Situation 1 (far-fetched): Pipe breaks, gas is still flowing right until the break. Inert gas flows out, and air flows in. When the system reaches 1600°C (which would admittedly take awhile: days/weeks?) the coating starts to degrade. It should be fairly easy for them to limit the amount of flow after a break though. Situation 2 (More likely): By either an act of terrorism, or some other accident, the containment vessel is ruptured a exposed to a fire. Basically you can crash a jetliner into a nuclear plant and still have a good chance at containment. A large truck bomb could take this out.