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Science Technology

Disposing Of Nuclear Waste As Nuclear Fuel 76

Posted by timothy from the nice-circular-ring-to-it dept.
Saige writes "Nuclear waste has been a contentious issue, recently culminating with fights in the government over Yucca Mountain in Nevada as a proposed storage site. Well, perhaps there's a better way to deal with nuclear waste -by using it in nuclear reactors. A nuclear scientist at the University of Maryland, has come up with CAESAR, a reactor that runs not on the standard U-235, but on U-238. U-238 makes up most of the fuel rods in current reactors, but doesn't contribute to the reaction, and ends up currently as waste." The Yahoo! story linked from this article doesn't seem to open, but here's a story at The Economist.
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Disposing Of Nuclear Waste As Nuclear Fuel More

Disposing Of Nuclear Waste As Nuclear Fuel

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  • So. What exactly does the U-238 become after all this? Also, how could the steam be used if these things are reacting, thus producing heat? Wouldn't the steam be affected by the extreme heat?

    I'm not trying to sound ignorant, I just know nothing of nuclear physics.
    • It sounds like the U-238 remains: So material now treated as waste could be used as fuel. The problem of disposing of spent fuel will remain, but CAESAR is, in effect, a form of waste storage that produces electricity.
      • No, it will be converted to much smaller atoms, so it stores the waste, extracts energy from it, and changes it's form. When CEASER is done with the rods, they will still need to be disposed of carefully, but the energy derived per unit of waste goes up greatly.

        So... there is less waste for the same amount of electricity, this is the advantage...
      • that doesn't make sense. The uranium has to split to generate energy, how can it be more uranium?

        • Re:So what does the waste turn into? (Score:4, Informative)

          by Gruturo ( 141223 ) on Monday February 03, 2003 @09:20PM (#5219801)
          that doesn't make sense. The uranium has to split to generate energy, how can it be more uranium?

          The point is not "more uranium". It is about putting U-238 (which is normally considered useless) to a productive use, instead of its more appreciated U-235 brother, which is much less common (0.7%). This way, instead of having to be disposed of as waste, it is used (and at the end of the process you have less waste than you started with).

          In nuclear processes, Uranium does not produce more uranium :-), it either gets split into krypton and barium, or absorbs a neutron and through some intermediate decays becomes plutonium.

          (apart from the other, naturally-occurring radioactive behaviour of uranium: it emits an alpha particle, becoming Thorium, which then becomes Radium, which becomes Polonium, which finally becomes Lead (not radioactive - it doesn't become anything else)).

        • We just went over this in Physics, so I will give it a shot..

          you dont start out with Pure U-235, only about 4%,
          (they get the percentage this high be making a Uranium gas, puting it in a large column, and skimming the lighter U-235 off the top)

          So, they still have lots of other stuff, including other types of Uranium.
          (only 9.7% of natural Uranium is U-235)

          only the U-235 is broken down by the neutrons that the current moderator, water, creating other radioactive elements such as Ba-141 and Kr-92 plus 3 more neutrons, or Xe-140 and Sr-94 plus 2 neutrons.

          There is still alot of Uranium in the fission fuel, just not the right kind

          (the "235" in U-235 represents its atomic weight, all Uranium has 92 protons in its nucleus)

          (you subtract 92 from 235 to find the number of Neutrons in a U-235 atom)

          (in order to get Plutonium, you start with U-238, add a neutron going the right speed, then you get U-239*[*=excited], which then beta decays, making a beta particle, a Np-239* and a neutrino. The Np-239* then beta decays and you get Pu-239, a beta particle and a neutrino. This is called "breeding" and is how nuclear bomb fuel is created)
    • The steam would be used to extract the energy as well. You would control the whole thing, by how you let water in and steam out, then you can control the density of the moderator, and through that the rate of the reaction, and the energy release.

      The energy release would modify the steam, but only it's temperature or, if it is at the right temp... it's water fraction, but this would probably not effect the moderation greatly since the mass of the steam is unaffected by temperature...

    • That was three questions (Score:5, Informative)

      by Spamalamadingdong ( 323207 ) on Monday February 03, 2003 @09:01PM (#5219691) Homepage Journal
      So what does the waste turn into?
      Unfortunately, the article is so deficient in technical details that it's impossible to answer that question without quite a bit more information. As just one example of how ill-written the article is, there is no explanation of how the reactor is supposed to accomodate the accumulation of neutron-absorbing fission products over its multiple-decade period of operation.
      What exactly does the U-238 become after all this?
      It becomes fission products. Some of the nucleons (protons and neutrons) become free neutrons which are not absorbed before they beta-decay to protons (hydrogen nuclei).
      Wouldn't the steam be affected by the extreme heat?
      That depends how extreme the heat is. You will also have some radiolytic decomposition of the steam, and everything else in the reactor. The displacement of atoms within metallic crystals causes "radiation embrittlement", which will put a limit on the run-time of such a reactor even if the fuel is effectively infinite.

      Fission products are lighter nuclei which result from the fission of heavier ones. Some fission products are themselves radioactive, some are not. Pretty much all of them are useless as nuclear fuel.

      Radiolysis is the radiation-induced breakdown of chemical compounds. A gamma ray or a fast neutron has more than enough energy to smash a water molecule apart, and this process will produce free radicals such as hydrogen and hydroxyl ions. If those radicals get together, you can get products such as hydrogen gas and hydrogen peroxide, and hydrogen peroxide decomposes pretty quickly to oxygen and water again.

      You'd be better off reading an intro on the web, but I hope this whets your appetite for more learning.

      • Why not create a reactor that reacts Uranium hexaflouride. There could be no embrittlement since UF6 is a gas. Since it is a gas, the pressure could be used to moderate the neutrons right? Or mix it with steam if you need to.

        In a reactor, the by-products would be ( probably automatically ) chemically seperated from the gas maybe as a solid ( idunno what substituting a barium/krypton/thorium/whatever in a UF6 molecule would yield: hopefully not fluorine gas but prolly Krypton+Fluorine is not a molecule. Maybe Krypton will decay into something more reactive fast enough to take up the excess flourine or maybe one of the other byproducts will be able to do it.)

        In a rocket, why couldn't you just shoot the ( superhot ) crap out the ass end?

  • Darn links... (Score:3, Informative)

    by Saige ( 53303 ) <evil.angelaNO@SPAMgmail.com> on Monday February 03, 2003 @05:49PM (#5218255) Journal
    Damnit! I accidentally left the wrong link in the original paragraph. I meant to include the Economist link, but I was submitting this to Plastic.com and Slashdot both, and managed to cut and paste the wrong part from Plastic here... *sigh*

    If anyone cares, the link to Plastic wasn't what was intended and can be safely ignored.
    • I was wondering at the apparently non-sensical link. If that's not proof the editors at /. don't even bother to read the articles, I don't know what is.

  • Cool, but isn't the real problem... (Score:5, Insightful)

    by bscott ( 460706 ) on Monday February 03, 2003 @05:50PM (#5218259)
    (caveat - I haven't had time to read the article yet, so I'm spouting off without much backing to my opinions, except that I live near Rocky Flats...)

    Isn't the real "nuclear waste problem" not just the fuel rods, but the kilotons of contaminated building materials, protective clothing, screwdrivers, air ducts, semi-trailer trucks, topsoil, reactor coolant, baseball caps, human remains...

    I'm sure this is a great advance for many reasons, but it's barely gonna scratch the surface of how to deal with contaminated material - or am I wrong?

    • Re:Cool, but isn't the real problem... (Score:5, Informative)

      by MacAndrew ( 463832 ) on Monday February 03, 2003 @06:20PM (#5218495) Homepage
      IIRC, that stuff called LLRW (low-level radioactive waste) is not that big a deal, in either strength or half-life radioactivity. It is a lot of volume, and can be toxic, but the really nasty long-lived stuff that must be sequestrered carefully is mostly products of the pile itself, like radioactive plutonium and strontium and so on.

      It's fairly hard to make something radioative by exposure. The LLWR is largely stuff that has come into contact with radioactive material, as in processing, hence comtamination.

      The biggest problem with LLRW is political -- people don't want it in their back yard. And I don't blame them -- given a choice of your yard or mine, I'd pick yours. :) But the health hazard is exaggerated.
    • No, the real problem is the hard stuff -- the witches-brew of unstable radioisotopes.

      Check out http://www-formal.stanford.edu/jmc/progress/anti-n uke.html

      The interesting part is that we aren't talking about the half-life of Uranium. We are mostly talking about the half-life of what happens when Uranium splits. Which does NOT have a particularly long half-life.

      So, really, what you are looking at is 500 years before it is less radioactive than the ore it came from, and a few thousand years before it is truly safe. Not the millions and millions of years that people always talk about.

      Of course, a few thousand years is still quite some time.

  • Weapons (Score:5, Interesting)

    by MacAndrew ( 463832 ) on Monday February 03, 2003 @05:54PM (#5218299) Homepage
    I think the really interesting thing here is that the reactor cannot be used as a breeder. That would make it an excellent candidate to require nations like North Korea to switch to. (If you think that's the way to go. NK is VERY energy poor now that the Russians no longer send oil. 60 Minutes did an excellent piece on them last night.)

    All reactors IIRC produce some plutonium, from bombarded U-238 (virtually all Pu is manmade). Breeders produce a lot. The "waste" which is U-235 depleted but plutonium enriched must be further processed to produce weapons-grade material. For 25 years we have banned reprocessing even to the level needed for use as fuel because of the concern is could be stolen and further enriched. Some countries like France and Japan disagree and do reprocess. The scare in Japan last week illustrates the risk. Most people here would agree there' no such thing as perfect security, esp. with the universal hazards of corruption, accident, and incompetence.

    Even if the thieves were unable to purify the material, it would make excellent "dirty bomb" material. Pu is not especially radioactive, absent heavy chain reaction, but it is very toxic and dangerous to ingest or inhale where it might lodge and expose sensitive tissue to prolonged damage.

    It's a shame nuclear policy is so constrained by weapons issues.
    • Perhpas it would be an excellent candidate to require ALL nations to switch to. But I guess some are more equal than others.

      BTW, 60 Minutes is populist shit. I hope they someday do a piece on something you really know, like your hometown, or something, then you'll see. They will distort the truth just to make whatever they are talking about sound interresting. I saw a piece on Finland where they (for real) explained that Finns don't do anything but dance tango. No exceptions, just tango, in all of their spare time, because Finns can't speak to eachother due to pathological shyness. So bollocks to their NK-piece.

      What crap.
      • That softball Finland piece -- which I saw -- is hardly ammo enough to take out every program they do. The quality of a piece depends a lot on who produced it.

        The striking and non-interpretive thing did was simply a space photo of the Korean peninsula at night -- all dark above the border and bright cities to the south. Relevant here was the fact that without Russian aid they are quite desperate, something that has been reported widely and should be accounted for in the eventual negotiated settlement we will need unless we really want to make them a colony. N. Korea I *do* know something about and I didn't detect anything horribly wrong with their report.

        Finland -- I agree, that was pretty silly! I think it was intended to be "fluff." I tend to assume all media reports could be totally off the mark, but think 60 Minutes does OK for TV journalism.

    • Re:Weapons (Score:5, Interesting)

      by DuckDuckBOOM! ( 535473 ) on Monday February 03, 2003 @06:38PM (#5218636)
      I think the really interesting thing here is that the reactor cannot be used as a breeder.
      ...huh? Quote from The Economist article:
      By adjusting the configuration of the core in the right way--by judicious positioning of graphite, for example--almost any civilian reactor can be made to produce plutonium, and thus to make weapons. Access to the core is not necessary with CAESAR, as it could run for decades without any need for refuelling. Thus it could be sealed. Countries could then adopt the design to show that their nuclear intentions were entirely peaceful.
      The way I read this, the difference with CAESAR is that the fuel doesn't require periodic replacement, so the reactor could be sealed to prevent, e.g., terrorists from making off with plutonium-laden spent fuel. The key word here is "could". Nothing appears to prevent a gov't from designing a CAESAR to allow harvest of plutonium, or from covertly retrofitting a sealed reactor. (Granted, it almost certainly wouldn't produce as much Pu as quickly as would a fast breeder, but those are purpose-built devices that the nuclear powers would discover & stop in a heartbeat. And you don't need all that much Pu to build a bomb.) The sealed-fuel concept would work well if the government controlling it were trustworthy, or if verification methods were foolproof. Unfortunately, in the case of an Iraq or a N. Korea, those are mighty big IFs.
      • OK: cannot READILY be used as a breeder :)

        For purposes of monitoring -- and unless we are going to ban all distrusted powers from using nuclear energu, monitoring will be required -- should be easier here. Knowing nothing about reactor design, this one included, I don't know was kind of tamper-proofing could be used. I'm picturing a great big seal.... Anyway, I suspect it would be a lot easier than what we've been doing with cameras and stuff, and less invasive, too.

        • Re:Weapons (Score:3, Insightful)

          by DuckDuckBOOM! ( 535473 )
          I'm picturing a great big seal....
          A REALLY big seal, with sharp teeth and attitude...kinda like an elephant seal after exposure to some of those spent fuel rods... :)

          I agree that it would probably be easier to detect tampering with a sealed reactor, if only because they'd need a fair amount of time & equipment to crack it, and you could build some of the detectors inside the sealed part where they'd be hard to defeat. The solution N Korea used - throw out the inspectors, shut off the cameras, ignore the griping until you've accomplished what you want to do - does, though, make me a tad nervous re the viability of any preventive measures that have to be carried out on foreign soil. Personally I'd have preferred we built the nukes in S. Korea, and ran a big power line across the DMZ.

          • Re:Weapons (Score:2, Insightful)

            by MacAndrew ( 463832 )
            I don'y usually respond to a response to a response, but...

            I like your idea of delivering power from SK. It's probably practical -- power in the U.S. is sold over very long distances. The problem is that NK would not accept it, not least because it would give us a big on/off switch to push them around with.
    • Just a nitpick. You wrote: "...is that the reactor cannot be used as a breeder". Well, with some modifications it could be used as a breeder. The benefit of this design is that it doesn't need to be refueled for long, long intervals and thus can be sealed and easily monitored.

      Also, you wrote parenthetically: "virtually all Pu is manmade". All plutonium is manmade. All the natural plutonium on the Earth decayed long ago.

      • On the first point, I made the mistake of semi-quoting the article. You're right, but I think the writer reasonably meant to indicate this one offered substantial advantages for monitors; perhaps use as a breeder becomes impractical. Someone else already complained about this.

        On the second, there is a natural reactor [pu.org] in Africa that years ago produced some plutonium. I included "virtually" to avoid nitpickers who know this, but forgot about the nitpickers who do not. :)

        The pu.org site has other interesting info about, guess what, Pu.
        • You're right. Thanks for correcting me.

          Although, I haven't yet found any good information on how much Pu is left at Aklo. But it's also the case that there's trace amounts of Pu to be found--some is generated from spontaneous fissioning of U-238 with subsequent reabsorption of a neutron.

    • Enough with the misconceptions already! (Score:5, Informative)

      by Spamalamadingdong ( 323207 ) on Monday February 03, 2003 @09:30PM (#5219862) Homepage Journal
      Breeders produce a lot.
      Well... no, not really. I'm told that near the end of a fuel cycle, a conventional pressurized water reactor (light water, not a CANDU) is producing the majority of its power output from plutonium fission. The breeder's claim to fame is that it can breed more fissionable fuel than it burns.
      The "waste" which is U-235 depleted but plutonium enriched must be further processed to produce weapons-grade material.
      Well... no, not a bit. Spent PWR fuel contains quite a bit of plutonium, but it is essentially useless for making bombs. A PWR cycle lasts a couple of years, more or less, and bombards the fuel like mad. U-238 absorbs neutrons and becomes U-239, which beta-decays to Np-239, which beta-decays to Pu-239. While some of the Pu-239 gets fissioned further down the line, some more of it captures a passing neutron and doesn't fission. It becomes Pu-240, or even Pu-241. These are isotopes with very different half-lives (much shorter) and much higher spontaneous fission rates.

      This is all-important for making a bomb. U-235 has a half-life of around 700 million years, and making a bomb with it is easy: squeeze together a prompt-supercritical mass, and wait a few milliseconds. Pu-239 is tricky, because its half-life is only about 25000 years and you have very little time to get it into a prompt-supercritical configuration before a spontaneous fission starts the reaction going. If the reaction starts too soon, the bomb blows itself apart into a sub-critical configuration before releasing much energy and all you have is a fizzle. Now imagine dealing with a substantial fraction of Pu-240 (half-life 6564 years [jaeri.go.jp] or Pu-241 (half-life 14 years [jaeri.go.jp]).

      Bomb-grade material is made in special reactors which allow the fuel to be irradiated relatively briefly at a low level, and then removed and processed to remove the plutonium. This is specifically to avoid the production of enough higher isotopes of plutonium to be a problem. The stuff coming out of a power reactor after a full fuel cycle is dirty as hell, but amateur proliferators are not going to be able to make a serious bomb (as opposed to dirty weapon) out of it. This is why we had few objections to building pressurized-water reactors for North Korea; they are essentially proliferation-proof.

      For 25 years we have banned reprocessing even to the level needed for use as fuel because of the concern is could be stolen and further enriched.
      I doubt that it's quite that simple. The real problem is that the plant required to refine fuel-grade Pu from spent power reactor fuel uses the exact same chemical processes as the plant which refines bomb-grade Pu from depleted uranium rods held briefly in a neutron flux for transmutation purposes. If you have a world full of people reprocessing it would be very hard to put a finger on the ones who are making weapons, so the US decided we had enough uranium to put the kibosh on all reprocessing just to set a good example.

      I think we should have gone with the Integral Fast Reactor [berkeley.edu], but it seems to have succumbed to the fundamentalist anti-nukes (who probably couldn't figure out that there are medical and explosive grades of nitroglycerine either...).

      • Re:But misconceptions fuel great arguments! (Score:4, Informative)

        by MacAndrew ( 463832 ) on Tuesday February 04, 2003 @12:20AM (#5220608) Homepage
        Thanks, that's interesting. Of course, it conflicts with a lot of what I've read, but a lot of what I've read conflicts, too. :)

        My understanding from many sources is that a breeder can produce material of 20-30% Pu. Yes, it "can breed more fissionable fuel than it burns" but that new fuel is (as I understand it) exactly the Pu-239 we fear. All Pu comes from reactors, anyway, it's just a question of technique, esp. removing the material after a brief bombardment by appropriate-speed neutrons.

        Bombarded Pu-rich reactor fuel is not the only problem, there's also the fuel-grade Pu after reprocessing. I've seen a couple of accounts of fuel-grade Pu bombs [ccnr.org] detonated, and I assume if one had the facilities to purify the fuel it would be even easier.

        There are serious technical hurdles to engineering the actual bomb, but here we want to deny them even the fuel. Plenty of countries have surmounted the techincal end, anyway, such as Pakistan [fas.org]. Even a sloppy detonation would be bad enough. BTW, I'm not thinking about terrorists, unless they somehow stole a complete weapon. They'd take the surer low-tech path of a dirty bomb or flying a plane into a building, etc. Terrorists with nukes are a Hollywood thing for now.

        On policy, here [nei.org] is a rather different account of why we don't reprocess -- economics. According to this account, Reagan vacated the Carter order in 1981. Truth?
        • I'd take the CCNR stuff with a grain of salt; they are an advocacy organization. Going through their main web page [ccnr.org], I find a lot of stuff which practically screams "NO NUKES!", such as opposition to any site selection for disposal of nuclear waste.
          • I don't represent that page as the gold standard, but do notice it is headed:

            ~ excerpted from the US Department of Energy Publication ~
            Nonproliferation and Arms Control Assessment
            of Weapons-Usable Fissile Material Storage
            and Excess Plutonium Disposition Alternatives

            It's an excerpt of a Lawrence Livermore publication, and believe me *they* ("the bomb factory") are not anti-nuclear. Anyway, what it says is in accord with what I've read elsewhere, and the critical bit of data is the feasibility of a fuel-grade Pu detonation, that is, the proliferation risk.
            • It's an excerpt of a Lawrence Livermore publication
              Don't ignore a couple of possibilities:
              1. The excerpt was chosen specifically to misrepresent the tenor of the whole report; it wouldn't be the first time selective quoting has been used to re-write the truth.
              2. The excerpt may be accurately quoted, but the report might be put out there as bait. Nobody but a proliferator is actually going to try this, and the more effort a proliferator wastes on something which can't work, the better.
              Just a note from a confirmed cynic. ;-)
      • I don't know if it's quite that simple. Even with mixed isotopes of plutonium, it is possible to build a bomb. It's not going to be a terribly efficient device, but it would get the job done. If you're satisified with working in the range of a few tens of kilotons, then you can be quite sloppy with thirty or forty kilos of mixed plutonium isotopes and still be effective. (It goes without saying that this would be a large device. No suitcase bombs, nor something that could be delivered on a lightweight missile--maybe something that would fit on a regular shipping pallet.)

        Uranium bombs are a different beast. Without highly (isotopically) enriched uranium, it is flat out impossible to produce a useful explosion.

        With chemically separated plutonium, you can build an inefficient bomb. With chemically separated uranium--you can warm your toes.

        All that said, a terrorist group would still be better off leaving plutonium or uranium dust in the streets or in a major building. Much bigger fear factor for the masses.

        • Even with mixed isotopes of plutonium, it is possible to build a bomb. It's not going to be a terribly efficient device, but it would get the job done.
          I've heard of one test device made using Pu from spent power-reactor fuel. It wasn't PWR leavings, either; it was from a British Magnox reactor, using a low (by current standards) burnup of about 20,000 megawatt-days per ton IIRC (current practice in PWRs is about 50,000 MWD/ton, again IIRC). And its "success" was supposedly iffy.

          I'd almost be inclined to let the most determined of the bastards get some spent fuel and try. The most likely outcome is that they'd kill a lot of their people from the radiation without producing anything useful (to them), whether their aim was a nuclear explosive or just a "dirty bomb". Their activities would not be that easy to hide; gamma detectors, xenon sniffers and such would make them much easier to find than people making anthrax or VX.

          Uranium bombs are a different beast. Without highly (isotopically) enriched uranium, it is flat out impossible to produce a useful explosion.
          Use neutron irradiation to breed thorium-232 into uranium-233, then separate chemically. Bingo, you've got a highly purified fissionable isotope (but another bugger of a problem with short half-lives, not terribly much better than plutonium).
          With chemically separated uranium--you can warm your toes.
          If that uranium comes from a thorium breeder, you can do both.

          BTW, I got a nuclear physicist to weigh in on the plausibility of the parent article. He says that it pins his bogosity meter (my words, not his).

  • U 238 absorbs nucleons (can't rememebr whether it's neutrons or Alpha particles) and transforms into plutonium. The plutonium is then used as fuel. Isn't the problem with refining 235 out of the 238 that 238 is a neutron absorber so it limits the reaction?
    There have been a few test reactors - notably at Douneray in scotland and an experiment in Japan.

    Has this guy come up with a hitherto unknown reaction for power generation? Or does nobody on Slashdot remember their basic nuclear physics.
  • If they are going to use Radioactive waste as an energy source, more power to them.

  • Great idea! (Score:5, Insightful)

    by Tuxinatorium ( 463682 ) on Monday February 03, 2003 @06:10PM (#5218416) Homepage
    This is a great idea, but thanks to the morons in washington, reprocessing spent nuclear fuel to recover useful fuel like plutonium has been illegal in the U.S. since the 1970's because they're paranoid that reprocesses plutonium could fall into the wrong hands and result in nuclear proliferation. So instead, instead of reprocessing the waste and thereby getting many times more energy out of the same amount of mined uranium, they store all that stuff underground. Personally, I think if we recycled the damn stuff it would be less likely to fall into the hands of terrorists because there wouldn't be so much nuclear waste crap everywhere. Which do you think is going to be more heavily guarded: a buried nuclear waste dump or a plutonium reprocessing facility. The U.S. law against reprocessing is idiotic and terribly wasteful.

    • Well.... (Score:4, Interesting)

      by MacAndrew ( 463832 ) on Monday February 03, 2003 @06:30PM (#5218582) Homepage
      Our policy doesn't look so idiotic after what happened in Japan last week, with North Korea right in their back yard and well-known for its infiltration missions. In the U.S. we may feel more secure, but we've had plenty of nasty surprises in the last few years.

      Stolen reprocessed plutonium would be very useful for a dirty bomb. It would have to be enriched before making a bomb, but it would be a headstart and obviate a nucleare reactor to make the Pu.

      I don't think the problem is that we're overflowing in nuclear waste. The problem is we've had so much trouble confronting where exactly to put the stuff because of political opposition. Countries like Japan and France have far less oil, and this desperation makes the nuclear power sacrifices less daunting.

      Old nuclear fuel is sequestered in ceramics and buried. There is not that much need to guard it. Nuclear fuel grade uranium is only 2-3% enriched U-235 to begin with, so its a -long- way from being particularly useful. Old weapons-grade Pu is mixed with material like U-238 to make it useless. With Iraq or N. Korea, the big challenge they face is not getting material, but purifying it.

      So ... I think our policy is debatable but not idiotic.
      • If the terrorists had the facilities to enrich plutonium, they might just as well mine their own uranium ore and enrich that instead of going to the trouble of sending in a lot of commandos to attempt to steal some plutonium.
      • to separating uranium isotopes from each other. Getting the U235 out of natural uranium is so difficult because U235 and U238 are the same chemical element; any separation process has to distinguish the isotopes based on the small difference in atomic weight. That means centrifuges, gaseous diffusion, lasers, and other cumbersome means.

        Separating plutonium from U238 in spent reactor fuel is much easier. Plutonium and uranium are different chemical elements and can be separated by chemical processes. It's not something you can do in your kitchen, but atom bomb designer Ted Taylor, in John McPhee's excellent book The Curve of Binding Energy, compares its difficulty to that of building, say, a large scale drug lab.

        We know perfectly well that criminal organizations manage enough chemical engineering to produce refined heroin and cocaine by tens of tons even without any governments supporting or protecting them. Separating Pu from U in quantities of only a few kilograms, as an official project of the local rogue government, appears quite achievable in the face of that knowledge.

        • Yes, Pu from U is relatively easy. One of the sources said it can be done with nitric acid using a machine about the size of a refrigerator. If you're not worried about safety, it's a lot easier.

          Isotopes are a different problem, and I didn't make this clear. Pu isotopes have different fission properties. [ieer.org] For a weapons, the Pu-239 alone is desirable; the other isotopes, particularly Pu-240, tend to spew neutrons that cause pre-ignition of the reaction, resulting is a "dirty" nuclear explosion and inefficient conversion of plutonium. Non-Pu-239 isotopes also make the material harder to handle because of spontaneous decay.

          A commercial reactor, esp. light-water, produces a significant proportion of the "undesirable" isotopes. Separating isotopes, as with U, is a lot of work; it's better just to produce the Pu right in the first place, that is, get the right reactor in the right configuration, and run it correctly for the purpose. But if you're a hard-up tinpot dictator and can settle for a 1-kt boom, or at least poison a water supply, and your real goal is to extort aid or concessions from other countries, then several kilos of dirty Pu will tide you over.

          I'd like to know what kind of efficiency the Indian and Pakistani bombs have. I read somewhere that the device to approach 100% fission would be a very large H-bomb, and so the small "neutron bomb" of the 80's was pitched misleadingly.
          • is probably enough to get a fusion bomb lit. It took a comparatively long time for the US and USSR to make fusion bombs because it wasn't clear at first that it was even possible. But China and apparently Pakistan got fusion bombs working pretty quickly.

  • Wow. (Score:5, Informative)

    by DuckDuckBOOM! ( 535473 ) on Monday February 03, 2003 @06:13PM (#5218449)
    If there's anything to this, it's simultaneously VERY big and VERY scary. Enriching uranium, even to power-plant specs (only 5% or so U235) is extremely difficult, expensive, and both physically & environmentally hazardous, not to mention that the end product must be replaced every few years (resulting in tons of high-level waste) as that small percentage of U235 is "burned up". A workable CAESAR would eliminate nearly all of this, drastically lowering the cost of building & fueling reactors while increasing their fuel supply by a factor of 98 or so. (This of course excludes the artifically inflated costs of the nightmarish regulatory/legal labyrinth builders/operators must run in many countries.)

    The dark side of all this is, of course, that a lowered cost of entry makes it just that much easier for "nuclear club" wannabe countries to produce plutonium for less benign applications. The author of the Economist article notes that countries could seal their CAESAR reactors (thus, I assume, burning the created plutonium for power alongside the U238) "to show that their nuclear intentions were entirely peaceful." Yeah, right. I'm sure Saddam Hussein and Kim Jong would be perfectly content to have their CAESARs crank out power, with nary a thought to the goodies sealed therein.

    Yet another two-edged sword, but a damned intriguing one.

    • Read the article. They aren't enriching or reprocessing the spent fuel rods. They're using a new process to fission U238 directly, using steam as a moderator to control the speed of neutrons so they'll split the atoms.
      • I did read the article, and I get what they're trying to accomplish. What I don't get is your point. Assuming that it has to do with the non-proliferation aspect of CAESAR, the article made it clear that, even though it isn't necessary to remove and reprocess fuel, there's nothing that prevents a government bent on building Da Bomb from doing so anyway. A CAESAR built, say, by France or the US for a third-world country could be sealed to complicate access to whatever fissionables would be generated therein. But the fissionables will be there, and, short of encasing the reactor in invulnerable unobtanium, they can be removed. My point was that the integrity of that "sealed" reactor depends heavily on the trustworthiness of the government on whose territory it sits. And I suspect you can think of more than a couple of governments not deserving of such trust.
      • "They aren't enriching or reprocessing the spent fuel rods."
        The article I read emphasizes that spent fuel--which no longer is sufficiently enriched--can be used in this reactor. That is a big part of its whole design. I'm not sure which article you read.
  • IANANP (...Nuclear Physicist), but it sounds like this reactor would be an extremely poor candidate for breeding plutonium. To sustain any reaction, it needs the moderation carefully tuned so that the neutrons fission U-238. That doesn't sound very compatible with neutrons moderated to be absorbed by U-238 (thus breeding it into Pu-239).

  • Building a Reactor Using U-238 Is No Big Deal (Score:4, Interesting)

    by kmellis ( 442405 ) <kmellis@io.com> on Monday February 03, 2003 @07:17PM (#5218910) Homepage
    ...but building a reasonably efficient version for power generation probably is.

    At any rate, Fermi's reactor in Chicago (the first) used natural uranium (almost all U-238) as fuel. There wasn't any other choice. Enrichening U-238 to higher quantities of U-235 is a really big deal. Natural uranium contains only about one half of one percent U-235. Fermi's design used highly purified graphite in a honeycomb pattern as the moderator. The Russians, before they got the plans for our reactor, looked at a U-238 design that used heavy water as the moderator (the Germans were going that way, too). Anyway, as this guy has shown, it's all about finding the correct moderator in the right configuration.

    • Actually, it is a very big deal (Score:5, Informative)

      by Spamalamadingdong ( 323207 ) on Monday February 03, 2003 @09:48PM (#5219961) Homepage Journal
      Fermi's reactor in Chicago (the first) used natural uranium (almost all U-238) as fuel.
      But the only part that was actually producing energy (fissions) was the 0.7% which was U-235; the 99.3% which was U-238 was just along for the ride (and eating up the occasional neutron).

      There are ways to get energy directly from fission of U-238, but they require very fast neutrons such as are created in a deuterium-tritium fusion reaction [iadfw.net].

      The Russians, before they got the plans for our reactor, looked at a U-238 design that used heavy water as the moderator...
      Then the Canadians must be smarter than the Russians, because the Canadians actually did it [carleton.ca].
      • "There are ways to get energy directly from fission of U-238, but they require very fast neutrons such as are created in a deuterium-tritium fusion reaction [iadfw.net]." 75% of the yield from the first fusion bomb came from the fissioning of the U-238 tamper.

        • Precisely (Score:2, Interesting)

          by Spamalamadingdong ( 323207 )
          And the neutrons which caused the U-238 to fission did not come from the U-238, but from the D-T fusion reaction. This is why I'm skeptical about this U-238 reactor; I have no information to indicate that the neutron spectrum emitted by fissioning U-238 is capable of sustaining a chain reaction at all.

          • As I understood the article, the idea is that neutrons from other unstable fission products in the 'spent' fuel rods are to be moderated so that they can cause U238 fission (and so produce more fission products).

            Since we currently just store the spent fuel rods, this is a great idea if it works, we can 'store' them in a reactor and generate power.

            Of course, I'm not a nuclear physicist.

      • Of course Canadians are smarter than Russians. Also smarter than a fair few yanks and a lot of the rest of the world. And just to round it out, we're not the least bit egotistical to boot!
    • And we all know Graphite is the way to go

      Just ask the Russians about how good it burns as well

  • Safer too (Score:4, Interesting)

    by HalfFlat ( 121672 ) on Monday February 03, 2003 @11:03PM (#5220322)
    IANANP but ...

    It seems that this design not only allows the complete removal of the whole enriching process, and the elimination of a good percentage of nuclear waste, but it seems safer too.

    The fission is only sustainable when the neutrons are just the right speed, which in turn can only happen when the steam is at just the right density. If anything goes wrong (e.g. steam escapes, etc.) the density changes and the whole chain reaction fissles pretty much to a halt.

    However when a good percentage of the U-238 has formed Pu-239, would this level of control still hold? (Thinking that Pu-239 undergoes fission with a wider range of neutron energies.)
  • So does this have any advantages (political or technical) over the Integral Fast Reactor [berkeley.edu]?

  • Seems like a lot of trouble (Score:3, Funny)

    by robert0122 ( 444085 ) <robert0122@nosPAm.yahoo.com> on Tuesday February 04, 2003 @12:44AM (#5220699)
    Couldn't we just compost [slashdot.org] the nuclear waste? ;-)

  • Similar to Boiling Water CANDU (Score:3, Interesting)

    by radtea ( 464814 ) on Tuesday February 04, 2003 @02:54PM (#5224741)

    IAANP, and from what I can tell from the article, the design looks similar in concept to the boiling water CANDU, which didn't quite live up to its promise.

    There are a couple of problems with steam-moderated designs:

    • maintaining a given density is not trivial
    • steam pipes tend to erode

    The first problem is a polite way of saying there may be reactor stability issues. If CAESAR uses super-heated steam this may be less of an issue, but otherwise the ratio of steam to liquid in the cooling circuit is a function of pressure and temperature in ways that can create problems if there is an unexpected excursion.

    The second problem is a major issue, especially when coupled with the long-term affects of radiation on materials. Intense neutron bombardment is a good way to introduce defects in metallic lattices. Defects are a good place for corrosion, cracking and other bad things to start. Ergo, the odds of a sealed reactor lasting for more than a decade or so are not good. Retubing of reactors is an ongoing maintenance problem even in conventional designs.

    Nuclear power is an option that we may in the end decide we have to go with, and it's good that advanced reactor designs continue to get consideration, but the engineering challenges are still severe and the proliferation potential is large.

    --Tom

    • Retubing is most certainly not a normal maintenance item in conventional reactors, and they last for over 30 years. Neutron embrittlement is an issue, but its impact is largely that heat up/cool down has to be very carefully controlled to avoid brittle fracture. The interesting thing about this proposal is that U238 has a zero fission cross section for thermal neutrons, but a stable cross-section of about 1/2 barn for the range of 2-6 MeV (and between 6 and 8 MeV, cross section rises to around a barn). That compares with a similar 1 barn plateau in U235 between 1 and 5 MeV rising to 2 barns between 5 and 11 MeV. While several orders of magitude lower than the thermal cross sections which form the basis for conventional water-cooled reactors, it's certainly in line with U235 and P239 profiles for fast reactors, and one could hypothesize that a fair amount of the fissions in a fast reactor might come from U238 fissions. In that case, the steam is only the coolant, not a moderator (only slight moderation is required) and steam density wouldn't be much of a concern for control.

  • CAESAR page at U of M (Score:3, Informative)

    by toddzilla ( 321757 ) on Tuesday February 04, 2003 @02:58PM (#5224783)

    Here's the U of Maryland page on the CAESAR project.

    http://www.caesar.umd.edu/ [umd.edu]

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