If the nuke is already in orbit, harvesting it for fissile fuel seems like a sensible way of decommissioning it. They you can power your NTR (or RTG if you must

If the nuke is already in orbit, harvesting it for fissile fuel seems like a sensible way of decommissioning it. They you can power your NTR (or RTG if you must) from its fuel. It'll require some in-orbit metallurgy work, to get it in the proper shape and composition.