Operating limits for most normal operation conditions and postulated accident scenarios for the Savannah River Site heavy water production reactor (SRS-HWR) are based on a different criterion than that used for boiling water reactors (BWRs) and pressurized water reactors (PWRs). The critical heat flux (CHF) is the most frequently used limiting criterion for BWRs and PWRs. Operating powers for the SRS-HWR, which runs at significantly lower pressures than commercial reactors, are limited by the onset of a parallel channel flow instability (OFI). The CHF and OFI criteria for PWRs, BWRs, and the SRS-HWR are compared.