It is fueled by plutonium-238. The fuel cask provided thermal insulation and added structural support to the fuel capsule. 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.
Some isotopes are unstable.
Unstable isotopes can be radioactive-they radiate energy and eject particles from their nuclei until they reach a stable form. This heat is converted to electrical energy by the thermoelectric element.