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Why is artificial transmutation not started?

  1. Jun 1, 2008 #1
    Why is artificial transmutation not started? Why is the Integral Fast Reactor not considered?
    I mean, its good isn't it, not producing transuranic waste? What about creating some reactor to consume all the transuranivc wastes or elements?And this is just to know, are fusion reactors being put to use?I mean, the half-life of the radioisotopes produced by fusion are lesser than those produced by fusion.
     
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  3. Jun 1, 2008 #2

    vanesch

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    The IFR, as are several other comparable projects of fast reactors like Superphenix, have hit a triple wall:
    - it is the ideal target for anti-nuclear activists, because such progress would undo most of their critique of nuclear power
    - it is for the moment technically more involved, and as long as there is cheap fresh uranium, there's no economic incentive to switch to fast reactors
    - the fuel reprocessing is by some considered as a proliferation issue.

    IMO, the transuranics are a fake problem, as most of the studies of deep geological storage show that if *something* gets out in thousands to millions of years, it is not going to be the transuranics, but rather some small amounts of long-lived fission products, such as I-129, or Sn-126, and which are for all practical purposes innocent at that time, because of the low doses.
    The transuranics have chemical properties which makes their migration in most soils almost impossible (they get very quickly sorbed by many materials).

    However, it is correct that a fast reactor will have "cleaner" waste than current thermal reactors, which produce more transuranics.

    There's a whole new (public relations ?) business now on ADC (accelerator-driven systems) which will be able to create a flux of fast neutrons (just as a fast reactor is...) to burn transuranics. In how much this is going to be a practical solution on a large scale is a different matter, and I always question their actual utility...

    Fusion is not possible yet. And it will, in the most optimistic case, still take decades before a technological demonstration can be given of its feasibility - this leaving aside whether commercially it makes any sense. But apart from that, the waste problem of fusion is indeed much smaller than for fission, given that there isn't any direct radioactive waste, but only indirect (activated structural materials).
     
  4. Jun 2, 2008 #3
    By "cleaner", do you mean that the amount of wastes produced will be the same but the level of harm it will cuse is lesser? Why not fast breeder reactors?It is a fast reactor which produces more fissile material than it consumes, doesn't it?So the fissile materials can be used for the thermal reactors! That's a double advantage isn't it?

    Thanks!
     
  5. Jun 2, 2008 #4

    vanesch

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    The essential fission products are still there, but there will be less minor actinides, and less of the nasty plutonium isotopes like Pu-240 or Pu-241.

    Now, the fission products are of course what makes the radioactive waste of a nuclear power plant so hot initially, so the *essential* waste is still there, and just as dangerous. But there are less transuranics present. Now, as I said, transuranics are not really a problem for geological storage. But without them, we can start thinking of a purely human construction dealing with the waste, without the geology. For the fission products, we only need a confinement for a few hundred years, and that's feasible with human constructions.
    So it is more on the "how to convince the others that geological storage is fine" side that this is an advantage, in that you can now say that *no matter where you put it*, you can engineer a structure that will confine it for, say, 600 years, and you can do away with all the sophisticated geology - even though that geology, where it has been studied, doesn't indicate serious problems for the transuranics in any case.

    Well, to me a fast reactor is in any case a potential breeder, and it is a design choice to have the ratio of Pu produced over Pu consumed slightly larger, equal, or slightly smaller than 1. There are several problems with using breeders to feed thermal reactors. The first one is that you produce pure Pu-239, and if you are going to separate that for a thermal reactor, you have a proliferation issue at that point. So it is best to keep the produced Pu-239 inside the fuel elements as long as possible, and try to consume it "on the spot", meaning fuel elements can stay WAAAY LONG in the reactor. For a breeding ratio = 1, this would mean that you could in principle use the fuel element "entirely", but of course damage to the structure, as well as accumulation of fission products will make you need to reprocess it a few times.
    Also, I don't know how high you can make the breeding ratio. I know that 1.2 has been demonstrated with a Russian reactor, and Superphenix also did so. This means that if you have a 1 GW breeder, you can only feed one single 200 MW LWR with it. That would be a waste: better keep that fuel to set up another breeder after some years...

    Have a look here: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fasbre.html
     
  6. Jun 2, 2008 #5
    So ultimately, deep geological storage is the only decent choice? Artificial transmutation is out of the question? But fast reactors sounds appealing, every thing has its own dis-advantages, but as you say instead of making a reactor which produces less energy than the thermal ones and its main purpose is to produce fissile material to run thermal reactors, what's the point? It is just as good as what's happening now. I thought of it as a choice because of the cleaner waste thing.But now that ther's no problems with deep geological storage...But isn't artificial transmutation like bombarding the wastes with neutrons and stuff.What about that choice?Instead of waiting for centuries, why not do it in hours.Why not take all the radioactivity of the actinides and stuff and finish it off right away?Nice link!
     
    Last edited: Jun 2, 2008
  7. Jun 2, 2008 #6

    vanesch

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    The MAIN source of radioactivity in the waste, for the first few hundred years, is a large array of fission products. They are not transformable. Well, you could pick out a few of them, and apply a specific transformation technique, but for the bulk of it, there's no transformation going to be possible: there are hundreds of them! Just putting things undifferentiated into a neutron flux will maybe improve some, and activate others. So the main source of radioativity is in ANY CASE there. Transmutation only applies to a few long-living nuclides that we might want to get rid of, and after a lot of separation work, get them concentrated and do something about it.

    The obvious target are the minor actinides, such as Am-241, which are still pretty active, and have relatively long half life times. There are also a few trace fission products who live long (and hence are only a little bit active), like I-129 and Sn-126. Point is, they are only a little bit radioactive. For instance, the La Hague reprocessing plant in France just dissolves the extracted I-129 directly in the seawater, because in that way, it is immediately isotopically diluted and the dose one can obtain from it is hundreds of times smaller than the natural background dose.

    The whole "problem" with the actinides is that they only decay significantly after 10 000 years, a time span which is too large for an engineer to build something that he can guarantee will survive that time span. On the other hand, we know that actinides migrate very slowly through most geological layers, and in any case, their activity 10 000 years from now is about 10000 times less than the activity of the waste NOW. But there remains the (psychological ?) problem of having it confined by geology, and not by human engineering.

    The long-lived radioactive products are only a few, and THEY can be selectively transmuted. Transuranics undergo fission in a fast spectrum (and hence are reduced to fission products), and there are also a few solutions for these few small long-living fission products like I-129 and Sn-126. The only question is: does it solve a genuine problem ?
    THIS is what transmutation is about.

    But, again, there's no hope to get rid of the (high) radioactivity of the bulk of the fission products during the first few hundred years. That remains the essential waste. BUT, one can engineer canisters which will withstand time for a few centuries.

    No no, you have it wrong there. A fast reactor is just as well energy producing as a LWR or a BWR, while it is breeding. But the main advantage of a fast reactor is that it can use U-238 as fuel (well, first breed it into plutonium, and then consume it), while a LWR (or any thermal reactor) can do this only marginally. This means that a fast reactor can use mined uranium about 100 times more efficiently than a LWR. THAT is the main advantage of a fast reactor.

    A LWR reactor uses only the U-235 component of uranium (0.7%), and "breeds" only about 60%, of which it consumes 30% on the spot, meaning, overall, about 1% of the actual natural uranium is used as a fuel. A fast reactor can use most of it, once it has some plutonium to get started with.

    In fact, all the current "waste fuel elements" still contain 95% of their energy, and all the depleted uranium (5 to 10 times more, used to make the slightly enriched fuel of LWR in the first place) is still 100% "energetic". It's a big waste to throw all that away. We could still extract in principle about 100 times more energy from the uranium than we did up to now. If we've been running for 30 years, that means that, JUST USING THE "WASTE", we can still run for about 3000 years at the same power level if we were to use everything in breeder reactors (producing electricity).

    As I said, the bulk of the short term activity, the fission products, cannot be transmuted. We're only talking about the relatively low activity on long times, caused by just a few components.

    EDIT: a fast reactor has two "waste advantages" over a LWR. The first is that it produces way less minor actinides than does a LWR. As such, one of the longest-living (but by far not the most active) components in the waste are seriously diminished. But the main advantage of a fast reactor is that it can burn plutonium efficiently. LWR can only burn Pu-239 and Pu-241, and produce Pu-240 and Pu-242 in the process. That means that only a certain fraction of the plutonium could eventually be used up in a thermal reactor, and we would still end up with a lot of Pu-240 and Pu-242 as waste, together with a lot of Americium and curium (and if one pushes, even californium). And that's far worse waste than the original minor actinides, because now the decay periods run into the 100 000 years, and moreover the activity and the quantities are larger than in the case of the minor actinides. On the other hand, a fast reactor can burn about any plutonium, so there's no build-up of "bad plutonium".
     
    Last edited: Jun 2, 2008
  8. Jun 3, 2008 #7

    Astronuc

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    Unfortunately, DU has found military use in armour (Abrams tank) and armour piericing shells. A lot of it has been used in Kuwait, Iraq and Afghanistan.

    Apparently DU is also used in counter-weights in large commercial aircraft.
     
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