i think maybe short lived isotopes at least a year but have to be cheap but none are affordable look em up sr90 po210 cs137 co60 cm244 cm242 pm147 ru106 pu238 ce144 pm147 sr90 last 30 yrs only 7200 but po210 2/5 a year same price only two grams (more than enough)
only option is european breeders (reactor for the pupose of fallout) us could make some they tried in the early 21st centuary but gave up on it
and try using metric ur reali confusing
its actualy around 170,000 cars 340,000 rtg each car has two( i used a calculator this time ) one year of mass production on ford level