This work involves steady-state high heat flux removal infusion reactor beam dumps, first walls in compact fusion reactors, and other applications with smooth surfaces and/or irregular coolant channel cross sections. In such applications, the coolant pressure is required to be low (≈ 1.0 MPa), and the coolant channels are moderately long [length-to-diameter ratio (L/D) ≈ 100]. The present experiments have resulted in high heat flux data in a region where only sparse data existed. Subcooled flow boiling measurements were performed for the critical heat flux (CHF), local (axial) variations of the coolant channel's heat transfer coefficients, and pressure drop for horizontal, uniformly heated tubes. The tubes had inside diameters of 0.3 cm, a heated L/D ratio of 96.6, and were made of amzirc (zirconium-copper). The coolant was degassed, deionized water. The exit pressure and the inlet water temperature were held approximately constant at 0.77 MPa and 20°C, respectively. From experiment to experiment, the inlet temperature varied slightly (±1.5°C) from 20° C. The actual measured inlet temperature was used in reducing the experimental data. Measurements of the above quantities were performed for the mass velocity and exit subcooling, varying from 4.6 to 40.6 Mg/m2·s and 30 to 74°C, respectively. For these ranges, (a) the subcooled flow boiling CHF varied from 625 to 4158 W/cm2, (b) the heat transfer coefficients during fully developed nucleate boiling varied from 30 to 400 kW/m2·K (Nusselt number = 150 to 1700), and (c) the overall pressure drop varied from ∼1.75 to 0.9 times the adiabatic pressure drop. For the flow conditions and geometry parameters given above, least-squares equations for the CHF were developed in terms of both the liquid Reynolds number and the exit subcooling. The average percent deviation of the developed equations from the data was <12.5%.