderate the fuel by spraying a fog nozzle https://en.wikipedia.org/wiki/Fog_nozzle you might be able to cause the single fuel bundle to go critical on its own. (For this reason, the spent fuel pool zones at a nuclear power plant do not have fog nozzles anywhere near them).
  8. Jul 24, 2015 #7
    How does a fog nozzle overmoderate a fuel bundle? Wouldn't void coefficient be large? We store new fuel in liquid water at <100F and are far from criticality.

    A new core with no fission products present might not (probably won't) fission at all. Start up sources (americium and californium IIRCC) are used to provide neutrons in a new core. Otherwise is would just sit there.

    Am I missing something?
  9. Jul 24, 2015 #8


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    A fresh does not need startup neutron sources to go critical or beyond critical. One can remove control elements (control rods in BWRs or PWRs) or dilute the boron in PWRs, and the core can attain keff > 1.

    Startup source are there to provide neutrons to cause fissions so that there a sufficiently strong neutron flux for the detectors as the core approaches criticality. There are tech specs on the counts per second.

    Primary sources are now Cf-252, while older ones might have been Pu-Be, Po-Be, or Ra-Be. Secondary sources are Sb-Be, where Sb-123 absorbs neutrons to form Sb-124m, which decays to Sb-124, which decays by beta decay to Te-124, either yielding a 1+ MeV photon that causes a photoneutron reaction in Be-9.
  10. Jul 24, 2015 #9
    Astronuc I'll take your word for it. Our SRM spec is 3 CPS.

    I don't have data to back my statement except what I have been told...

    Obviously we have never attempted a startup without sources or irradiated fuel. I can say without a doubt, however, one fresh bundle does not achieve criticality in water without boron or control rods. I still wonder about the fog.
  11. Jul 25, 2015 #10


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    The purpose of the source is not to enable criticality but to make its onset predictable and repeatable. With no source you might withdraw too much negative reactivity before a random decay triggers the reaction. Since the initial reaction is a cold reactor can be very hazardous if not handled correctly, predictability is highly valued.

    Does irradiated fuel make the source unnecessary? Never thought about that; probably yes. Still, the engineers need to be able to make a verifiable calculation. How much delta-k per second can the operator make, and how many seconds can elapse after criticality and before the onset of the reaction? They may prefer the known properties of a source to the uncalibrated decays of irradiated fuel.
  12. Jul 25, 2015 #11


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    For some utilities, a sourceless startup means not having to deal with hardware in the fuel. I know about start,up sources in PWR fuel, but I understand, there are not startup sources in BWR fuel, but they are in components somewhere in the core among fuel assemblies near the in-core instrumentation.

    A sourceless startup requires irradiated fuel of a certain burnup, probably on the order of 30+ GWd/tU, because there has to be a certain amount of transuranics, e.g., isotopes of Pu, Am, Cm, that undergo spontaneous fission which provides the neutrons necessary for the detectors. I'll have to check with some core designers, but I believe that twice-burned fuel is preferred, and that may depend on core power density and core design strategy.

    Reactivity changes (Δk) come from the normal operations: withdrawal of control rods/blades, or dilution of soluble boron in PWRs, and changes in fuel/moderator temperature and density.

    Another aspect of a fresh core at first approach to criticality, there is not much in the way of delayed neutrons, since there isn't essentially any fission to produce the precursor nuclides.
    Last edited: Jul 26, 2015
  13. Jul 27, 2015 #12


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    Light water reactor fuel has two peaks in the moderator curve - one at typical cold water density and another at very low density similar to foam. Also note that new fuel is stored in water chock full of boron.

    As others have stated, primary/secondary sources do not affect reactivity. They do not increase the multiplication factor or make a reactor critical. They just provide a measurable indication of how close to critical you are.

    The purpose of secondary sources is not so much for actual startups. As long as you have some burned fuel, when you get to around k=0.99 you have plenty of subcritical multiplication. What secondary sources are important is for detecting unintended criticality - e.g. from a boron dilution or misload accident. Without secondary sources, even with burned fuel you may not be able to tell the difference between k=0.8 and k=0.98 - depends on the specifics of the plant though, how they load their fuel, the sensitivity of the source range detectors, etc.
  14. Jul 27, 2015 #13
    Our reactor is a BWR and the water in the fuel storage pool is not borated -I am sure. I don't think the storage racks contain boron or halfnium but I will find out.

    The double peak in the moderator curve is very interesting! Is it also true for BWR fuel? It would have to be wouldn't it since water is the moderator in both cases or is it due to the boron?

    We use voids (similar to foam) to reduce reactivity.
    Last edited: Jul 27, 2015
  15. Jul 27, 2015 #14


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    Oh BWR fuel is different, the assemblies are smaller. PWR fuel is like 2x2 BWR assemblies. The double hump moderation curve is characteristic of water-moderated LEU systems regardless of design.

    Yes, increasing void ratio reduces reactivity - up to a point. When you get to very low densities there is another peak of optimum moderator density.
  16. Jul 27, 2015 #15
    As others are saying the sources in the core is for predictability.

    BWRs typically don't use startup sources, even after relatively long shutdowns. More than a year or two may drop your SRM counts too low (<3 cps) where you would need to insert startup sources. My plant's sources are in the spent fuel pool and we haven't used them since the 90s. After our last large refuel outage (over 45% of the core was replaced), even after a 3-4 weeks subcritical with almost half the core replaced we were still in the 10^2 range on SRM counts. We also have the ability to make discriminator adjustments to raise count rate > 3cps for meeting OPERABILITY of SRMs. We (typical GE BWRs) don't have any source components in the fuel or in the core. Counts are all from fission products detected by the in-core SRMs.

    If you don't have indication of criticality, you would not know you were there. You could, in theory, create a very supercritical condition that wouldn't be caught until power rapidly increased and returned to the source range. This could lead to core damage. This is why SRMs are required.

    As for the 'fog' nozzle for fire spray, I actually don't know the theory on it really well, we train on it though. Never ever fog spray fuel in the (dry) new fuel storage pit. Ever. BWR fuel is not borated after it arrives. It gets placed into the spent fuel pool after all inspections are done. The spent fuel pool racks may use BORAX plating to preclude criticality in the spent fuel pool and maintain keff<0.95 as required. The high density storage racks that many plants use almost all contain boron plating.
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