The rods are indeed capable of melting the metal sheaths around each rod of Uranium becasue of the way it was shut down. Critical mass could only occur when the exposed fuel then melts and compresses on the floor of the reactor.

A scram shutdown with no coolant circulation can contain enough residual heat to damage the "protective blankets" around the Uranium.

WHile you are technically and perfectly correct Jack, the truth is there remains a possibility of melted fuel that could proceed to criticality..

If you want a number I would put it at 10% or less.

A meltdown would still last indefininately. Far longer than a Gulf oil spill.

Neither alternative is anything less than catastrophic.