A heat flux of more than 10 MW/m2 has to be removed in high heat flux components such as the divertor/limiter and the NBI beam dump. Water in the subcooled flow boiling regime is thought to be the most promising coolant for this purpose. This paper aims to give a reliable prediction method for the critical heat flux which determines the cooling capability. Subcooled flow boiling in a round tube with a uniform heating condition was selected as it presents fundamental information. An empirical correlation is proposed, based on experimental data at ambient pressure measured by the authors and on existing published data at high pressure. The validity of the correlation was confirmed. The Weisman-Pei model was also confirmed to have good accuracy. Based on both the authors' and the Weisman-Pei correlations, the pressure and exit quality of water required to remove a heat load of more than 40 MW/m2 were determined. The conditions are, in general, a pressure of more than 1 MPa, mass velocity more than 10000 kg/m2 s, and quality around −0.25. The effects of tube length and inside diameter on the critical heat flux are also discussed.