They are actually extremely inefficient since they use a thermocouple. They only convert about 5% of the thermal energy to electricity, the other 95% of the hea

They are actually extremely inefficient since they use a thermocouple. They only convert about 5% of the thermal energy to electricity, the other 95% of the heat must be rejected somewhere. A steam turbine nuclear reactor is a Rankine cycle heat engine, their efficiency is above 40%. The reason RTG are used is simplicity and no moving parts, but efficiency greatly suffers. If we could figure out more efficient thermocouple all sorts of opportunities open up, haven't seen much work in this area.