system
Given the following post, respond as if you're commenting on Reddit:
human
Why is water such an effective radiation shield?

What are the properties that make water(H2O) effective at shielding/absorbing radiation?
gpt
Simply put, it is dense and cheap.

There are lots of different types of radiation - photons, electrons, alpha particles, neutrons, and more exotic types.  A "shield" blocks radiation by interacting with it.  The rate of interaction depends on many things - the energy of the radiation, the composition of the shield, atomic number, etc etc.  But it also depends on the amount of atoms it has to cross.  Water is fairly dense (1 g/mL), therefore you can pack a lot of atoms between it and whatever you want to shield.

Concrete is an effective shield for the same reason.

There are many other reasons why certain materials are good at shielding certain types of radiation.  But if you have a strong, mixed source (like spent nuclear fuel), water is pretty effective as a general shield.