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Which actinides are not fissile?

  1. Aug 1, 2013 #1
    Well, which are not?

    Uranium 238 is not. It fisses at a very significant amount under influence of fast neutrons; yet the fission cross-section even for fast (fission spectrum) neutrons is low enough that infinite multiplication factor is below unity even in bare metal, unmoderated U-238. Yet it is big! When uranium 238 is irradiated with fast neutrons, like from fusion, the convergent fission chain reaction produces much more energy than the energy of the initial neutrons.

    Thorium 232 is another actinide isotope known to have small fission cross-section even at high energies.

    But are there any others?

    Just because a nucleus will not fiss under influence of a slow neutron does not mean that its fission under influence of fast neutrons cannot sustain chain reaction. Example: plutonium 238. It will not fiss due to slow neutrons. Yet its cross-section for fission under fast neutrons is larger than the cross-section of plutonium 239 - so much so that the critical mass of plutonium 238 is smaller than that of plutonium 239!

    Plutonium 240 and 242 are less liable to fission - yet both are fissile! The critical mass of plutonium 240 is 33 kg - significantly less than that of uranium 235. And even plutonium 242 has a critical mass of under 90 kg.

    So how about the other even isotopes - which do not fiss due to slow/moderated neutrons but can fiss due to fast/fission spectrum neutrons?

    What is the infinite multiplication factor of plutonium 244?
    And how about the other long-lived even isotopes of uranium - uranium 236, 234 and 232?
  2. jcsd
  3. Aug 1, 2013 #2


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    'fiss' is not a word. 'Fission' can be either a noun or a verb.
  4. Aug 1, 2013 #3


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    This article might help you with a list of 'fissile' isotopes versus 'fissionable' isotopes.

  5. Aug 1, 2013 #4


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    that's horrible! :frown:

    (though i must admit my dictionary agrees with you :redface:)

    unfortunately, there doesn't seem to be a latin-derived verb (in english) that goes with fission

    from eg commission we have the verb commit, from solution the verb solve, but the equivalent verb from fission would be find (pronounced finned, from latin findo findere fidi fissum, to split) …

    and unfortunately there's already a (germanic) with the same spelling!​

    so i think the best course is to use the word "split" :smile:
  6. Sep 17, 2013 #5
    No, that´s where I started. It does not cover the less common isotopes - exactly when they can undergo fission and when they cannot.
  7. Sep 20, 2013 #6
    IIRC U-234 and 236 are not good at fission. With (epi)therman neutrons definitely no.
    (Of course, they, as any other actinide, will fission with sufficiently fast neutrons).

    U-232 is short-lived (70 years) and has strong gamma daughters. Not funny.

    Neptunium has only two long-lived isotopes, 236 and 237.
    Both are fissile.

    Np-237 is produced in ton quantities by current power reactors (about 1 kg of it per ton of fissioned Uranium, meaning a few kg per one reactor core reload), but AFAIK it is not currently separated on a significant scale by anyone.
    It is about as good bomb material as U-235.

    Np-236 has a very small critical mass AND long half-life (154kyr), making it much better than Pu-239 for bombs (!!!) but it is very difficult to produce in separated form. Perhaps it's good, we don't need real pocket nukes to exist...
    Last edited: Sep 20, 2013
  8. Sep 20, 2013 #7


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  9. Sep 21, 2013 #8

    Only three isotopes with long enough half-lives:
    Am-241 432.2 years
    Am-242m1 141 years
    Am-243 7370 years

    All of them are fissile (in the "nuclear bomb" sense), not better than U-235.
    Am-242m1 is fissile in "thermal neutrons" sense too, comparable to Pu-239.

    Americium in spent nuclear fuel is about 100 grams per ton, mostly 241 and 243.

    Curium has many interesting relatively long-lived isotopes, longest is 15 million years (!).
    However, only a few kg of it was produced in total during last 50 years.
  10. Sep 21, 2013 #9


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    Some terms and definitions:


    The coloured actinide isotopes (Z=89 bottom) in the attached picture are more or less fissionable, and those of red, reddish brown or brown may be considered fissle.

    Image from http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf (zoom 3, or finer to see nuclides, and centering with http://www.nndc.bnl.gov/chart/reCenter.jsp?z=97&n=150). That page arrives from selecting σ(n,F) after selecting http://www.nndc.bnl.gov/chart/

    Some of the coloured or grey cells next to the fissile isotopes, where neutron absorption and subsequent beta decay(s) produces a fissile isotope would be considered fertile, otherwise they are not fissile or fissionable.

    Attached Files:

    Last edited: Sep 21, 2013
  11. Sep 22, 2013 #10
    Yet neither are Pu-238, 240 and 242...
    Yes. So does U-238.
    U-238 does NOT undergo chain reaction with fast, unmoderated reactors. Pu-242 DOES.
    So how about U-236 and U-234?
    Yes, and Pu-238 is also short-lived (87 years). And although Pu-238 does not fission with slow neutrons, it is notorious for having a smaller critical mass than Pu-239.
  12. Sep 22, 2013 #11
    The point is, how would one go with building a nuclear device with a material which gets almost red hot from its own decay?

    Even if that is somehow overcome (some fancy active cooling?), what to do with very high gamma doses in case of U-232?
    What to do with decay products rendering the device non-functional after only a few years in storage?

    I don't think building a nuke from an isotope with half-life of less than about 1000 years is attractive to anyone, regardless of how small its critical mass is.

    If we absolutely must have a mini nuke the size of big grenade, my hunch would be to separate Np-236. It would be immensely costly, though.
  13. Sep 22, 2013 #12
    I was rather aiming to check which actinides are completely safe against chain fission in the worst conditions (unmoderated bare metal) - radioactive decay and heat being the only problem.
    Pu-238, 240 and 242 certainly can go critical. How about uranium?
  14. Sep 22, 2013 #13


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    U-235 was used in some early nuclear weapons. U-233 could also be used.

    One has to be clear on the terminology. Fissile implies 'thermal' neutrons, i.e., fast fission neutrons or source neutrons that have been moderated to 'thermal' energies on the order of 0.025 eV. Fast neutrons have energy on the order of 1 MeV.

    In the case of weapons, one considers 'prompt', i.e., fast fission neutrons, and consideration for system criticality by prompt neutrons is very different than for moderated/reflect or delayed neutrons. BTW, that's not a subject for elucidation/elaboration in the public domain.
  15. Sep 23, 2013 #14
    I think U234 and U236 can't go critical, alone or with any moderators.
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