One reason is "the last level of containment held" doesn't give engineers the safety margin they want, or the public-reassuring PR they'd want. Much better to be able to say that the last level of containment is never even relied on, but is purely insurance.
Thank you. If the fission reaction has indeed stopped, I wonder if whatever heat is still being generated (especially if the fuel rods are exposed and enough heat exists to melt the cladding) would still pose some danger to the pressure vessel?
