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If one of my modern-day characters, a nuclear power-plant worker decides to take a swim in one of these pools, what will happen?

Would he die, get sick, or not much?

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    $\begingroup$ Solved by XKCD what-if.xkcd.com/29 It's the first result on google when searching your question title. $\endgroup$ – Trevor Jan 31 at 17:11
  • $\begingroup$ I personally feel that even though xkcd has addressed this, it's still worth answering this question. The answer given there, while awesome, is largely of non-quantitative and only talks about one specific incident. There's more data out there to use. (For what it's worth, I've tried to do this in my answer, but I'm still sure there's even more information out there that could be useful.) $\endgroup$ – HDE 226868 Jan 31 at 17:47
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    $\begingroup$ @HDE226868 Seems pretty broad to me. What will happen? Anything could happen. And question shows no research effort prior to asking $\endgroup$ – Trevor Jan 31 at 17:51
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    $\begingroup$ Don't forget XKCD's ultimate answer to the question: "The security guards will shoot you as you try to get into the pool....." $\endgroup$ – Thucydides Jan 31 at 23:15
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    $\begingroup$ @TrevorD I could agree with you about not enough research effort, but not sure I see the too broad aspect. Either the diver will get sick (and maybe die), or not, and using a typical pool, we can figure that out for most reasonable scenarios. $\endgroup$ – HDE 226868 Feb 1 at 17:52
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xkcd covered this a while back, actually. It turns out that water is an excellent radiation shield (which is one reason the spent fuel is put in them in the first place). This means that if you swim near the top of a pool, you're going to be just fine (and might, in fact, receive an even lower dose of radiation than if you stood around outside the pool).

The amount of shielding from, say, gamma rays depends on something called the half-value layer. A slab of material of this thickness will reduce the amount of radiation traveling through it by half. For water, the half-value layer depends on the energy of the radiation. Say a pool is 12 meters deep. For, say, gamma rays, we have

$$\begin{array}{|c|c|c|c|}\hline \text{Energy (keV)} & \text{Half-value layer (cm)}\\\hline 100 & 4.15\\\hline 200 & 5.1\\\hline 500 & 7.15\\\hline \end{array}$$ At the surface, the fractional intensity is tiny.

That said, if you go close to the fuel, yes, you can and will get sick and perhaps die. Water doesn't stop radiation from reaching you; it just decreases the dosage with increasing depth. If you swim a few feet away from the rods, you'll indeed get a high dosage, which could kill you. This has happened on multiple occasions, although it appears that the affected divers survived because emergency measures were immediately taken.

The risk increases with repeated dives, and so divers who work at these sites have their radiation levels monitored. Their exposure in each dive may be small (on the order of a few millirem), but over the years, that can cause problems. For this reason, they're limited to 2000 millirems per year.

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