In Reactor, you are inside a ship that is trapped inside the heart of a nuclear reactor. While you would like to leave, that is not an option. Positrons, neutrinos and photons make up some of the nuclear particles that are attempting to destroy you. At the same time, the reactor's core is approaching meltdown.
Using your ship as a bumper car, you can ram the particles into the kill walls to destroy them, or into the control rods. When the control rods are hit by a particle with enough force, they will be destroyed. When all of the control rods are destroyed, the reactor's core will temporarily shrink in size and you'll move on to the next level. In later levels the core is replaced with a swirling vortex that can suck your ship in and crush it.
Your ship is also equipped with decoys, which are used to temporarily lead particles away from your ship. You begin the game with three decoys and three lives. An extra life is earned for every 10,000 points, and an extra decoy is awarded each time you destroy both control rods.