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Disposition of Spent Fuel - Separation option

  1. Feb 23, 2015 #1
    But nikkkom, that makes no sense.

    1: Vitrification is not cheap; I doubt that even superficially it is significantly cheaper than reprocessing; you know perfectly well that one does not simply dunk spent fuel rods in molten glass and bobsyouruncle. You first have to get the relevant materials out of the problem material, and convert it to workable form, you then have to work out into what kind of glass (LOTS of glass, please note, it is not just one part Pu, one part glass, one eye of newt and one tongue of dog!) and how to incorporate it as a homogeneous mass. That will COST!!!! Certainly it does not cost as much as isotopic separation, but that is a separate concern; we don't have to separate isotopes until we need to harvest isotopes for purposes that will pay for the process.
    2: That process itself produces waste, generally chemically dangerous and generally unacceptably radioactive as well, and not easy to vitrify. More cost!
    3: Lots of very smart people have been busting guts (or cerebral vascular systems) to produce ceramic matrices that can be relied on not to dissolve or leak under various underground conditions, and considerable credibility gaps have grown concerning some at first promising-looking materials. Cracks, crystallisation, solution, you name it. So we have a nice new one that won't leak? This year? This century? This millennium? So much nicer than storage in re-usable form against the time that we could have reprocessed the cooled, unmodified fuel assemblies after the demand had grown and the techniques improved? Do tell...
    4: Storage of minute volumes of UN-reprocessed high-activity waste, literally cubic metres rather than thousands of cubic metres, in a sealed chamber, accessible for extraction on demand would be a fraction of the cost of such treatment, and it might be expected to be undertaken within decades rather than centuries, after a few cycles of which it would have paid its way and and also would be much safer than the vitrified time bombs that could be expected to remain useless and unacceptably active after many millennia.
    In sum, I regard the cost of a rational storage and process-on-demand as far more economical, and probably far safer than vitrification.
     
  2. jcsd
  3. Feb 23, 2015 #2
    I do not understand your point. You seem to imply that with actinide removal, some steps of current process can be removed. That is not true.

    IIUC current "state of the art" reprocessing goes as follows:

    - fuel pellets are dissolved (say, in nitric acid)
    - uranium and Pu are extracted using some organic solvents tailored specifically for these elements. This removes a large fraction of the solution, since spent fuel still has ~95% uranium.
    - the remaining solution is calcined (dried into powder)
    - the powder is melted with glass (something like 30-50% of power, the rest is glass) and poured into stainless steel vessels

    If you want to remove actinides, you can't avoid any of the above steps. You also likely can't just modify a step. You need to *add* another step (probably it will have to be inserted after U/Pu removal). That inevitably increases cost.

    In case you are arguing for not doing reprocessing at all, yes, it is an option which seem to be followed by US right now. If it means just storing fuel, it is okay. It even has economic benefits, since less radioactive fuel is probably somewhat cheaper to reprocess.

    However, it feels like just postponing a decision to either reprocess or bury it as-is. Eventually you will have to choose what to do.
     
  4. Feb 25, 2015 #3
    Firstly nikkkom, removal of actinides immediately changes the game. It counts as reprocessing, which is anathema to the anti-nuke brigade. What are you going to DO with those actinides (actinoids, whatever)? Build bombs right? No? A likely story! But if not then what? Fuel MORE power stations? HAH! Just as we thought, breeding more deadly fuels that will poison us all. None of your nonsense, we want all that deadly stuff vitrified and stored safely for at least a few million years, and NIMBY at that!

    Well, then we would need to vitrify the actinoids as well, no? A very different kettle of fission.

    My proposal is:
    * Usual procedure after removal from the core until things cool off till heat is no longer a significant factor. Use whatever you find it profitable to use (meaning pretty well nothing at present AFAIK.)
    * Store the cool material sanitarily under conditions such as we have agreed on as being hard to hijack, and with careful cataloging of history and status. This storage to be continued ad Kalendas Graecas or otherwise convenient occasion. There is no point to processing them before such time, and the longer they are left, the smaller the content of troublesome volatile components such as iodine or krypton. (Unless of course, you WANT iodine or krypton!)
    * When the occasion does arise, select the most suitable items for the current requirement and process them more or less as you have described. Very likely the desired materials will be actinoids, but that doesn't much matter. The consumers will determine what they want and when.
    * Having disposed profitably of the desired products such as Am-free Pu, you extract as much as practical of the non-radioactive residue for safe disposal, and return the rest to clean, dry, sealed, (re)cataloged storage.
    * Should it be determined that certain residues for some reason never could be regarded as worth retaining, even as thermal fuel for disposable space modules, and that it remains sufficiently dangerous to be worth genuinely destroying, expose them to a suitable neutron flux to render them inactive or so radioactive that they soon will decay to inactivity.

    Such procedures wouldn't waste anything much nor risk anything much, nor cost much over and above the storage and processing costs that clients would be happy to pay anyway.

    No?
     
  5. Feb 26, 2015 #4
    I don't think you can economically destroy fission products this way. While neutrons would transmute, say, Cs-137 to Cs-138 which quickly decays to stable Ba-138, it will also transmute stable Cs-133 to radioactive Cs-134.
     
  6. Feb 26, 2015 #5
    Your remark might be correct, but I suspect it is over pessimistic.

    * The approach might prove unnecessary if uses were found for enough isotopes
    * Burning also might prove unnecessary because the final volume of problem isotopes would be too small to make permanent storage unattractive.
    * If we did opt for isotope burning, one would have to consider which isotopes are of practical interest. Look at Si-32. Nasty, very nasty! Half-life of 700Y B-emission; intolerable! Active enough to be dangerous, but too stable to make it acceptable to wait for it to decay. And it might be difficult to burn anyway with such a neutron excess. And yet, I don't know how much we produce or whether we would want use it for anything, but it is easy to immobilise chemically if we don't want it. It is not as mobile as say I-131, or as readily absorbed or retained, so it is not under realistic circumstances a major concern. Each element and each isotope can be considered in its own contexts of its halflife, its biological effects, its mobility in the environment, its projected inventories, and its burnability.
    *Also consider your examples of Cs isotopes. We need not take them too seriously because the example is academic, but I think it is illustrative. One might not bother to "burn" them at all. Cs-137 has a half-life about 30 years; only about .1% would be active after 300 years and it is not a very powerful source anyway. One MIGHT simply bury stocks where they would be safe for 1000 years. (About 33 half lives, right? About 1 eight-millionth residue?) Cs-134 shouldn't be too much of a problem even if you did get a lot of it (not all that likely, yes?) because its half life would reduce 99.9% of the inventory to stable Ba in less than 30 years.
    * In fact that example should remind one that the point of burning troublesome isotopes works two ways: produce less-active or inactive isotopes, or produce MORE active isotopes that will decay to stability faster, as you instanced in the Cs-138 example, even if there were troublesomely large amounts of Cs-133 in the input.
    * Note too the tendency for decay by neutron capture; the trend is to produce isotopes that are more neutron-rich and less neutron-greedy. One naturally would need to tune treatments for best results, but that trend is helpful.
    * A lot of the treatment could be simply by storage of the concentrated wastes in specially neutron-rich regions on the periphery of active fission plants. Note that "burning" non-concentrated wastes would make no sense anyway. We probably would be interested in burning only trace quantities if any at all.

    I am not arguing that isotope burning would inevitably be necessary; in fact it goes against the grain for my part, but if it were to prove necessary, I suspect it would be practical.
     
  7. Feb 26, 2015 #6

    mheslep

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    If the total fission product waste stream mass from a reactor is 3%, and the radiotoxcity decays below that uranium ore in ~300-400 years, it seams to mean burial is a reasonable option. It seems to me therefore that the focus of waste discussions should be on a solution for the actinides, on fast spectrum reactors or thorium fuel cycles that leave no actinides, while at the same time greatly expand the fuel supply.
     
  8. Feb 26, 2015 #7
    Now, THAT sounds like something else. I like it. My only reservation is that I distrust burial. I prefer storage. The storage COULD take the form of burial I guess, but I would like to be sure that we know what is where at all times.
     
  9. Feb 27, 2015 #8
    It's possible to separate waste into individual elements, but this costs a lot.
    Theoretically it's possible to separate isotopes too, but this costs astronomical sums if done on many kilogram scales.

    IOW: both are non-economical.
     
  10. Feb 27, 2015 #9
    There are cases when it's not economical. The final product of vitrification is completely useless. Storing it will just make us spend money on it for millennia. Why not bury it in boreholes in a subduction zone? What's wrong with that?
     
  11. Feb 27, 2015 #10
    You will notice that I avoided discussion of isotope separation because I assumed that it would be too expensive.

    In contrast however, separation of elements is not generally dramatically expensive, though of course it is not something one does just for laughs, particularly when radioactivity is a factor. (OH, and BTW, when I speak of "elements" I am not speaking of pure elements, but convenient compounds for handling storage or disposal, such as oxides, fluorides, carbonates, magnesium salts etc).
    As for its being uneconomic, it might be if the alternative is to store it a few centuries, but if you get the material in a residue from which you want particular elements or isotopes, or if you want to reduce bulk to be stored, it could very cheerfully be economic. I reckon that a blanket dismissal is inappropriate.
     
  12. Feb 27, 2015 #11
    I agree about vitrification products generally being useless and the same for ceramic compaction, and see no point to either. Burial in subduction could work just as well in corrosion resistant tough metal capsules, because once it is on the down elevator, who cares about a few leaks during the next few million years, by which time who cares anyway; that is a LONG way down the half life ladder.

    But what gives you the idea that burial in a subduction zone is simple, safe, and cheap, compared to storage in a near-surface, secure underground chamber? How deep do you want to go, and how much do you want to bury? I trust you don't think that a few hundred feet into the bedrock would be sufficient?
     
  13. Feb 27, 2015 #12
    Wrong. When radioactivity of the source material is in MILLIONS of rems per hour, as is the case here, there is a BIG difference. Reprocessing as currently done is essentially removal of two elements: U and Pu. It is a fact that it is expensive.
     
  14. Feb 27, 2015 #13
    Wrong. Metal capsules are neither sufficiently tough nor sufficiently corrosion resistant over millennia in hot, high pressure environments.

    You need *waste itself* to be in a chemically inert, insoluble form, so that hot water won't leach fission products out of it.

    Do you really think people who designed reprocessing are idiots?

    3-4 kilometers would do. Oil and gas exploration industry routinely drills much deeper than this.

    Gibbpic2.gif

    As to difficulty of burial, here's the end product of La Hague vitrification plant: a cylindrical canister of ~40 cm diameter:

    Example-of-a-universal-canister1.jpg

    I don't see why lowering these down a borehole would be a problem.
     
  15. Feb 27, 2015 #14
    In the context of "it is uneconomic" as someone asserted, there is a difference between "expensive" and "dramatically expensive". The expression that I used was the latter, right?

    It also depends on what is being done, and what for.

    To do ANYTHING at that level of activity is expensive, which is one good reason for not reprocessing fuel until you really need to (by which time in dealing with commercial enterprises, you should be years down the line). It does not follow that it would be unprofitable, which is a totally different matter. Right?

    But even if you are dealing with barely-cool fuel to extract the U and Pu, which obviously was worth the expense, or you wouldn't be doing it in the first place, then continuing to extract residual materials apart from those two elements is not a monumental overhead if the other materials in question are something you badly want. Much as it was worth extracting the U and Pu originally. Opportunism they call it.

    Note that vitrification of those items would have been expensive too, and you would have got nothing out of it; not now, not later. This way you would have got, say Tc and Pm isotopes, saving the costs of mining them, which would have been REALLY dramatically expensive.

    Right?
     
  16. Feb 27, 2015 #15

    mheslep

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    Two different waste streams are in play in this discussion: i) the tradition nuclear spent fuel with actinides and mostly U238 by mass, and ii) fission products only, i.e. the waste from a fast or thorium fueled reactor, or separated out after reprocessing from traditional spent fuel. Geologic disposal is arguable for the first, but not the latter.
     
  17. Feb 27, 2015 #16
    Firstly nikkkom, how did you arrive at that no doubt well-founded conclusion?
    How hot?
    Since when does say a few hundred K (which is a heck of a lot more than anyone would be drilling in) imply that mild steel couldn't stand it? What would the content of water and acid be in that environment?
    What pressures are you discussing that we should be keeping out? We are not trying to protect fragile items from collapse, but essentially solid material that would barely deform in the absence of shear forces after it had finished compacting, even if it were made of talc. And what source of shear forces would a say, 150cm-long, 50cm-thick capsule (edited to come a bit closer to the canisters that you posted) encounter that would affect it at all below about 1700K? Plastic rock doesn't swirl like rapids and at those temperatures all the common rocks are pretty well molten. Our dime-store mild-steel pipe would represent a nice rigid rock in the current.
    You surely didn't think that the drill would go down to to magma did you, even if the capsules landed up in magma an aeon later in its process of subduction? And at that time who would care? Even if they were down the hole along with the capsule?
    And if the course of subduction did bring the capsule into rock at say 1500K (which is already pretty hot for magma, though not as hot as some), and the capsule did start to deform, then once more, who would care; it would already have travelled several km downwards, which would take millennia or a good deal longer, and though it would deform, the iron still would not have melted.
    I think a bit of calculation plus some rational specification of the plausible conditions would be in order before asking me about whether the hypothetical engineers are fools or not. Have you actually discussed any of these points with them before attributing opinions?
    Just asking...
     
    Last edited: Feb 27, 2015
  18. Feb 27, 2015 #17
    Sounds good to me. I for one wasn't discriminating between those two, because my argument is independent of that point. I was expressing my suspicion of vitrification and subduction (or similar permanent disposal of high-intensity waste) in general.

    Conversely (and, I admit, irrelevantly) I remark that the anti-nuke brigade would not be happy with less than subducting the whole caboodle, including the concrete of the buildings, and I bet I could find some parties who would be happy to fund an injunction against nuclear pollution of subducting magma as well! (Is anyone betting?) :)
     
  19. Feb 27, 2015 #18
    No you don't. All you need is a container that won't let water in at shallow levels so cool (below about 400K) that liquid water could exist around it. By the time that becomes relevant your capsules will be another km or two deeper. I live near where living humans work voluntarily in mines deeper than 3 km with no more than some air conditioning, and not a vein of boiling water in sight.
    Not that *I* would volunteer there, you understand, but at least hundreds of thousands do, year in and year out since before I was born.
    By the time you get deep enough to compromise the canister (say another few km) there would not be much liquid water anyway and we would be out of sight of anything resembling ground water, but if you were nervous of your canister dissolving, you could line it or lacquer it with cheap high temperature nylon or polyimide.
    And why chemically inert? If it comes to that, HOW chemically inert? Fuming nitric acid or explosives would be poor prospects for such treatment, but when was the last time anyone suggested disposing of such materials in such a manner?
    But what about say, reactive materials such as metals and common salts such as sulphates or carbonates? How do you see them leaching out? If they did, how far from the parent capsule would you expect them to leach in densely compacted rock? A metre? How?
    And insoluble? In what? Molten rock?Even your vitrification material would dissolve in really deep magma, right?
    I don't think you or your sources have worked this out very carefully. Sorry!
     
  20. Feb 27, 2015 #19
    Sorry, you lost me there. Who suggested that people who designed reprocessing are idiots?
    I do accept that deep burial of vitrified high activity waste is an idiotic idea, but that is a totally different matter.

    That is a comforting assurance; where did you get the figure, and who ratified it? I assume that you have seen diagrams of what happens at the interface between the subducting plate and the overlying plate? Perhaps there are regions where such depths would suffice, but what sort of brinkmanship would be acceptable in playing chicken to see whether the canisters get truly subducted, or scraped off and exposed near the surface?

    However, the point is trivial in comparison with the question of what it costs to drill so deep into hard rock. Is that your idea of a solution to the disposal problem, to rival the cost of near-surface storage? Even for a few centuries?
    Suppose our store had a few thousand square metres of storage space; how much drilling (3-4 km deep forsooth!) would it take to dispose of the potential capacity of a store on such a scale?
    "Out of sight is out of mind" is a tempting principle, but if it turned out that something had gone horribly wrong, such as say, the process of subduction not behaving as expected, where would you rather have the waste? Up here, or no longer quite so "down there"?

    I find that statement confusing. I thought that I had explained that the main problem is the cost of drilling, plus loss of control thereafter; a drilling engineer would no doubt be able to explain a few in addition. However, I DO hope that you are not under any illusion that it is just a matter of drilling and dropping! Just the delivery to the bottom of the hole, plus sealing them in place would be a fairly fraught procedure, compared to storage as I have described it.
    What exactly had YOU thought?
     
  21. Feb 27, 2015 #20
    The acidity of the underground water in the future can't be easily predicted. You should assume the worst (that it, that it will be acidic). Hot acidic water dissolves steel on a timescale of a decade. Look at photos of any abandoned mine.
     
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