What's the goal of your rebuild (RPing isn't really a build goal (imo), because pretty much any build could be explained away), general teaming, Running TFs (Exemping), End Content, Solo, other challenges, etc...? I think that will help a lot with the advise given. Gobbledegook's build is a good one, but some of the power selections come late in the build if you're looking at running lower TFs or teaming with friends/SGmates that are running lower content. And, I'd agree overall with what others have said, I also find Energy Mastery to be a good Epic choice for Rad Tanks. I do however run Mu Mastery on a number of them as well. I have build for 45%+ Melee Defense on several of my Rad Tanks. and still been able to maintain high resists. I have a build for Rad/Fire/Mu that I'll be trying out, when I get time, that shifts the Def to S/L to see how I like that/how it compares. But cycling between Meltdown and Barrier should cover most of the bases.