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Homework Help: Pu-239 + 1N fissions into what?

  1. Jul 1, 2009 #1
    I'm working on my very last question of the last exam of the last class for my corespondents high school program. Almost done!!! But I need help on this one. I've spent too much time on it already but I still don't know how to answer it.

    1. The problem statement, all variables and given/known data
    "Complete each equation. Classify each as nuclear fission or nuclear fusion:

    (a.) 239/94 Pu + 1/0 N -----> ________________ + 3 (1/0 N) + _________________

    (b.) 2/1 H + 3/1 H ------> 4/2 He + __________________ + _________________ "

    The above numbers (x/y) is a super-script and sub-script respectively.

    2. Relevant equations


    3. The attempt at a solution

    Classifying them is easy enough, (a.) is fission and (b.) is fusion. I also found from Wikipedia's Fusion article what I think is the right answer. In the first blank I have 1 neutron and in the second blank I've given the total energy released of 17.6 MeV. I don't know if I should try and show how that would be calculated, by finding the mass defect and using E=mc^2.

    part (a.) is more difficult because from the research I've done the is no single pair of isotopes that Pu-239 will always split into. There are dozens of different elements and isotopes it could break into. All I know is that the protons & neutrons on the right side should add up to the left side. My text book only has one chapter on Nuclear Physics and does not give more than a few paragraphs on fission. So it doesn't really give me the information of how to solve this.

    So since there is no one answer for it I thought I would try and find a generic example of a common (most probable) way a Pu atom could fission, and use that. I never could find anything spelled out (unlike for U 235 which I've found many example equations), just some probability graphs from Wikipedia's Fission product yield article. I've tried Xe and Zr or Tc and Sb or Rh and Sn, but I can't find any real isotopes of those that actually add together to balance the equation out. Is there any two isotopes that could be used to "fill in the blanks" in question (a.) or is it more likely to be 3 isotopes and I need to add a "blank" to their equation? Also should I include the energy released from the reaction, like I did with (b.)?

    Thanks for the help.
  2. jcsd
  3. Jul 1, 2009 #2


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    Homework Helper

    Right, well once the numbers add up, all are correct. But if you should check if you are given the masses of certain elements. Because if you are, then you should try to use those elements.

    Why don't you use U-235 and Deuterium [itex]^2 _1 H[/itex] ?
  4. Jul 2, 2009 #3
    Sure, I could just invent some isotopes and call it good but I wanted to find some real isotopes that commonly result from a Pu-239 fission event.

    If I used the products from a U-235 fission equation they wouldn't add up, since I'm supposed to solve it for Pu-239. The things that I guessed might work from the fission products graph add up right for the protons, (each pair adds to 94) but no isotopes of those elements has enough mass to add up to 237. That's looking at wiki's "Fission products (by element)" article.

    I'm not sure what you mean by using deuterium.

    Thanks, Alan
  5. Jul 2, 2009 #4


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    Homework Helper

    well in that case, I guess you have to research like you are doing now

    Left side is 239+1=240

    Right side= 235 + 3(1)=238 if you add [itex]^2 _1 H [/itex] you'd get get the 240.

    and the atomic numbers would add up as well.
  6. Jul 2, 2009 #5
    There's still a missing proton. Uranium is 92, hydrogen is 1... that's 93 but Pu is 94. So like the others that I've tried one part matches (U-235 and D add to 237) but the other doesn't.

    Should I just invent some isotopes then? I guess the graders would have to have a list of possible isotopes a mile long if they were only going to only accept real observed reactions. Maybe they are just checking peoples ability to add???

    I still would like to know if there is a "correct" answer. Could I post this in the High Energy, Nuclear, Particle Physics forum?

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