It generates electricity using EXOT and ELEC. Read the description on the actual save's webpage for a more complete description.
To be more specific, what keeps your reactor running is the fact that your energy-only wall prevents the WARP inside (which is generated when EXOT explodes, and is a gas, so can't pass through energy walls) from escaping.
Since the WARP from an EXOT explosion instantly corrodes new EXOT into WARP, it perpetuates the explosion when the WARP can't escape and new EXOT is flowing in.
Actually the WARP doesn't do that. It's the ELEC given off by the the CLNE that reacts with EXOT when it flows in as the WARP's life degrades to zero and begins to disappear. Thus generating more WARP from the further reaction s of ELEC and EXOT. If I removed teh CLNE to stop the emission of electrons, the reactor would NOT run forever.