Reactor?
I wasn't sure how they would manage this, thinking that a reactor (such as a PWR) needs water/steam, turbines, generators etc to work, and would be an effort to get to the moon, but it seems like they have a few idea on how they want to go about this.
This is why I'm not a rocket surgeon and I'm some IT guy.
https://ntrs.nasa.gov/citations/20120000789
https://ntrs.nasa.gov/citations/20060011236