What creates radiation zones?

[Captain]

I got the achievement hazardous environment, but I didn't leave any fuel cells?
Is there something else I can do to make it more radioactive in the future?
What can I do to prevent it, can I go back in time and fix it somehow?
Thanks.

[Lost in Days Studio]

it might have been that you dropped a fuel cell some time in the past, it will create a green cloud