A tetryon reactor is a type of power generator that produces epsilon radiation.
The Nacene utilized tetryon reactor technology. The Caretaker's array utilized several tetryon reactors. It was believed that they were destroyed with the array, but this later came into question.
It's probable Suspiria's array also utilized tetryon reactors.
Tash's graviton catapult used a tetryon reactor. While assisting him repair the catapult, Seven of Nine suggested that he stole the tetryon reactor from the Caretaker - though this theory was later found to be caused by paranoia, brought on by Seven's use of a cortical processing subunit. (VOY: "The Voyager Conspiracy")
It is worth noting that while Tash didn't seem to have stolen a tetryon reactor, the tractor beam Seven finds in the visual record of the destruction of the Caretaker's array is never explained.