[aaronmdjones]

The comment I replied to said decay/interactions. Full control rod insertion stops all interactions, reducing the output power of the reactor by over 90% within typically two to five seconds, and only downhill from there.

You're right that the existing fuel continues to decay (and this produces some heat, which is why you need an operational reactor cooling system even if you've shut it down, in order to prevent a meltdown), but it doesn't produce enough heat to meaningfully produce any power (via a steam turbine), and thus it could be argued that you have successfully stopped the core of the reactor from doing its job faster than you can pick a lock.

Off hand I imagine red-teaming a nuclear power station wouldn't actually go this far; victory would end at demonstrating merely that you could have (e.g. by being in a position and possessing the requisite equipment to compromise a temperature or flow sensor in the cooling system, leading the reactor controller to conclude that the cooling system has failed, triggering an emergency SCRAM).

Still it's interesting to think about.