A nuclear analysis using transport theory was made of an open cycle gas-core reactor for assumed operating conditions. Calculations were made for cavity diameters from 2.44 to 4.88 m, for hydrogen (cavity) bypass variation from 0 to 99%, for reflector thickness from 0.61 to 1.07m, and for both isotopes 235U and 233U as fuel. The results for these configurations indicated that 233U and some bypass hydrogen may be necessary to keep critical mass levels low enough to give system pressures of <1000 atm.