Without quantifying "possible" and "irreparable" this is nothing but fearmongering. What would it take to accidentally or intentionally breach the containment

Without quantifying "possible" and "irreparable" this is nothing but fearmongering. What would it take to accidentally or intentionally breach the containment vault on a reactor? What are the chances of that happening? Ignoring paranoid overreactions, what is the worst ecological damage that could be done? How is it "irreparable"?