I'd suggest a fission reactor. In my experience, they're a lot simpler to build and automate than HTGRs and, if you're smart, you can make the uranium fuel pellets using 2 AC engines and the proper gearing. Its a bit hard on the steel, and requires a fair amount of indium, cadmium, and silver, but its well worth it. It won't be fully uploaded for another few hours, but (WARNING: SHAMELESS PLUG AHEAD) episode 33 of my LP has me actually going about building a reactor that, later on, can even be used to make tritium. Its not active, but I'm working with one of Reika's own, personal designs, so it should be fairly good. If ya want, the link to my LP is at the top of my sig, expect to see the video before 3-5pm Eastern Time.
[ETA]
Actually, it seems my upload rate has spiked a bit, so my estimate is now off by about two hours. It should online by 2pm EST at the latest.