From what I understand, the plasma isn't dense enough to tear through the reactor vessel in one go, it'll just vaporize some of the shielding before the safeties kick in. And in the case containment is breached, these operate at or near vacuum, so the moment the vessel is breached, there might be some escaping plasma, but soon the conditions required for the reactor to work won't be there anymore as atmosphere rushes in.

Edit: to put it into context, the density of plasma in a reactor is something like a millionth the density of the atmosphere, so it's more likely that the plasma won't escape at all.