SNAP-27 APOLLO RADIOISOTOPE THERMOELECTRIC GENERATOR AND FUEL CASK
The SNAP-27 radioisotope thermoelectric generator is one of several atomic-powered electric generators developed jointly by the National Aeronautics and Space Administration and the Atomic Energy Commission. (SNAP stands for Systems Nuclear Auxiliary Power.) It is fueled by plutonium-238. The fuel capsule, containing 3.8 kilograms (8.36 pounds) of fuel, was carried to the Moon in a separate Fuel Cask attached to the side of the Lunar Module. The fuel cask provided thermal insulation and added structural support to the fuel capsule. On the Moon, the Lunar Module pilot removed the fuel capsule from the cask and inserted it in the RTG.
The SNAP-27 Radioisotope Thermoelectric Generator (RTG) converted the heat energy from radioactive decay to electrical energy. Five such RTGs provided electric power for the Apollo Lunar Surface Experiment Packages (ALSEP) left on the Moon by Apollos 12, 14, 15, 16 and 17. These stations transmitted information about moonquakes and meteor impacts, lunar magnetic and gravitational fields, the Moon's internal temperature, and the Moon's atmosphere for several years after the missions. After ten years, a SNAP-27 still produces more than 90% of its initial output of 70 Watts.
Isotopes are forms of an element that are chemically the same but have different atomic masses. Some isotopes are unstable. Unstable isotopes can be radioactive-they radiate energy and eject particles from their nuclei until they reach a stable form. For instance, an atom of plutonium-238 radiates a helium nucleus 2 protons and 2 neutrons -- to become uranium-234. The collisions of these helium nuclei with the walls of the fuel chamber heat the inside of the SNAP-27 to 600oC (1100oF). This heat is converted to electrical energy by the thermoelectric element.
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