Yes 5 RTG pellets per radio isotope generator, but it is 16 EU/t forever. You would end up having to make a bunch of them, but they run from the leftovers of running reactors, so you need to being running reactors for a while to get them. Kind of a nice bonus to a semi limited resource.
If the goal is RTG pellets, it's pretty easy to maximize fuel usage in a reactor. I ended up putting in 9 cells at a time with 9 heat vents so I'd get 1 plutonium per cycle. I'm sure there's more rapid ways of wasting fuel for rtg pellets, but I didn't have a lot of spare iron lying around for more fiddly reactor bits.