I couldn't place blocks at either 127 or 128 when doing a nuke test. It was a wool block too, so I doubt it was an added limit.
I do have a rather perfect test environment; effectively 'creative mode' built to see how much load it could induce; still took two users the better part of thanksgiving weekend to build (I don't even want to think about how many months of gathering and fabricating it would take for real). Enough solars to cause noticeable client lag at the moment. I very much look forward to the upgrade to see if that removes the bottleneck from client/server.
Also yes, Nukes are under-powered and short duration in comparison to the possible trickle energy offerings.
Maybe an even more temperamental integral fast flux (waste reducing) reactor that was good for breeding and a 40% chance that cells that would otherwise evaporate at the end of cycle instead become some 'weak isotope' in need of concentration instead of simple re-enrichment.