Is there any way to make Fission Pellets and Fission Fragments less ambiguous? For instance, the Casaba antimatter engine, according to its right-click menu, burns "Fi P". The tank that holds this however, is labelled a "Nuclear Pellet" tank. It also doesn't help that another tank (which looks quite similar mind you) called the fissionable tank holds "Frag" which is burned by the Asimov engine. It also definitely doesn't help that B9PartSwitch labels Frag as "Fissionable Particles" and there is an entirely separate other tank for "Fi P" which I guess is pellets. Also the Vulcan smelter labels Fi P (pellets) as FiPellets and Frag/FissionableParticles as FiParticles. I'm not quite sure what my main point is, but I think one could make it a lot more clear / newbie-friendly. Not a big issue tho, and I won't complain if you don't fix it
There is not, to my knowledge, a way to make NUK. (Though I'm not quite sure why you would pick this engine; it has rather low efficiency, high cost to run, and as you said, no ISRU refueling capabilities. I'd recommend a less powerful/efficient engine that can be refueled, or just using a terrifyingly efficient engine like the X-6 Clarke if your up for it. These are just my two cents though, you do you! )