Complex sentence: On these spacecrafts, Pu-238 dioxide fuel undergoes natural decay like in a nuclear reactor to produce heat that SiGe thermocouples/unicouples convert to hundreds of Watts of electrical power.
Split into simpler sentences: On these spacecrafts, Pu-238 dioxide fuel undergoes natural decay. The SiGe thermocouples/unicouples convert this heat to hundreds of Watts of electrical power.