Winzer, I think Fearless is giving you bad advice. (And would encourage you to read his other posts in this section and the community's reaction to his advice).
Betting one's education on a particular unproven technology being successful is foolish, because by definition, you don't have enough information to make an informed choice - that's what you want the education for. "Unproven" is perhaps a little too polite - "couldn't make it work so the program was canceled - twice" is maybe closer to the truth.
One also needs to look at the issues involved - is the problem that we don't know how to make nuclear reactors on the ground, or is the problem that we don't know how to make them fly? Given that, does it make more sense to study how to build better reactors on the group, or does it make sense to study how to make things fly?
I think you also have to look at potential employers - is NASA or JPL more likely to hire someone with an aero/astro degree or with a nuke E degree? One can make a lot more progress in a field with the right job and the wrong degree than the wrong job and the right degree.