Reed College, an undergraduate institution in Portland, Oregon, maintains a research reactor. The FAQ says that tours can be arranged for groups; the page does not say if an individual may join.
My son attended Reed 2000-2004, and I took a tour during that time. Cherenkov Radiation was indeed visible, and interesting!
As you only have limited time and it is likely a busy time in winter, I think prebooking tours at this time of year makes sense. But when I was there in September, a few years back, I waited till I was there and could book in one of the many offices.
Whether regular buses or a rental car is suitable for you is more a matter of opinion and I do not answer ...
The Technical University of Munich operates the research reactor FRM II (official website).
It is a very small reactor with only a single fuel rod (fotos, panorama), so there is not much Cherenkov radiation to see though. Furthermore, sometimes it is shutdown for maintenance. Nevertheless a visit is certainly worth it.
They offer guided tours, mostly to ...