Steady-state subcooled water flow boiling experiments were carried out in a uniformly heated horizontal circular channel with an exit pressure of 1.66 MPa and with the mass velocity G varying from 4.4 to 32.0 Mg/m2·s. The test section, which was made of high-strength zirconium-copper, consisted of a tube with an inside diameter of 0.3 cm and a heated length-to-diameter ratio (L/D) of 96.6. The coolant was degassed and deionized water. The inlet water temperature was held constant at 20°C. These experiments are related to high heat flux removal in fusion reactor beam dumps and first walls in compact fusion reactors. For the chosen values of L/D and exit pressure, the measured critical heat flux (CHF) values are higher than any previous values for smooth tubes in the literature. The effect of increasing the pressure from 0.77 to 1.66 MPa is to increase the CHF progressively from 2.0 to 19% as the mass velocity is increased from 4.4 to 25.0 Mg/m2·s. The percent increase in the CHF dropped to 10.0% as G increased from 25.0 to 32.0 Mg/m2·s. Below 25.0 Mg/m2·s, the relationship between the CHF and the mass velocity is linear. Further, an increase in the exit pressure resulted in an increase in the slope of this relationship. However, the local heat transfer coefficient actually decreased as the pressure increased, for the same power level and mass velocity.