1) plutonium fuel cells can't take prod modules
2) the recipe only produces 1
3) they burn up very very fast, not sure how fast, maybe 30 seconds? certainly much faster than a vanilla uranium fuel cell as comparison
4) getting (enough) plutonium is hard/annoying and very slow
( 5) yes the reprocessing does take prod, but... per default it's 5 -> 1 plutonium if my math it correct so even with some prod it doesn't change a whole lot)

All that means that you basically can't reliably run the reactor. It took hours to get enough plutonium for a few cells which ran the reactor for a few minutes to be able to get a bit of lithium. And nothing more.
Thankfully getting power from it is kinda optional, solar is good enough to launch a rocket (even though you'll be constantly on low power).

But... at first you think getting the reactor is cool and a game changer, but instead I just run it for a short time to get a bit of lithium. Importing a fusion reactor feels worth it (long term, haven't been to Aquilo). Which is silly.
I guess getting the reactor to legendary would help a bit because it would waste less heat, but that's crazy.

In short: I just feel some balance is off there. The burn time could probably be doubled and it would still be hard to run it consistently at this point.
Personally - without doing any more math here and I'm not saying that's what should get changed, those are just some thoughts - I would 1) allow prod for the fuel cell recipe, 2) double the fuel cell recipe output from 1 to 2, 3) double the burn time in the reactor. Something like that.

Getting the reactor is hard enough, getting plutonium is annoying enough. The reactor should feel amazing and not just the means to get lithium (which you only need for lubricant for the electric engines for the rocket silo which is also a bit silly).