About cells and reflectors I meant crafting of double cells..copper isn't quite fit for that (but I may be mistaken)
Why do you deny new reactor components so..strictly? ~_~
I thought of something - when we stop reactor, we use graphite(?) rods for that. A new reactor chamber crafting component would make sense. Maybe 2 compressed carbon plates per 1 rod and 4 rods per chamber?
About fuels - there is not much to add to nuke reactor.. Some endgame fantasy-based maybe. I think, you can brainstorm your FusionR in matters of innovations, a lot can be added there.
I had an idea, which isn't tech at all and doesn't even fit in your add on, but maybe - some medicine and drugs? (chemistry tech tree is Involved)
Good thing to add is red stone block and some storage for coal dust (maybe, battery-type item?)