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Nuclear waste and depleted uranium research paper (No math!)

  1. Mar 2, 2015 #1
    1. The problem statement, all variables and given/known data
    (This isn't a typical problem here so feel free to suggest an alternative forum category or website.)

    I'm trying to do research for a paper about nuclear waste and depleted uranium storage by the DOD. I have a lot of questions I need to answer before I can get too far into this since this is a bit off the beaten path in terms of what has been taught in my nuclear weapons course.

    (I'll bold all of my questions to make it easier to read)

    Before I begin the paper I need to know the processes used by defense nuclear reactors in order to create the waste and depleted uranium. I know that in order to make 235 the reactor bombards 238 for some amount of time (I don't know how long, and can't find it anywhere but I suppose it depends on several physical factors) and in the process makes heavier isotopes like 239/240/241. When we speak about "waste" I'm under the impression that these heavier isotopes make up at least part of it but I imagine that there is a significant amount of 238 still left over from the process. Is u-238 a significant byproduct of the production of u-235 and, if so, is it recycled for use in another reactor or considered "waste" afterward? Or, is the 238, now with a much lower concentration of 235, considered "depleted" and kept for use in depleted uranium armor or weapons? Or, both?

    The process of creating 235 within a reactor is different than a reactor we think of that creates electricity. What is the difference in waste/byproducts between the two types of reactors? I imagine that there is probably a lot more heavier isotopes within the reactors used for electricity than those made for producing 235.

    I know that reactors use chemical reprocessing to extract plutonium and other isotopes for mox-fuel that is used in thermal reactors. Does anyone know if the reactors used by the defense department does anything similar when it comes to chemical reprocessing?

    As for storage I see that most of the facilities use a "pond" or pool to contain whatever waste they are holding. I have read the this is for reducing radiation outside the pool and keeping the spent fuel rods cool. What is the primary source of the excess heat and radiation? I know that Pu-238 has a pretty intense decay heat and a short half-life but I don't know what sort of concentration it might have within the spent fuel.

    Finally, I have been going through the Defense Nuclear Facilities Safety Board publicly available documentation and the DOE technical standards but, as anyone who has ever read such things, it can be tedious. I was wondering if anyone had any suggested reading materials or sources that would be handy for a paper like this?

    Feel free to correct me if I have any misunderstandings or mistakes. It's important to me to really understand this as I find it all pretty fascinating. It will not be a highly technical paper (no equations unless I see a real need) but rather a paper about the risks of storing these materials.
    2. Relevant equations
    N/A

    3. The attempt at a solution
    N/A
     
  2. jcsd
  3. Mar 3, 2015 #2

    SteamKing

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    Hold it right there. We've encountered the first major gap in your knowledge.
    U-235 and U-238 are isotopes of the same element, uranium. Both isotopes have 92 protons in the nucleus, but a different number of neutrons:

    http://en.wikipedia.org/wiki/Isotope

    Both isotopes occur naturally, but U-235 is quite rare, only about 0.7% of all the uranium found in the crust of the earth.

    Unfortunately, it is the U-235 isotope which fissions in a nuclear reactor, not U-238, and the natural ratio of U-235 in a sample is too small to sustain the chain reaction which occurs in a nuclear reactor. Therefore, the amount of U-235 in reactor fuel must be increased in one of several different processes known as enrichment:

    http://en.wikipedia.org/wiki/Enriched_uranium

    Fortunately, the amount of U-235 need only be increased to about 3%-4% to work in a reactor, so while the process is arduous, it can be achieved fairly readily. For use in nuclear weapons, a sample of uranium needs to contain at least 90% U-235, which makes production of such highly enriched uranium very expensive.

    The enrichment of uranium does not occur inside a nuclear reactor, but is performed in special industrial plants which make reactor fuel. There are several different ways in which uranium can be enriched for use as reactor fuel.

    Enriching uranium to make it more suitable as a reactor fuel still leaves a lot of U-238 lying around, which unfortunately is inert as nuclear fuel in a reactor. Uranium which has less than 0.3% U-235 is called depleted, and because of its high density, it can be used to make things like bullets for ammunition for conventional arms:

    http://en.wikipedia.org/wiki/Depleted_uranium

    When the nuclear arms race was in full swing, special reactors were dedicated for the production of plutonium for use in making weapons. The earliest facility for such production was built at Hanford, Washington during WWII to make the fuel for one of the first atomic bombs. However, AFAIK, the US is no longer making plutonium for use in nuclear weapons or for other purposes, but is instead using existing stocks of this material, some of which comes from old nuclear devices which have been phased out under arms control treaties or which are otherwise rendered obsolete by the march of time.

    The production reactors at Hanford, Washington and the enrichment facilities at Oak Ridge, Tennessee were shut down long ago, and these facilities were demolished, along with other parts of the weapons-making infrastructure of the US.

    After uranium fissions in a reactor, it leaves behind other chemical elements as the products of this fissioning. These products are themselves radioactive and must be stored safely to prevent people and the environment from being contaminated with radioactivity. When you store a bunch of this material in one place, it must be cooled, and storing it under water is a cheap way to do this. The water also helps to absorb some of the radioactivity, as well.
     
  4. Mar 3, 2015 #3

    DEvens

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    Wikipedia is your friend. I will try to avoid any of the links that SteamKing has already given, but I might double up on some.

    http://en.wikipedia.org/wiki/Nuclear_reactor
    http://en.wikipedia.org/wiki/Nuclear_fission
    http://en.wikipedia.org/wiki/Neutron_activation
    http://en.wikipedia.org/wiki/Decay_chain
    http://en.wikipedia.org/wiki/Radioactive_waste
    http://en.wikipedia.org/wiki/Nuclear_reprocessing
    http://en.wikipedia.org/wiki/MOX_fuel
    http://en.wikipedia.org/wiki/Strontium
    http://en.wikipedia.org/wiki/Caesium
    http://en.wikipedia.org/wiki/Iodine
    http://en.wikipedia.org/wiki/Uranium
    http://en.wikipedia.org/wiki/Plutonium

    To extend what SteamKing said about storing fuel:
    When reactor fuel first comes out of the reactor it has a variety of isotopes in it. There are two basic kinds in addition to the material that went into the reactor that has not changed during use. The first is fission products. When a fissile nucleus splits, the parts it breaks up into often have too many neutrons to be stable. So they tend to be radioactive, often with medium to short half lives. Also, their decay may produce another isotope which is also radioactive.

    The other set of isotopes are those produced by neutron capture and later decay. U238 can catch a neutron to become U239, which can beta decay to Neptunium 239, and again to Plutonium 239. Just for example. There are lots of other possible chains. These materials have quite a range of half lives.

    Radioactive materials that are biologically active can be quite troublesome. Iodine, for example, is very readily absorbed by many living things, including humans. And in mammals it is concentrated in the thyroid. And Iodine is a significant fission product.

    Short half life isotopes mean a lot of radiation in a short time, then quick decrease in amount. Such material needs to be stored for a short time, usually for something like 20 half lives or some such amount of time. So a material with a 5 day half life would only need to be stored for 100 days or so to decrease its amount by a factor of 1 million. During that time you need to be very careful about storage. After that it becomes much less dangerous. Used fuel is stored in water pools until much of the short lived material decays away.

    Long half life isotopes mean very little radiation in any given time, and very slow decrease in amount. U238, for example, produces only a small amount of radiation because its half life is 4.468 billion years. It can be quite easily stored and handled. You simple need to avoid ingesting it.

    Medium length isotopes are some times the most troubling. Caesium 137 has a half life of 30.17 years. So, to get it to 20 half lives would require over 600 years. During that time it produces an intermediate amount of radiation. And it is somewhat biologically active as well. Strontium 90 has a half life of 28.9 years. And it tends to deposit in bones. And both of these are produced in non-trivial amounts as fission products. Used fuel is planned to be stored in long term storage, such as underground vaults, until the medium length stuff is decayed away.
     
  5. Mar 3, 2015 #4
    I understood that. Can you point out where you see me misrepresenting that? I just want to make sure I have my vocabulary correct since you see a mistake. I know that U-238 goes in the reactor and, after being bombarded by neutrons, begins producing a variety of isotopes including U-235. Once a certain amount of U-235 is created they take the whole thing out and, through some chemical procedures, turn the mixture into uranium hexafluoride so that it can go through the enrichment process. My question though, is that since a reactor designed to produce weapons does not run as long as other reactors (the rods are taken out at a certain time in order to get the largest amount of U-235), is there a meaningful difference between the ratio of the other isotopes/fission material between the two reactor types? As I said, since the rods stay in the other reactor longer I expect a larger amount of other isotopes besides U-238. The core of my inquiry is whether the waste coming from reactors whose purpose is to create weapons have more dangerous amounts of radiation or less. It seems like the reactors made for weapons production would actually have less incidental radiation but I want to be sure before I proceed.

    I know that one of the fission byproducts coming out of the reactor is plutonium. But that really wasn't my question. Rather, I'm trying to discern whether the U-238, or other usable isotopes, are being discarded as unusable after only one trip in a reactor or if they are being reprocessed and sent back to reactors to produce more U-235. That is, are they recycling U-238 after U-235 enrichment?

    As for the the question about the source of excess heat I think I was being more specific than you understood. The question stems from the fact that I don't really know the chemical composition of the material they are putting in these pools. U-238 doesn't produce a large amount of heat on its own, so the heat is coming from some other isotope. I know that some plutonium isotopes emit a lot of heat but I don't know if they constitute a large enough percentage of the material to be blamed for the excessive heating.

    My trouble so far has been trying to find out exactly what materials are produced by the reaction process. Typically I see things like U-238 in and U-235, Pu-239, Other stuff. My interest is in the other stuff and their amounts. I found a bit of information from the fission products page about this so I'll be going over it a bit. However, the information you've shown me about the varying half-lives of these products has me wondering if they store waste by half-life or if it is all stored together. What, chemically, is stored in the barrels at the bottom of these ponds?

    Thanks for the responses.
     
  6. Mar 3, 2015 #5

    SteamKing

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    And you apparently have not understood what I wrote in my initial reply.

    U-235 is not created in nuclear reactors, by bombarding U-238 or any other substance.

    U-235 is separated from U-238 in natural uranium at an enrichment plant by non-nuclear processes, before it goes into a nuclear reactor.

    The three chief processes pioneered by the Manhattan Project were:
    1. using centrifuges to separate the lighter U-235 from U-238
    2. using electromagnetic mass separation devices called "calutrons"
    3. using gaseous diffusion to concentrate uranium hexafluoride gas (UF6) so that progressively richer amounts of U-235 were obtained. Once the UF6 gas was sufficiently enriched, then metallic U-235 was produced using chemical means.

    U-235 is not a product of reactor operation. U-235 is the fuel with which the reactor operates in the first place. It's the fissioning of U-235 which creates the energy output of the reactor.

    Again, your lack of understanding of basic nuclear reactor operation is going to give you problems understanding nuclear waste. Plutonium, while mildly radioactive, is highly carcinogenic and must be handled using special equipment to prevent contamination of workers and work areas. Things like glove boxes and special suits are used when plutonium is handled.

    U-238 is present in nuclear reactors only because it is very expensive to use nearly pure U-235 as fuel.

    As far as the fission reaction in a reactor is concerned, the U-238 is an inert material, which contributes no energy because it does not undergo fission.

    Some countries do recycle nuclear fuel rods after they have come out of a reactor, but the US has suspended such operations because of the plutonium which is contained in the waste. The Ford and Carter administrations banned the reprocessing of nuclear fuel rods (especially the separation of plutonium) for fear of nuclear weapons proliferation. Although this policy was later reversed, no reprocessing took place due to a variety of reasons, not least of which such activity was no longer subsidized by the government:

    http://en.wikipedia.org/wiki/Nuclear_reprocessing

    The storage ponds are designed to contain the waste which is highly radioactive, not necessarily the U-238. Because of the radioactivity, it is expensive and difficult to separate out the various elements contained in the waste. Some of the radioactivity naturally dissipates while the waste is sitting in the holding pond as the isotopes with short half-lives decay. Once the radiation intensity dies down, the waste material cools and it can be stored in a metal container. It's the other stuff with the long half-life which must be stored for hundreds, if not thousands of years in a secure facility, to prevent radioactive contamination of the environment should it spill out.

    These articles describes the typical composition of the nuclear waste produced in a reactor:

    http://en.wikipedia.org/wiki/Spent_nuclear_fuel

    http://en.wikipedia.org/wiki/Radioactive_waste

    http://www.whatisnuclear.com/articles/waste.html

    Most of the stuff coming out of a reactor is U-238 and unfissioned U-235, with a smattering of elements like Np, Pu, Am, etc. plus the products of fission, like radioactive barium.
     
  7. Mar 4, 2015 #6
    I see my mistake. I was confusing the production of Pu-239 with U-235. So, the only step to making weapons grade uranium is the conversion to uranium hexafluoride and then enriching 235. Thank you for clearing that up for me. I knew that 235 occurred naturally but was thinking putting 238 into a reactor would create more 235 manually. Understanding that my second question still stands (but replace U-235 with Pu-239). Is it the case then that the amount of Pu-239 (and other Pu isotopes) follows a bell curve over the lifetime of a reactor and is not as radioactive as the reactors made for creating plutonium weapons (since they take the rods out at the top of the curve)?

    This has been a great help. I've got about 15 wiki pages and various other agency pages up that I'm going through. Though asking questions on here makes it easier for me to understand and retain. I'm not ignoring the reading suggestions, just supplementing them with questions here.
     
  8. Mar 4, 2015 #7

    SteamKing

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    I think for the purposes of your paper, you don't need to worry about reactors which make Pu-239 for weapons. In the US, at least, all such facilities were shut down years ago when the Cold War ended and they have been dismantled. The existing stocks of Pu-239 have come from recycling obsolete nuclear warheads or was leftover inventory which was manufactured before the Cold War ended. This material is not being used to fuel power reactors, thus it must be stored in secure facilities not only because it is an environmental hazard, but because this material could possibly be used by another state with a functioning nuclear weapons program, like North Korea, to make nuclear weapons if reprocessed.

    Because most of the uranium which is used in power reactors is the inert U-238 isotope, there have been various proposals put forward to convert U-238 into Pu-239 to make reactor fuel instead of nuclear weapons. These fuel-making reactors are called breeder reactors, and there are several different processes which are used to make Pu-239:

    http://en.wikipedia.org/wiki/Breeder_reactor

    Making Pu-239 for use as fuel in nuclear reactors was once an attractive idea because it was feared that existing uranium stocks were quickly being depleted by using this material as both reactor fuel and in making weapons. However, new sources of uranium bearing ore were found, and fears of a uranium shortage were eased, so the breeder reactor idea fell by the wayside, except for research purposes.
     
  9. Mar 4, 2015 #8
    My only interest in the plutonium production is the amount and type of waste that we created while it was in production. Also, are not some fusion weapons still using Pu-239? Or are these weapons being phased out for pure plutonium?
     
  10. Mar 4, 2015 #9

    SteamKing

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    Plutonium is still being used in nuclear warheads, but my point is that this material is no longer being made.

    Over the years, various arms control treaties have reduced the number of nuclear warheads which the US and the former Soviet Union are permitted to retain in their arsenals.

    As warheads have been decommissioned and taken out of service, the nuclear material from the bomb device is removed and put into storage. If any plutonium is needed, it is pulled from storage, rather than manufactured in a reactor, all of which I understand, have been taken out of service and dismantled.

    Some of the devices in the nuclear stockpile use fusion in the warhead to boost yields. The fission component of the warhead, whether it uses uranium or plutonium, must have the fissile isotope purified to >90% purity in order to function, which is called "bomb-grade" material, so your comment about using "pure" plutonium already applies.
     
  11. Mar 4, 2015 #10
    By pure plutonium I just meant to ask if uranium was used for both the fission and fusion reactions. Do you have any sources on plutonium no longer being made? I'm trying to find anything saying this but can't. Googling "dismantle plutonium reactor", "plutonium no longer made", etc doesn't get me anything. Perhaps my "google-foo" is off.
     
  12. Mar 4, 2015 #11

    SteamKing

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    All fusion reactions in nuclear weapons are initiated by detonation of a fission device. The actual material which fuses after the detonation of the fission component is a material called lithium deuteride. Deuterium is an isotope of hydrogen where the nucleus contains one proton and one neutron.

    Some fission devices use small quantities of tritium (or hydrogen-3) to boost yields, but tritium has a short-half life (about 12 years). Tritium also must be produced in a nuclear reactor, and the facility which produces tritium is located on the Savannah River on the border between Georgia and South Carolina:

    http://en.wikipedia.org/wiki/Savannah_River_Site

    Here are some articles on plutonium and the place where most of it was made in the US:

    http://en.wikipedia.org/wiki/Plutonium

    http://en.wikipedia.org/wiki/Hanford_Site

    This article discusses the facilities which once produced the US nuclear weapons stockpile:

    http://en.wikipedia.org/wiki/Nuclear_weapons_and_the_United_States
     
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