In the UK, large stocks of civil nuclear waste contain significant quantities of americium-241. That makes the fuel not only long-lasting but also readily accessible. Instead of building new reactors to produce plutonium, agencies can extract Americium from existing waste, a form of recycling at a planetary scale.

Using it seems way more preferable to just letting it sit in casks.

Traditional RTGs utilize thermoelectrics, which are reliable but inefficient, often achieving only five percent efficiency. Stirling engines can convert heat to electricity with an efficiency of 25 percent or more. [...] Stirling engines introduce moving parts, which also raises reliability concerns in space. However, Americium’s steady heat output enables RTG designs with multiple Stirling converters operating in tandem. If one fails, the others compensate, preserving power output.

That seems a little ridiculous though. All that friction requires a lube that'll last "generations." In space, without gravity, and at incredibly low temperatures.