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A How is iodine-131 produced in nuclear reactors?

  1. Dec 19, 2016 #1
    It seems everybody knows that iodine-131 is produced in nuclear reactors, but so far, I couldn't find any example of nuclear reactions starting with U-235 and leading to I-131...!
    Directly, undirectly, going through other fission products, going through beta decay... Nothing. I mean, nothing which starts with U-235 nucleus and ends with I-131 nucleus. Seems to me incredible.
    Has anyone any information ?
  2. jcsd
  3. Dec 19, 2016 #2

    Andrew Mason

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    Hello Andrew1949. Welcome to PF!

    The U235 nucleus will never decay to I131. I131 is produced when U235 undergoes fission in a nuclear reactor. Fission causes the U235 nucleus to split into two parts. The split is somewhat random so you end up with various elements as fission products. I131 makes up about 3% of U235 fission products.

  4. Dec 20, 2016 #3
    Hello Andrew,
    Sure, I131 doesn't come from U235 by decay. But the question still is that I have no example of nuclear reaction leading to I131.
    It could be U235 -> I131 + Xx + n neutrons; but then, what is the X element? ; what is the number n? Giving that we must have : 235 = 131 + x + n.
    It could be U235 -> X + Y + n, and X or Y -> I131, either by nuclear reaction or by decay; but then, what are X and Y?
    What surprises me, is that I found impossible to have, explicitely written, a reaction chain of the form U235 -> ... -> ... -> I131, starting from U235 and eventually leading to I131. I just found vague considerations such as "I131 is formed as a product fission"... I know that. But how?
  5. Dec 20, 2016 #4

    Andrew Mason

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    It could be U235 + n -> X + Y + mn and X could decay to I131 or it I131 could be one of the direct fission products: U235 + n -> X + I131 + mn. It is rather hard to study the actual reaction because it occurs inside the reactor. Why are you wanting to know exactly how the reaction occurs?

  6. Dec 20, 2016 #5
    Because everybody says that I131 is formed in nuclear reactors and is hazardous (so many exercises for students in physics are about Chernobyl or Fukushima and include calculations on Cesium 137 or Iodine 131), but noboby seems to know how I131 is formed in the reactors... It amazes me. And I do want to know.
  7. Dec 20, 2016 #6

    Andrew Mason

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    We know generally how it is formed. It is one of many radioactive elements created in a reactor resulting from the fission of the U235 nucleus, or from the fission of U236 or P239 and possibly other elements (ie. elements heavier than U235 created in the reactor by bombardment with neutrons) and their decay. There may be several pathways.

    Last edited: Dec 20, 2016
  8. Dec 20, 2016 #7
    "There may be several pathways." For sure ! But no one is clearly described... In this table, as an example "https://www-nds.iaea.org/wimsd/fpyield.htm#T2", I-131 is not listed as a possible direct fission product of U-235. So, I-131 does appear, of course, but the chain of reactions leading to it remains a mystery ; isn't it surprizing ?
  9. Dec 20, 2016 #8


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    The table only lists stable or very long-living products. I-131 is an intermediate product not listed there.

    U-235 -> I-131 + Y-xx + a few neutrons is a possible reaction where xx is about 100 (rare process)
    U-235 -> Te-131 + Zr-xx + a few neutrons, and Te-131 decays to I-131 with a half-life of 25 minutes (ground state) or 30 hours (excited state).
    U-235 -> Sb-131 + Nb-xx + a few neutrons, and Sb-131 decays to Te-131 with a half-life of 23 minutes
    and so on.

    Here is a calculator, the production via Sb-131 dominates, but fission to Te-131 and Sn-131 is also relevant.
    Last edited: Dec 20, 2016
  10. Dec 20, 2016 #9
    That's perfect. That's what I was looking for.
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