"Crucible energy core" is a title based upon conjecture.
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The Crucible energy core was the power source of the Crucible. The core consisted of Z-Neutrino energy.
A shaft ran from the vessel's bridge down to the core. The TARDIS, its defences taken down, was dropped into this shaft and fell into the core. The newly-created Meta-Crisis Tenth Doctor dematerialised the TARDIS before it could be destroyed by the core. (TV: Journey's End)