The xenon poison fraction in a 1000-mwe molten salt breeder reactor power plant (MSBR) was calculated as a function of sparging cycle-time and the rate of diffusion of xenon into graphitic structure in contact with the fuel salt. lt was found that the xenon poison fraction can be held to 0.005 neutrons per neutron absorbed in fuel atoms by passing the fuel stream through a helium sparge-chamber at the rate of 10 ft/sup 3//sec if the coefficient of diffusion is no greater thsn 10/sup -//sup 7/ cm/sup 2//sec. or 21 ft/sup 3//sec if it is as large as 10/sup -/>s5