I will apologize ahead of time but to have answered the question I had to use some technical/science terms.
A definition you will need:
Thermocouple: a kind of thermometer consisting of two wires of different metals that are joined at both ends; one junction is at the temperature to be measured and the other is held at a fixed lower temperature; the current generated in the circuit is proportional to the temperature difference.
This is a great question and I have to admit that I had to do some research in order to answer it because it is different from how nuclear power plants work (which makes total sense). Turns out that Cassini is powered by three...wait for it...radioisotope thermoelectric generators. Big word but here is a simple explanation of what it does: an RTG uses heat from radioactive decay in order to generate electricity by use of thermocouples. In Cassini's case the heat was from the energy coming from the radioactive decay of plutonium. The explanation of the process can get quite technical but I will simply put it that an RTG relies on temperature differences in order to create a voltage.
An RTG consists of the fuel, plutonium, which is kept in some sort of container. Thermocouples are hooked up to the container and the heat produced from the radioactive decay allows for electricity to be generated. You can see the RTGs at the bottom of the Cassini spacecraft on the image below.
RTG's can use fuel other than plutonium (i.e. strontium, polonium, curium, etc.) however, plutonium requires the least amount of lead shielding (2.5mm) in order to contain the radiation. And when you are designing a spacecraft, the weight of everything on board is extremely important. Plutonium also had an optimal half-life (87 years) for the mission...meaning that it should have a continuous release rate of energy for an optimal amount of time (in Cassini's case it needed to power for at least 11 years).
I hope this helps!
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