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Well, we don't know exactly why the reactor was manually shut down. Did the person actually know that the control rods had dropped? Or was it reported by sensors? I suspect the procedure us to initiate shutdown in response to any reports of uncommanded rod behavior. I doubt they would wait around to see if this was a sensor or reporting system error.

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The dropped rods already shut down the reactor in the sense that it is sub critical. It still has enough leftover heat that it will continue to produce steam for a while, but it would have been almost instantly clear to the operators what had happened.

[Correction] as noted below, this type of reactor -- unlike US Navy ones -- can remain critical even with dropped rods. So a better statement here would be that the operators would immediately notice a reduction in criticality. thus power and heat. Then it's a process/policy question about shutdown timing.

Navy reactors are physically tiny. A rod in a reactor the size of a garbage can means much more than a rod in a reactor the size of a studio apartment.

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