https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge... Most RTGs use 238Pu Unlike

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge... Most RTGs use 238Pu Unlike the other three isotopes discussed in this section, 238Pu must be specifically synthesized and is not abundant as a nuclear waste product. At present only Russia has maintained consistent 238Pu production[dubious – discuss], while in the US, no more than 50 g (1.8 oz) were produced in total between 2013 and 2018.[15] The US agencies involved desire to begin the production of the material at a rate of 300 to 400 grams (11 to 14 oz) per year. If this plan is funded, the goal would be to set up automation and scale-up processes in order to produce an average of 1.5 kg (3.3 lb) per year by 2025.