I know, stupid question, but is radiation, and its negative effects, in the game yet? The wiki says it is not but I think it may be out of date.

No, it is not - chances are it won't be until 1.13, as there's quite a lot on the to-do list :)

Aww, I am really looking for a mod with radiation. Will you be back porting to 1.12?

Probably not, no - I don't really have the time to build for two versions simultaneously :(

@DonMegel Radiation was added in v2.11b :)