Revenge is a pretty powerful motive. Running with that idea, I've got a character for your nuclear-sabotaging antagonist:
He lives in the vicinity of the facility and has been campaigning to get the plant shut down. But when he (or someone he cares about) develops cancer - he discovers the facility is purposely exposing its neighbors to harmful radioactive chemicals as part of a top secret study.
Outraged, he sets out to shut the facility down.
You could then throw in a twist that he only realises at the very end that the trial the facility was conducting was part of groundbreaking research that could have actually developed a cure for cancer - and that, while unethical to test on people that hadn't consented, his actions now mean the entire thing is in jeopardy...