It's all based upon two things:
1. Critical mass/critical geometry resulting from any shifting of material causing a mass of fissile material to undergo fission again.
2. Decay heat from the fissile material's fission products. They'll continue to produce heat at about 7% of the total power levels immediately prior to the stopping of the fission process. Those fission products will continue to produce heat dependent upon their half-life. That heat needs to be cooled somehow - and that may involve doing things that could inadvertently result in additional fission reactions occurring.
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