Pu-238—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-238
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.
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