All substances "trap radioactivity" by simply absorbing the radiation. The question is always how much stuff you need to put between yourself and the radiation source to be shielded from it. I find this

SFR is located 55 metres below the bottom of the sea and is accessed from a tunnel opening on the edge of the vast man-made cooling lagoon outside Forsmark nuclear power plant (in which it's damn nice to bath in the summer as the water is 30 degrees hot).

positively irresponsible so I took it as a joke for techies. You definitely do not want to swim in a nuclear power plant's cooling lagoon [geiger counter readings and chemical composition of the water might convince me otherwise], or in the Clab pool.


Nothing is 'mere'. — Richard P. Feynman