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HELP! I'm not sure how to do this problem.

  1. Apr 22, 2007 #1
    Alright... this question is on my homework for my accelerated physics class. I have a test tomorrow and I'm lost on how to do it so
    Here is the question:

    A very small sample of fuel for a nuclear fission reactor contains .012 kg of Uranium-235. Suppose that each fission event from this fuel produces Barium-141 and Krypton-92, along with 3 additional neutrons. The nuclear reactor is then able to provide power to homes, which uses an average of 7.2x10^9 J of energy every month.
    a) Determine the number of fission events possible from this fuel sample.
    b)How long could the Uranium-235 fuel sample provide power for the average home?

    Uranium-235 = 235.0439 u
    Barium-141 = 140.9144 u
    Krypton-92 = 91.92515 u

    Here are some conversion factors:
    1 u = 1.66x10^-27 kg

    I'm not exactly sure what equation to use and how to use it. Here is the equation
    E = mc^2

    This will sound weird but I already know the answers.. Our teacher gives us the answers but she only takes points for the work and plus I need to know how to do it for the test. Here are the answers:
    b)119 months

    I would really appreciate it if you could help me out and give me pretty good detail so I can figure it out. Thanks!
  2. jcsd
  3. Apr 23, 2007 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Part a) is fairly simple in that you know the mass of a single uranium-235 atom and you know the mass of U-235 in the fuel rod. If each atom is one fisiion event then all you have to do is find how many atoms constitute a mass of 0.012Kg.

    For part b) you have to find the mass defect between the parent nucleus and the products of fission. You can then find out how much energy a single fission event releases and multiply it by how many events there are. Then you will compare it by the energy consumed per month by the households.
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