If the perfect quantity of fuel were added to a slightly subcritical mass to create an " exactly critical mass ", fission would be self-sustaining for one neutron generation ( fuel consumption makes the assembly subcritical ).

If the perfect quantity of fuel were added to a slightly subcritical mass, to create a barely supercritical mass, the temperature of the assembly would increase to an initial maximum ( for example: 1 K above the ambient temperature ) and then decrease back to room temperature after a period of time, because fuel consumed during fission brings the assembly back to subcriticality once again.

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