The reactor room of Nunton Experimental Complex contained the facility's reactor core .
Under the Kastrian's influence, Sarah Jane Smith took the hand of Eldrad into the reactor room where it absorbed the radiation to restore itself, keeping Sarah free from harm as it did so.
The Fourth Doctor gained access to the room by passing through a cooling duct from cooling systems control. He pacified Sarah and she dropped Eldrad's ring.
When Driscoll was sent to collect the hand, he discovered the ring and came under Eldrad's influence, leading him to return to the room with the hand and carry it into the reactor core. Driscoll was vaporised by the radiation and Eldrad was fully restored. (TV: The Hand of Fear)