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Why is U235 used for chain reaction but not U238

  1. Mar 24, 2006 #1
    i want to ask why only U235 is used for the chain reaction but not used U238
  2. jcsd
  3. Mar 25, 2006 #2


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    U235 is considered 'fissile', which means the nucleus is readily or more easily fissioned upon the absorption of a neutron, and it tends to more readily fission with neutrons of low energy (< 1 ev). U238 is considered 'fissionable', which means it may fission but that it requires an energetic neutron in order to fission - generally several MeV.

    The other fissile nuclei also have odd masses, U233, Pu239 and Pu241, as well as some heavier nuclei.

    The stability, or rather instability, to fission has to do with the binding energy of the excited nuclear once the neutron is absorbed. For example, the binding energy (excess energy) of the last neutron in U236 is approximately 6.4 MeV, but the critical energy for fission is only 5.3 MeV, so when U235 absorbs a neutron, the resulting U236 nucleus has an excess of 1.1 MeV above the energy needed to cause fission.

    U238 will absorb a neutron to form U239, which will eventually decay by beta emission to Np239, and this nuclide then subsequently decays by beta emission to Pu239, which is fissile. This is the process by which Pu239 is produced.
  4. Mar 25, 2006 #3
    U235 can decay after absorbing a very low energy but U238 needs a large energy to decay, so that a chain reaction used U235, right??

    other questions, why U238 needs a large energy to decay but U235 only needs < 1eV. The stability of U235 is lower than U238 ?? or other reasons ~?
    and a whole chain reaction only used U235? any other isotopes or elements and how can they decay?(absorbing the energy that produced by U235?)
  5. Mar 25, 2006 #4


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    Firstly, lets distinguish between 'fissioning' and 'decay'.

    Fissioning implies the atom 'splits' or 'fissions' into two nuclei (fission products). Decay implies the emission of a particle, e.g. beta or alpha particles, or gamma-ray.

    When a nucleus of U235 absorbs a neutron and becomes U236 in an 'excited' energy state, it will most likely fission, but there is a chance that it will decay by gamma emission to U236 (relatively stable). The stabilities of U235 (t1/2 = 7.038E+8Y) and U236 (t1/2 = 2.34E+7Y) are less than U238 (t1/2 = 4.468E+9Y).

    See - http://wwwndc.tokai.jaeri.go.jp/CN04/CN024.html [Broken] for properties of U isotopes.

    The binding energy of the last neutron in U239 is ~4.9 MeV vs a critical energy for fission of ~5.5 MeV. So, U239 is more likely to spontaneous emit a gamma ray to dump the extra energy. Then at some point, the nucleus emits a beta particle to become Np239.

    A chain reaction is sustained as long as the rate of neutrons absorbed by fissile species, e.g. U235, and causing fission is constant. In a nuclear reactor, some of the U238 will be converted to Pu239 and that will also contribute to the fission process. This is also the purpose of a breeder reactor - which converts more fertile material, i.e. U238 to Pu239 than it consumes.
    Last edited by a moderator: May 2, 2017
  6. Mar 25, 2006 #5
    1) is the fissioned U235 caused the U238 to fission in a chain reaction (since i know that the % of U238 is higher than that of U235)?

    2) why can U235 fission by low energy, U238 need a large energy to fission. is it relative to the critical energy for fission?

    3)In a chain reaction, why not only used U235?

    4) what are nuclear reactor and breeder reactor?

    thx for ur answering !
  7. Mar 25, 2006 #6
    Odd numbered heavy nuclei, are naturally fissionable u don't need a lot of energy to split them, such as U235 or Pu 239, this has something to do with the repulsive and binding forces inside the nucleus. Usually natural uranium contain 0.7% only of U235 which is fissile, and the rest of U238.And that's why some times u need the enrichment process...
    U-238 can capture a neutron and decay to Pu239 which is also fissile, but u need some more energy to do that.

    It's a very hard chemical procedure to separate isotopes...

    There are fast breeder reactors and thermal reactors, fast reactors use fast neutrons (energetic), and thermal reactors operate with thermal (slow) neutrons...

    Thermal reactors also use moderators, which are intended to slow the neutrons until they approach the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher probability of fissioning U-235, and a lower probability of capture by U-238 than the faster neutrons that result from fission do.
    Examples can be Candu heavy water reactor, PWR(pressurized), LWR(light), BWR(boiling)..etc. Naval reactors can be PWRs.

    For fast breeders there's hardly any need for moderators, u only need highly enriched uranium (sometimes weapons-grade), or plutonium in order to reduce the amount of U-238 that would otherwise capture fast neutrons. Some are capable of producing more fuel than they consume, usually by converting U-238 to Pu-239.
    An example is sodium cooled reactor, and generally LMFBRs (liquid metal fast breeder reactors...

    I hope this has answered your question, i didn't try to get into much details...
  8. Mar 25, 2006 #7
    I think there's a question i've missed...

    I'm not sure if i quite understand your question, but U-238 is fertile, if it absorbs a neutron with the right energy it decays to U239>Np239>Pu239(beta decay), the last product is naturally fissile...

    Again fertile means that it can produce fissile nuclides under neutron irradiation, but it's not fissionable itself...
  9. Mar 25, 2006 #8
    Oh and fissile means, that the nuclide experience fission process by capturing a neutron...

    Fission means the product of 2 nuclei from one heavier nucleus + energy of around 200Mev and a number of neutrons...
    Last edited: Mar 25, 2006
  10. Mar 25, 2006 #9


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    Please read my previous posts which address some of the content of the questions herein.

    Some U-238 will fission in a neutron flux, but the probability is very low. As Nomy-the wanderer indicated, the odd nuclei (mentioned in one of my previous posts) are more readily fissionable (i.e. they are fissile) because the addition of a neutron adds energy in excess of what is required to fission the resulting nucleus.

    Most (>99%) of fissioning occurs in U-235 (or Pu-239) in a uranium fueled reactor. The U-238 absorbs neutrons and eventually becomes isotopes of Pu (239, 240, 241, 242), Am (241, 242m, 243) and Cm (242, 243, 244, 245) depending on time and neutron spectrum.

    U-236, with an excess of energy is much less stable than U-239. U-238 can fission, but generally requires neutrons of ~ 5 MeV or higher. Neutrons released from fissioning will have energies on the order of several MeV.

    I would like to address this question in the Nuclear Engineering forum since it seems to come up often.

    Well in some nuclear warheads, nearly pure U-235 was used. Modern nuclear warheads use Pu-239. Highly enriched U-235 or Pu-239 would be difficult to control in a reactor configuration.

    Uranium which contains about 0.71% U-235 and 99.2% U-238 (with traces of U-234) can be enriched to increase the concentration in U-235. Modern commercial plants used fuel with up to 5% U-235 and ~95% U-238 in the form of ceramic UO2 which is encapsulated in a metal tube of a zirconium alloy sealed at both ends.

    The cost of enriched U increases with enrichment, so utilities use as low an enrichment as possible. On the physics side, it is a matter of distributing the thermal energy in a reactor as well as maintaing safety margins which dictates the enrichments used.

    During the course of operation, fission products accumulate in the fuel, which perodically must be removed from the reactor, and fresh (unirradiated) fuel added.

    Fast reactors, which use fast neutrons (keV-MeV energy range), tend to use fuel with about 20% Pu-239 dispersed in U-238, also in the form of a 'mixed' oxide (PuO2-UO2). The cladding is usually stainless steel, e.g. SS316L (as well as other special stainless steels). Special breeder assemblies are used to produce extra Pu-239. Rather than water in the core, fast reactors typically use liquid metal, particularly sodium.

    A nuclear reactor is a device which enables the controlled fissioning of a fissile material (e.g. U-233, U-235 or Pu-239) to produce thermal energy. The thermal energy is then converted to mechanical energy (usually via a steam turbine) which is then transformed (via a generator) into electrical energy.

    The vast majority of commercial nuclear power plants are light water reactors (LWRs) of which there are Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Some reasonably good descriptions of nuclear reactors can be found here - http://en.wikipedia.org/wiki/Nuclear_reactor - but I must caution the reader that there are some inaccuracies in the Wikipedia article.

    Specifically -
    This claim is misleading since weapons-grade (WG) can mean highly enriched (> 70% U-235 or Pu-239) as well as the Pu isotopic vector. In fast reactors, as I pointed out above, the enrichment is typically about 20% fissile (usually Pu-239). If natural U is used as the diluant, then the fuel will also have 0.71% U-235 in the matrix with U-238 being the balance.

    The CANDU is a special type of pressurized water reactor which uses natural or slightly enriched (typ. < 1.5% enriched) U and heavy water D2O rather than light water H2O in the core as moderator.
    Last edited: Mar 25, 2006
  11. Mar 25, 2006 #10
    But Astronuc, my professor once said that they used to use weapons-grade (70 ~ 80% enrichment) fuel, before they decided- for safety reasons of course- to limit the enrichment to 20% only...

    So yes i think it's not a claim, it's a fact. Just doesn't work anymore.
  12. Mar 25, 2006 #11


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    Certain research reactors and small cores such at the TRIGA reactor have used fuel with 70% enrichment, but the Uranium dispersed in a U-Zr-H matrix, and they had relatively high leakage, and a different negative Doppler coefficient. To my knowledge, commercial cores never used 70% enrichment.

    Even fast reactors do not use, nor do they need fuel with 70% enrichment. MOX fuel with about 20% is satisfactory and essentially standard.

    Again, it depends on what is meant when the term WG is used. Fast reactors generally use fuel with Pu from reprocessed reactor fuel (or so-called reactor grade (RG) Pu).

    I believe some fuel made for graphite moderated gas reactors was enriched up to 90-93%, but again it was dispersed in a matrix. Such high enrichments are used when the physical density of the fuel is relatively low compared to 100% TD, and when the fuel is 'dispersed' in an inert matrix.

    So in a sense, some fuel may be derived from WG U or Pu, but it is inaccurate to say or imply that a reactor requires WG fuel.
    Last edited: Mar 25, 2006
  13. Mar 26, 2006 #12
    The fuel derived from weapon grade plutonium gets reprocessed and used as a mox fuel, That's a different matter. i'll ask my rpofessor again for more details about what he was talking about, he mentioned a type of reactor that used 70 ~ 80% (or weapon grade) uranium and not plutonium...But i forgot what it was.
    Last edited: Mar 26, 2006
  14. Mar 26, 2006 #13


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    Certainly 90% enriched U is considered WG, but IIRC the cut off is about 70%. HEU is considered 20% or greater.

    There is a discussion here - http://www.uic.com.au/nip04.htm

    But it seems different groups or organizations use the term somewhat loosely.
  15. Mar 26, 2006 #14
    That was a great link, it's a usefull site. thx Astronuc.
  16. Mar 26, 2006 #15


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    Another useful site is the World Nuclear Association

    Here is a paper on the US-Russian Agreement on HEU
    http://world-nuclear.org/trade_issues/tbriefings/heu/index.htm [Broken]

    On Weapons Grade material - see


    http://world-nuclear.org/uiabs96/haas.htm [Broken]

    Various forms of U and Pu and fuel cycle information.
    http://world-nuclear.org/info/inf14.htm [Broken]

    Military Warheads as a Source of Nuclear Fuel - October 2005
    http://world-nuclear.org/info/inf13.htm [Broken]
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  17. Mar 29, 2006 #16
    1)Would you give me more examples on 'fissile' and 'fissionable' elements? i want to know more ~

    2)and why we use U but not other 'fissile' elements in a chain reaction?
  18. Mar 29, 2006 #17
    For example thorium-232 is fertile if bombarded with neutrons, it'll decay several times to form Uranium that is fissionable material.

    Uranium 235 is fissile, when capturing a thermal(i.e slow) will experience fission and will produce 2 fission fragments and neutrons...
    The type of these fragments depends on the surrousing conditions.

    This is a matter of properties, for example plutonium can be used, also thorium, but it's all connected to irradiation behaviour, strength, required mass, economics, availability, related reactor materials such as coolants, moderators, cladding...Also their state(solid, liquid), form (powder, metal, ceramic?).

    You know why they developped a plutonium bomb instea dof a uranium one, cause this would make its size smaller, cause u basically need only 1/2 of the mass(when using uranium) to reach criticality.

    So it depends on what kind of properties you are looking for, the application, costs...etc
  19. Mar 29, 2006 #18


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    Nomy-the wanderer gave some examples fissile and fissonable, and I believe I mentioned others in previous posts, as did others.

    See this thread - https://www.physicsforums.com/showthread.php?t=107527 - in our Nuclear Engineering section.

    The reason uranium is used is that 1) it occurs in Nature, and 2) uranium ore has a much higher fraction of a fissile isotope (U-235) than thorium ore (U-233).

    from the publication cited below.

    On thorium, see technical publications at - http://www.thoriumpower.com/english/technologies/tech_publications.htm

    particularly - "On the Use of Thorium in Light Water Reactors" - M.S. Kazimi, K.R. Czerwinski, M.J. Driscoll, P.Hejzlar and J.E. Meyer -- Massachusetts Institute of Technology (April 1999)

    One can download pdf files. Just right click and use "save target as".
    Last edited by a moderator: Apr 22, 2017
  20. Apr 15, 2006 #19
    is 235 easyer to fission because theres less neutrons to keep the protons from expelling each other?
    or is that only a small part of a larger picture im missing?
  21. Apr 16, 2006 #20
    I think it has more to do with the odd number of neutrons in U-235 than the total number. Atoms with an even number of protons and/or neutrons tend to be more stable than those with odd numbers. A U-239 atom, for example, has about a 40% chance of fissioning if it captures a thermal neutron, versus a small fraction of a percent for U-238. All of our common fissionable isotopes (U-233, 235; Pu-239, 241) are even-Z, odd-N configurations.
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