Catalyzes the NADH-dependent reduction of rubredoxin (Rd), a small iron-containing redox protein (PubMed:10464233, PubMed:11398485, PubMed:15746356). Can also reduce other electron carriers, including 2,6-dichlorophenol indophenol, benzyl viologen and menadione, but rubredoxin is the most efficient electron acceptor (PubMed:10464233, PubMed:11398485, PubMed:15746356). Does not reduce ferredoxin (PubMed:10464233). Shows comparable activity with NADPH and NADH as electron donors, but NADPH is probably the preferred physiological electron donor (PubMed:10464233, PubMed:11398485). Is probably part of an oxygen detoxification system that protects P.furiosus during exposure to oxygen (PubMed:15746356). H(+) + NADP(+) + 2 reduced [rubredoxin] = NADPH + 2 oxidized [rubredoxin] H(+) + NAD(+) + 2 reduced [rubredoxin] = NADH + 2 oxidized [rubredoxin] Binds 1 FAD per subunit. kcat is 15000 sec(-1) with rubredoxin as substrate. kcat is 750 sec(-1) with benzyl viologen as substrate. kcat is 19 sec(-1) with 5,5'-dithiobis[2-nitrobenzoic acid] as substrate. kcat is 9 sec(-1) with H(2)O(2) as substrate. kcat is 341 sec(-1) with NADPH as substrate. kcat is 354 sec(-1) with NADH as substrate. Optimum pH is 7.0 for the reduction of rubredoxin. Thermostable. Monomer. NROR, unlike the vast majority of enzymes from hyperthermophilic sources, shows considerable activity at low temperatures. Belongs to the FAD-dependent oxidoreductase family.