Also, if plants needed UV light to survive, the would not survive in a greenhous, because glass is a very good UV filter. This is also why you can't sunbathe in a greenhouse, or car, or anything that is under glass.
Regarding the radiation... Even hypothetically speaking, if cacti (or any other plant) were capable of "absorbing" radioactivity of any kind, it would be rather dangerous to keep one, as the plant in question would then "trap" the radioactivity.
In fact, musrooms "trap" radioactivity, or rather they keep the radioactive substances in them for longer than other lifeforms. It is for this reason that it is not advised (in western Europe at any rate) to eat wild mushrooms, as they are still more radioactive than normal after the Chernobyl disaster.
Hope that helps understanding
