I didn't see a bug reported for this and I do know that the dependencies are listed on the downloads page, with that being said it appears that BigReactors dependencies aren't being checked when mods are loaded because I'm getting the error here rather than a dependency error. Cheers ^.^
http://pastebin.com/ZFTu5EjD

Ah. That's because I have CoFHLib listed in the dependency string instead of CoFHCore. I'll get that fixed, thanks.

Glad I could help.