> the TMI plant was producing about 6% of its output power when it was put into shutdown, which is about 50MW - i.e. the full power of a much smaller Navy reactor.

Are you taking heat density into account? An analogy comes to mind from summer outdoor-grilling season: A tiny chunk of glowing-hot charcoal doesn't produce nearly as much heat as does a bonfire, but the chunk of charcoal will still burn your hand pretty catastrophically.