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Why Unstable Uranium

  1. May 21, 2013 #1
    Hello. I have some questions regarding Uranium 235 and its instability.

    Whenever you hear about nuclear reactions, you almost always hear Uranium 235 linked with it as being quite suitable for splitting because it is unstable (At least I do).

    I have been vaguely exploring nuclear transmutations and what you might call "Nuclear Alchemy" and forming new elements via nuclear fusion and fission. I know the amount of protons in an atom determines what element it is and a certain isotopic ratio of protons to neutrons determines if it is unstable or not, and I always hear how Uranium 235 is the perfect element for nuclear fission because it is unstable and breaks easily.

    I know nuclear fusion and fission are quite hard to perform on normal larger atoms due to the amount of energy required. For example, all the energy in the sun is enough to only make iron as the largest element from nuclear fusion and it requires much larger events (Supernovas) to transmute larger elements.

    To my questions. Why are unstable elements such as Uranium 235 so easy to perform nuclear fission on and make two new random elements? Why can't other elements such as Lead, Iron and Copper be made unstable and thusly used in nuclear fission to make new elements? Are there any cheats or catalysts that would allow nuclear fusion to be accomplished without all the energy of the sun required to simply transmute Hydrogen into Helium?

    Thank you.

    (Edit: Additional question... What is the difference between Uranium and Uranium 235 specifically?)
    Last edited: May 21, 2013
  2. jcsd
  3. May 22, 2013 #2
  4. May 22, 2013 #3
    Thank you. But I know that separating atoms takes an incredible amount of energy considering they are bound by an incredible amount of energy. But why is Uranium 235 an exception? The Binding force for U235 must still be present in order to get the amount of energy we can from it (Atomic bombs, Nuclear reactors), so why is it so much easier to perform nuclear fission on?

    What I mean to say is, why can't we just send a nucleon into any old atom and get nuclear fission? I know it has something to do with being unstable, but what does the isotopic ratio matter anyways, neutrons don't even have a charge?
  5. May 22, 2013 #4
    It is not. It would take immense amount of energy to separate U-235 into the constituent protons and neutrons.

    However, instead of separating U-235 into constituent protons and neutrons, you can separate it into just two nuclei, so that each of these has a large binding energy - and their combined binding energy is bigger than the binding energy of the initial U-235.

    For example, you can separate U-235 into nuclei of Th-231 and He-4, and have energy left over.
    The energy gain from fission (and alpha decay) is bigger, and the barrier is lower.
  6. May 22, 2013 #5
    In reference to the binding energy per nucleon curve in the wiki page I linked above, all the nuclides on the right to the isotope Fe-56 are unstable and try to decrease their mass number (A) by undergoing either alpha decay or spontanious fission.

    Now we can "induce" fission by bombarding these heavey nuclides with neutrons. The fact that these neutrons are neutral makes them optimum since they can penetrate the atom without being scattered due to electrmagnetic interactions and hence can approach the nucleus.

    U-235 is a good nuclear fuel because it has a high probablity to interact with slow neutrons (the probability is generally termed cross section, and slow netrons are generally termed thermal). This means that it can udnergo fission if you bombard it with a slow neutron. As a result of the fission reaction, more neutrons will be generated. These newly born neutrons can almost always induce fission again in the remaining U-235 nuclides because they usually have an energy sufficient to induce the fission (recall thermal neutron is enough to induce fission in U-235). U-235 is not unique in this, heavy nuclides with odd mass number have the same property. These are generally called "fissile".

    On the other hand nuclides such as U-238 are called "fissionable but non-fissile" because you can induce fission in these nuclides but using fast neutrons (> 0.5 MeV).
  7. May 22, 2013 #6
    No, not all of them can gain energy by spontaneous fission. For example, an isotope with mass number 84 could gain energy by somehow turning into isotope with mass number 56 - but this does not mean it would gain energy by turning into isotope with mass number 42.
    What is important to note is that U-235 is one of the very few primordial radioisotopes - isotopes which are exactly unstable enough to decay at an important rate, yet so stable that they have lasted though the age of Earth.

    Between 100 milliard years and 100 million years, the isotopes are:
    K-40 (1,25)
    Rb-87 (49)
    Sm-146 (0,103)
    Lu-176 (37,8)
    Re-187 (41,2)
    Th-232 (14)
    U-235 (0,71)
    U-238 (4,47)

    And of these 8, 5 (all the low mass ones) decay into stable nuclei by single decay - one alpha for Sm-146, one beta for all others, incl. K-40 which also can undergo electron capture and positron emission.

    The 3 long lived isotopes on the isle of stability are unique in being long-lived isotopes that decay into short-lived isotopes undergoing a radioactive decay chain.
  8. May 22, 2013 #7
    So, it is just a property of Uranium 235 to break when bombarded with slow neutrons. I'm going to amuse this because it is unstable, which I suppose would mean I don't have a clear knowledge of instability in atoms. By what I have read, would an atom become unstable when it reaches a specific isotopic ratio? I assume Uranium isn't the element that can be made unstable, if so, what determine if an element can be made unstable, and if it could be made unstable, would it be fissile?

    I just noticed I'm asking a ton of questions, so forgive me if I annoy anyone with my lack of knowledge.
  9. May 22, 2013 #8
    Agreed.I was not very precise in my description as I was trying to give qualitative trends to the original poster.
  10. May 22, 2013 #9
    Exactly because of this I would recommend reading either the wiki page I linked above (and the wiki page of fissile materials) or alternatively you can read an elementary intrduction to nuclear engineering/physics such as:

    The first two chapters (and may be chapter 4 as well) can get you right on the track to learn the language of fission.

    Having said that, questions are always welcome :-)
  11. May 22, 2013 #10
    Thank you! I'll do some more research.
  12. May 22, 2013 #11


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    Staff: Mentor

    Actually, it's U-236* (= U-235 + n) which is unstable, the * indicating an excited nucleus. U-234* (U-233 + n) is similarly unstable, but there is a probablity that U-236* or U-234* will decay by IT (γ - emission), and remain stable. One can find trace amounts of U-234 in natural U, and a fair amount of U-236 in recycled U.

    U-235 is an isotope of U.
  13. May 23, 2013 #12
    There is a reason odd isotopes of uranium and plutonium are fissile and even isotopes are not. Pairing energy.

    Basically, even numbers of neutrons are more strongly bound than odd number of neutrons.

    The energy needed to fission U-236 is similar to the energy needed to fission U-239. But the energy released in adding a slow neutron to U-235 is larger than energy released in adding a slow neutron to U-238. So if a slow neutron is captured by U-238, the resulting U-239 does not have enough energy for fission, and has to get rid of the energy by emitting gamma ray. If, however, a slow neutron is captured by U-235, the energy is big enough to cause fission of U-236, and most of time does, though it still often emits gamma ray instead of fission.

    If a fast neutron is captured by U-238, the additional energy from the kinetic energy of the neutron may be enough to cause fission.
  14. May 23, 2013 #13
    GT: If you have not yet read here


    it will provide you some additional insights like this one:

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