system
Given the following post, respond as if you're commenting on Reddit:
human
How can a material remain radioactive after it has been exposed to radiation?

I was watching TV today and the topic of the Fukushima nuclear accident was brought up on the news and the reporter had mentioned "radioactive water" which got that way from exposure to radiation from the Fukushima nuclear plant. I just wanted to know how a substance, such as water, can remain radioactive just because it was exposed to either alpha/beta/gamma radiation. What is the mechanism that makes this work? For example, if I expose a glass of water to a bunch of xrays or gamma rays, why is that water no longer safe to drink? How does radiation stay within the substance?
gpt
It's important to recognize the difference between radiation, and radioactive material.

Radiation is the particles and waves which are emitted, which can be ionizing or non-ionizing. 

Radioactive materials are atoms which are unstable and emit radiation.

Some types of radiation can cause other materials to be radioactive. But in the case of Fukushima, the water itself is not what is radioactive. The water came into contact with the melted core material. The melted core material IS radioactive. Some of the melted core material dissolves in the water, and now the water carries the melted core material with it. So the water itself is clean, but it has dissolved radioactive materials and particles with it.

If you were to use ion-exchange and other types of filtration to separate the radiaoctive particles from the water, the water would come out clean and virtually non-radioactive.