The game Reactor is about stopping a nuclear reactor from melting down. The player's ship is inside the reactor and must avoid being hit by positrons, neutrinos, and photons that will try to knock it into the deadly kill wall. To win, the player must make these particles crash into the wall. As levels progress, the reactor core grows in size, making it harder for the ship to move around.
The gameplay involves knocking particles into the kill wall or using decoys to distract them. The ship starts with a limited number of decoys, which can be placed near the kill wall to make it easier to destroy the particles. Destroying control rods on either side of the screen also helps by reducing the reactor core's size and giving the player more time to play.