Honestly the best way to handle that would probably be to simply not mention it.
If directly asked about it, just start spewing jargon at them until they get bored and walk away. Just don't give them something that would make a good soundbite.
That sounds like it would backfire horribly. Honestly, it's a lose-lose situation. If you didn't mention it, they'd spin it as some sort of cover-up; if you spewed jargon at them, they'd just take whatever snippet sounded good to them and spin it into something unrecognizable that fits their agenda. Journalists never let truth get in the way of a good story.
You would need to pick your jargon carefully. The words that tend to set people off are pretty particular. Just avoid words like "radioactive", "isotope", any element name, "nuclear", etc. Don't call it a "reactor", call it a "thermal generator". Be honest and open about everything, but don't brag about it on twitter or in press releases.
The idea is to act like it is nothing special to give off the impression that it is nothing special. Make it all seem very routine and boring. Hyping actually boring aspects of the mission could also help. Do lots of things like making twitter accounts for rocks: https://twitter.com/N165Mars
Well, just look at the kerfuffle over the Cassini RTG back in 1997:
Reporter: "Could this probe fail on launch, re-enter the atmosphere, and explode, showering the U.S. with radioactive particles?"
Scientist: "Yes, but there's a one-in-a-million chance of that happening, and we have safeguards against that..."
Headline: "SPACE PROBE COULD EXPLODE AND SHOWER RADIATION EVERYWHERE!"
The key here is the "Yes, but..." -- if a reporter wants a sensational story, they'll find a way to get it, even if they have to play fast and loose with the truth to do it.
(Thanks for that Twitter account link -- I love NASA's sense of humor.)
Yeah, they really botched Cassini. However with the experiance they gained from that, and a healthy awareness that PR issues like that can happen in the first place, I think they could overcome any issues they might run into. The answer they gave was correct, and they were correct to not be concerned. They know RTGs are safe, but they forgot that the public doesn't know that.
If directly asked about it, just start spewing jargon at them until they get bored and walk away. Just don't give them something that would make a good soundbite.