This answer by PearsonArtPhoto mentions for spacecraft powered by an RTG there's "only a single rocket that can launch them (Atlas rockets)". Of course that probably doesn't apply to non-US launches, but the most recent nuclear-powered US spacecraft weren't all flown atop an Atlas:
- Galileo on STS-34 and Ulysses on STS-41
- Mars Pathfinder and both MERs (with RTG-like heaters) on Delta II
- Cassini on Titan IV-Centaur
- New Horizons and Curiosity on Atlas V
Now, given that the former three launch systems are all retired nowadays, it would look like the Atlas V is the only remaining option for anything, no matter if it carries an RTG or not. But then what about the Falcons or Delta IV Heavy (or possibly Ariane)?
Are there really some additional rules or regulations limiting the choice of a launcher for an RTG-powered spacecraft?
- Political/national security issues prohibiting the use of a non-governmental LV?
- Launch safety requirements requiring a "RTG-rated" launch vehicle?
- Technical reasons (the need to be able to install the RTG shortly before launch)?