I've heard that if you swim into a nuclear reactor spent fuel containment pool, you will actually measure the least radiation somewhere in the middle of the pool, thanks to shielding from the universe.
But just to be sure, I got in touch with a friend of mine who works at a research reactor, and asked him what he thought would happen to you if you tried to swim in their radiation containment pool.
“In our reactor?” He thought about it for a moment. “You’d die pretty quickly, before reaching the water, from gunshot wounds.”
I know someone who worked at a medical isotope reactor and there were no guns there, at least not in the reactor area. Security tends to be much much further outside ;)
After all, remember some things in a reactor don't react well to bulletsch.