If you think about that, a lot of fuel for in-space nuclear reactors will already have been launched, so, if a new peace treaty outlaws them, it'll be a boon to whoever operates fission reactors in space. Or wants to use them for propulsion.
Once in space, they can't be disposed of - deorbiting is a big no-no, as it's blowing them up.
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.
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.
If you think about that, a lot of fuel for in-space nuclear reactors will already have been launched, so, if a new peace treaty outlaws them, it'll be a boon to whoever operates fission reactors in space. Or wants to use them for propulsion.
Once in space, they can't be disposed of - deorbiting is a big no-no, as it's blowing them up.