The loaders don't load nuclear fuel to furnaces. They work for rocket fuel but not Nuclear Fuel. The loader storage chest which unloads fuel is empty. The loaders just don't connect to furnaces.

I don't know if it's related, but I have Burner Fuel Bonus, which changes crafting speed depending on the fuel's quality. From what I see, it replaces furnaces and burner inserts with different entities, which might be the root of this problem, but if this was a mod conflict, the loaders wouldn't work at all. Also, when they run out of fuel they revert to their vanilla stats.

Hi Necril! Thanks for reporting this to me, even though I'm about a month late in responding.

I'm not sure why this should be the case, but I'll definitely take a look at it. I know for certain that nuclear fuel works in other entities, but perhaps some obscure requirement in my code is excluding this specific use case.

I'll let you know if I can reproduce the bug and come up with a fix. As always, let me know if you have any more issues.