The primary objective of the study is to evaluate the safety of rFIXFc (recombinant coagulation factor IX Fc fusion protein, BIIB029) in previously untreated participants with severe hemophilia B. Secondary objectives are to evaluate the efficacy of rFIXFc in the prevention and treatment of bleeding episodes in previously untreated participants (PUPs), and to evaluate rFIXFc consumption for prevention and treatment of bleeding episodes in previously untreated participants.