This AAPG-EMD Special Publication (Memoir 101) is a comprehensive and integrated review of energy resources in the Solar System, including materials that could both sustain future manned expeditions as well as meet Earth energy and mineral challenges in the 21st century and beyond. It has been prepared by a group of well-known industry, government, and university geologists and engineers, including NASA astronauts and other professionals from a range of geotechnical specialties. Any long-range program of human exploration and settlement of the solar system must consider in-situ resource utilization, the vital role that extraterrestrial energy minerals and related resources must also support human civilization on Earth as well as on nearby worlds of the Moon, Mars and the Near-Earth Asteroids. See the assembly of Memoir 101 Chapters: http://web.i2massociates.com/search_resource.php?search_value=EMD+Memoir+101.
RTGs harness heat generated by the radioactive decay of plutonium-238 dioxide, which can be refined from the by-products of the enrichment process required to produce a heavier, more fissile isotope, plutonium-239, for nuclear weapons. The lighter isotope produces very little gamma radiation, which means it requires relatively little shielding in a spacecraft.
More more on RTGs see Chapter 9: http://i2massociates.com/downloads/Memoir101-CHAPTER09Rev.pdf