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Nuclear reactor

  1. May 11, 2013 #1
    I think about an interesting question - How many people can kill unshielded nuclear reactor in one second? Some averages, estimates ... Do you have any idea? And why? On what is it dependent?Thanks.
  2. jcsd
  3. May 11, 2013 #2


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    Most reactors have some shielding, if for no other reason than to contain the cooling fluid which is at some pressure above atmospheric pressure. Otherwise, shielding is incorporated in a design precisely to protect persons from radiation exposure, as well as high temperatures.

    It would take a high level of radiation to kill a person in one second, perhaps several tens of thousands of rads.

    The amount of radiation available depends on the source strength and distance. For a localized source of radation, the dose rate decreases roughly proportional to the distance squared.
  4. May 11, 2013 #3
    As the radiation increases so the minutes one has left to live decrease easy as that.
    Under normal conditions of operating I think no one would die in a long enough time of measurement.

    Look up Chernobyl accident , even though after the blast the reactor was more like a fast reacting nuclear bomb , sources say some persons survived 15 to 20 minutes and then died from the mind blowing amounts of radioactivity that was around the site after the accident and for many days to come.
    But those are not normal operating conditions so normally nothing like that happens.
  5. May 11, 2013 #4
    As others are saying, it depends heavily on the source term and other things.

    For large power reactors, even a handful of fresh fully unshielded fuel bundles can kill in minutes, as they can be well over 1 million Rem/hr (1E4 Sv/hr) on contact. A few feet of water greatly reduces this though, as every foot roughly reduces it by an order of magnitude.
  6. May 11, 2013 #5
    it depends on the surface area of your body too, fatter person takes greater radiation than thinner one if he was to stand infront of a radioactive source. For the sake of arguement, if someone was to stand in the middle of a nuclear reactor (between fuel rode) which is operating like a normal reactor, a radiation intake for one second is high enough to be considered a lethal dose. (In addition to the parameters that were mentioned by the guys before me the type of the reactor should be considered too when you want to make actual calculations, as a PWR has high radiation density than a BWR or AGR, plus the enrichement of the rods, etc).
  7. May 14, 2013 #6
    In operating reactor radiation flux is strong enough to generate ~50kW in every kilogram of fuel - after all, the reactor heats up exclusively because of the radiation, nothing else.
    50kW is approximately the power of 25 electric kitchen kettles.

    A human body in conditions like these would be cooked (as in "boiled meat") in a few seconds.
  8. May 14, 2013 #7
    Operating reactors' fuel emits radiation at about 50000 Gy/s = 180 million Grays/hour. A lot of it is absorbed - fission fragmens don't leave fuel pellets, all betas are absorbed, most neutrons are absorbed, many gammas are absorbed - but not all.

    And 180 million Gy/h is an enormous number.

    20 cm thick steel walls of PWRs attenuate hard gammas ony by about 250 times. The gamma radiation directly at the wall of the operating PWR should be seriously high, and some neutrons will also be there. Stab in the dark: tens of thousands of Grays/h?
  9. May 14, 2013 #8

    haha, tell me about it. I was considering nuclear radiation only, that's why I put the first sentence "for the sake of argument" :smile:
  10. May 14, 2013 #9
    That's what heats up the reactor - radiation. There are no other heat sources.

    (It's debatable whether recoiling fission fragments are "radiation", but if fast-moving He-4 nucleus is considered to be a kind of radiation, then they can be too)
  11. May 14, 2013 #10
  12. May 14, 2013 #11
    The Energy that heats up the reactor causes the water (or whatever inside) to evaporate comes mainly from the energy difference between uranium and the fission daughter atoms. The thread initiator I believe is considering radiation from gamma, neutrons, alpha, beta etc and their effect on human. (I actually don't like saying radiation cause gamma, neutrons and alpha are also energy)

    Nuclear fission Eq
    (235,92)U + (1,0)n-->(A1,Z1)F1 + (A2,Z2)F2 + X(1,0)n+Y+Energy
    Where X is an integer, Y is gamma.
  13. May 14, 2013 #12
    And that energy takes multiple forms. Some of it is gamma. The majority of it is kinetic energy from the fission fragments as they are bouncing around after being fissioned.
  14. May 15, 2013 #13
    Yes. Fission fragments are flying at several % of speed of light, so they are a "radiation" of sorts. And just like other heavy kind of radiation, alpha, they aren't penetrating.

    And, yes. Fission fragments' kinetic energy is 90% of total fission energy, gammas and (delayed) betas (and their attendant secondary gammas) are no more than 10% of total released energy.

    Even 10% of energy of operating reactor, when expressed as radiation dose, is enormous and far higher than anything survivable even for one second. Heck, even 1% is promptly lethal (it is about 500 Gy = ~50000 rem).
  15. May 15, 2013 #14
    I find this topic a bit strange. It only makes sense to me if viewed from a "I want to figure out the orders of magnitude of radiation fields in an operating reactor" POV.

    The idea of exposing people to such fields, even theoretical, is deeply unfunny. It's akin to asking "how quickly would a human die if thrown into a boiling water?". Stupid.
  16. May 15, 2013 #15
    in the eq I represented the gamma (Y) on its own, and the Kinetic energy is represented mostly by the energy term.
  17. May 15, 2013 #16
    That's what I meant, and I believe the question was all about the 10%
  18. May 23, 2013 #17
    In order to kill in 1 second you'd probably have to have a high enough dose rate to cause massive organ system damage to the heart, nervous system, etc. The threshold dose for nerve damage is about 50 Gy (depending on who you source), and around 40 Gy for damage to the heart, so I imagine 200 Gy in 1 second would instantly kill. Of course, this is just a best guess.
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