The technology is nothing new, the application is. I believe the last nuclear powered aircraft flew in 1957 and the program was canceled in 1961 [scientificamerican.com]

Just speculating outside of my area of expertise.
Let's consider the development of the traditional fission reactors. First we built huge, land based reactors and later submarine based ones. Never heard of an airplane or rocket- based one.
We were unable to sustain a fusion reaction exceeding a few milliseconds under the best, laboratory, land based conditions. No weight and size limitations, etc. From a strictly technical, engineering point of view I find it very unlikely that someone would start to work with fusion on a missile or rocket. Just consider, how complicated it is to contain the fusion pallets in a lab! Now, try to do the same under the extreme vibration of a rocket engine.

Isn't that cute, Donald Trump is claiming he knows something.

Donny Dipshit is dangerously stupid, but his supporters are worse.

I don't think Donald has a clue of what technology the US has. People in the know would be afraid to show him.