This question comes from Jonathan, who asks: What if I took a swim in a typical spent nuclear fuel pool?

Would I need to dive to actually experience a fatal amount of radiation? How long could I safely stay at the surface?

The short answer is that, assuming you're a reasonably good swimmer, you could probably tread water anywhere from 10 to 40 hours.

At that point, you would black out from fatigue and drown. This is also true for a pool without nuclear fuel in the bottom.

And it's not really the question Jonathan was asking. Spent fuel from nuclear reactors is indeed highly radioactive, and it's also hot.

Water is good for both radiation shielding and for cooling, so fuel is stored at the bottom of pools for a couple decades until it's inert enough to be moved into dry casks.

We haven't really agreed on where to put spent nuclear fuel yet. One of these days we should probably figure that out. But Jonathan was asking about swimming in the pool.

Here's the geometry of a typical fuel storage pool. The heat from the fuel wouldn't be a big problem.

The water temperature in a fuel pool can in theory go as high as 50°C, but in practice they're typically between 25°C and 35°C –warmer than most pools but cooler than a hot tub. But Jonathan was asking specifically about the effects of radiation.

For the kinds of radiation coming off spent nuclear fuel, every 7 centimeters of water cuts the amount of radiation in half.