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Homework Help: Decay Rates ,Radioactivity ,Half life easy question

  1. Nov 27, 2011 #1
    1. The problem statement, all variables and given/known data
    The radioactive nuclides released into the atmosphere by the Chernobyl nuclear accident
    in 1986 included both 131I (t 1/2= 8 days) and 137Cs (t 1/2= 30 years). Given that 5 times as
    many 137 Cs as 131 I are present in the reactor in steady state, and that 131 I is produced in
    roughly 1% of all fissions, calculate
    (a) the ratio of activities ACs/AI immediately following the explosion.
    (b) how many days before the activities are equal.
    (c) Given that the Chernobyl reactor produced 1000MW of power, estimate AI 4 days
    after the explosion.

    2. Relevant equations
    it would take me considerable amount of effort to find what means 1% of all fissions produce I 131 .

    3. The attempt at a solution

    it is A level to find that λ= ln2/ (t1/2) and A= λΝ as is probably the whole exercise.
    which gives Acs/AI easily since 5NI =Ncs is given,

    b) N=No e^-λt should suffice

    c) maybe among the other equations P=Po e^-λt has to be used once the power has been found 4 days after the accident , some other equation must be used to find AI ,note that AI initial is not given ,but only as a function of Acs.
  2. jcsd
  3. Nov 27, 2011 #2


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    Science Advisor

    AI initial can be calculated given the reactor power level and the fact that "131 I is produced in roughly 1% of all fissions". I suggest you put in the "considerable amount of effort" to figure out what that statement means.
  4. Nov 28, 2011 #3
    What about the other questions.Does anyone know the equation for power?
  5. Nov 28, 2011 #4


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    Science Advisor

    Think of it this way. If the reactor is operating at a certain power level, there are a certain number of fissions/sec, which you should be able to calculate if you know how much energy is released in each fission. The problem tells you what fraction of the fission events produce I-131, so you should be able to calculate the production rate of I-131. In steady state, the production rate equals the decay rate, so you should be able to calculate the steady state activity of I-131. Does this help?
  6. Jan 7, 2012 #5
    im afraid this confused me even more .Am i thinking correct for a) and b) ?
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