I love this mod and it's been working great! One item that I cannot seem to refine, however, is the Portable Fusion Reactor. Is this intentional?

Hi, yes it's intentional - for something to be refinable, all of the ingredients must also be refinable. Uranium-235 currently isn't, so portable fission reactor isn't, and transitively that also means that portable fusion reactor isn't.
The ingredients themselves are still refinable, so that doesn't stop you from also crafting legendary turbo belts relatively easily. Though while the mod discourages it since there's no point in them, being unable to refine uranium-235 is a balance decision that's remained for now

Understood, thank you for the explanation!