This paper investigates the attributes and limitations of noble gases and binary mixtures as potential working fluids for gas-cooled nuclear power plants with Closed Brayton Cycles (CBC). Compared are the heat transfer coefficient and pressure losses of helium and other noble gases and binary mixtures, at typical operating conditions in commercial power plants (7.0 MPa and 400-1200 K), for the same molecular flow rate and geometry. Also investigated is the impact of the working fluid choice on the performance of nuclear power plants with direct CBCs and single-shaft and multiple-shafts turbo-machines. The effects of the working fluid choice on the number of stages of the turbo-machines and the nuclear power plant's thermal efficiency and electrical power output are calculated and compared. Although He has high heat transfer coefficient and significantly lower pumping requirement, the heat transfer coefficient of the He-Xe binary gas mixture with a molecular weight of 15 g/mol is similar to 7% higher and the turbo-machines have 75% fewer stages than for helium. The pumping requirement for this binary mixture, however, is 3.5 times that of helium, decreasing the plant peak efficiency by similar to 2 percentage points. Thus, using He-Xe, over He, as working fluid for gas-cooled reactor power plants would have to be based on considerations of reducing the size, mass, and cost of the turbo-machines. (C) 2008 Elsevier Ltd. All rights reserved.