The performance of very high temperature reactor plants with direct and indirect closed Brayton cycles (CBCs) is compared and the effects of the molecular weight of the CBC working fluid on the number of stages and sizes of the axial flow, single shaft compressor and turbine are investigated. The working fluids considered are helium (4 g/mole), He-Xe, and He-N2 binary mixtures (15 g/mole). Also investigated are the effects of using low and high pressure compressors with intercooling, instead of a single compressor, and changing the reactor exit temperature from 700°C to 950°C on the plant thermal efficiency, the CBC pressure ratio, and the number of stages in and size of the turbomachines. For plants with direct CBCs, the effect of cooling the reactor pressure vessel with He bled off at the exit of the compressor is also investigated. The present analyses are performed for a reactor thermal power of 600 MW, shaft rotation speed of 3000 rpm, and intermediate heat exchanger temperature pinch of 50°C. Copyright © 2010 by ASME.