Want a Master's in nuclear engineering... what should I get my Bachelor's in?

UMD offers a minor in Nuclear Engineering as part of the ME program.

http://www.umd.edu/catalog/index.cfm/show/content.section/c/127/s/1826

If you’re interested in nuclear fuel management (or just materials in general), then you may want to look at Materials Science Engineering (and check to see if they would approve a NE minor with a MSE degree).