The reactor is a closed system which uses the Element 115 as its fuel. The element is also the source of the gravity-A wave which is amplified for space/time distortion and travel.
The disc is one of nine, given to the American government in an "exchange" program in the early 1970's. The makers of the craft and providers of the fuel were from the Zeta Reticuli star system. What we exchanged for the technology is not known. A back engineering program began in 1979 of the remaining hardware and technology.
The power source is a reactor which uses element 115 as its fuel. In this reactor element 115 is used as a target and is bombarded with protons in a small, highly sophisticated particle accelerator. When a proton fuses into the nucleus of an atom of 115, it is transmuted and becomes an atom of element 116. Although we too can transmute elements here on earth, it is typically not done in this fashion, or at anywhere near this level of efficiency. Furthermore, we have yet to produce anything heavier than element 112.
As soon as each atom of 115 is transmuted into 116, it immediately decays and produces a radiation unlike that which we normally observe in nuclear decay. Each atom of element 116 decays and releases two antiprotons (anti-hydrogen), a form of antimatter. Antimatter can be produced in particle accelerators here on earth, but only in minute quantities and only stored for short periods of time.
The flux of antimatter particles produced in the reactor are channeled down an evacuated, tuned tube (which keeps it from contacting with the matter that surrounds it) and reacted with a gaseous matter target. This Total Annihilation reaction is the most efficient and energetic nuclear reaction there is. The more familiar nuclear reactions are Fission, producing energy from the splitting of atoms as used in nuclear reactors & atomic bombs, and Fusion, the fusing or combining of atoms (typically hydrogen nuclei) to release even more energy. Fusion is the reaction that powers the sun and other stars and is what gives hydrogen bombs their "punch". These two more common nuclear reactions are dwarfed by the power and efficiency of the annihilation reaction used in the alien reactor.
The reaction between the gaseous matter target and the antimatter particles produces a continuos release of tremendous amounts of heat. This heat is converted directly into electricity by the use of a thermionic Generator. The Thermeonic generator used in this reactor is so efficient, that there is no detectable waste heat produced. This is an apparent violation of one of the basic laws of thermodynamics. Similar, but not nearly as efficient or powerful, thermionic generators are used as power sources in our satellites and space probes.
As amazing and efficient as all this seems, it is only secondary to the primary function of the reactor. The antiparticle flux emitted from the transmuting element 115 is not the only energy radiated during operation. This is the point at which the gravity A wave is first produced. The gravity wave emitted by the 115 reaction appears on the hemisphere of the reactor, propagating up the tuned waveguide in a fashion very similar to the way microwaves behave.
All of the actions and reactions inside the reactor are orchestrated perfectly like a tiny little ballet, and in this manner, the reactor provides an enormous amount of power used to amplify the gravity A wave so it can cause the requisite space/time distortion for space travel..