[zamadatix]

If one is using a nuclear reactor for long term power or propulsion you shouldn't need to be disposing of it in the Earth's vicinity anyways - there is plenty of solar in Earth orbit. Not that peace treaties around nukes will inherently ban reactors.

[rbanffy]

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.