A nuclear reactor was a device which used provided power through radiation.
Hundreds of years ago, the humanoid natives of a planet in the universe of superspace used a nuclear reactor to power their civilization. It was left running after the people were forced underground by their war machine.
In 2267, a man landed on the planet in a spacecraft and was attacked by the reactivated war machine. He fled into the ruins of a city and discovered the reactor, which contaminated him with radiation. Realizing that he was going to die anyway, the man set the reactor's controls to overload, causing it to explode and destroy the machine. (TOS comic: "Siege in Superspace")