The Outer Space Treaty restricts nuclear weapons and WMDs specifically. Nobody's bothered to figure out if reactors count as weapons or not (I'm betting on not), even though a sufficiently damaged reactor in orbit is basically a slow-motion dirty bomb, because nobody builds reactors for use in space. There's been maybe one or two very tiny reactors, and a bunch of RTGs (generators powered by heat from decay, rather than artificial fission reactions), none of which come anywhere near the scale they're proposing here. No need to legally get ahead of technology that doesn't yet, and may not ever exist.