Fuel- Pu-238 Is the Radioisotope of Choice for RTGs
Although other radioactive fuels have been considered for RTGs, plutonium-238 (Pu-238) has been used most widely. Pu-238 is a radioactive isotope—a form of plutonium that gives off energy as rays and particles.
It continues to be the radioactive fuel of choice today and in planned future missions.
What qualities make Pu-238 a good choice for fuel in an RTG? Its half-life is one of the most important. Halflife is the time it takes for half of the radioactive material to decay.
Polonium-210, which was used in the early SNAP-3 RTG, has a half-life of 138.4 days. At the end of that time, the amount of radioactive material remaining is half of the original amount. This means there is only half the heat available for conversion into electric energy.
Longer space missions require a radioisotope with a longer half-life. Pu-238, with its half-life of 87.7 years, fills the need. For example, after five years, approximately 96 percent of the original heat output of Pu-23 is still available.
Safety Issues
Because the nuclear fuel in RTGs is radioactive, safety is a critical issue. As it decays, Pu-238 emits radiation mainly in the form of alpha particles, which have a very low penetrating power. Only lightweight shielding is necessary because alpha particles cannot penetrate a sheet of paper. Radioisotopes producing more penetrating radiation, such as beta or gamma particles, would be more difficult to handle safely and would require heavier shielding, a distinct drawback
on space missions.
Smaller and Lighter Power Supply
The weight and volume of solar panels can cause problems on some space missions. With RTGs, weight and volume are far less of a concern. Pu-238 has a relatively high power density, and a given volume or weight of Pu-238 can produce a relatively high number of watts of power for long periods of time. These qualities lead to smaller and lighter heat sources than comparable power levels from other sources. This makes Pu-238 fuel an efficient power producer for the space it occupies and the weight it adds to a mission.
