After being irradiated in a reactor,a nuclear fuel produces various kinds of fission products and,among them,cesium isotopes and their compounds are substances of interest due to their high radioactivity and heat load.Generally,the cesium exists in the form of oxide and can be combined with other elements such as uranium,tellurium,and molybdenum depending on the conditions surrounding it.One of the promising back-end technologies of nuclear fuel cycles is a pyroprocessing using high temperature molten salts as process media instead of water[1,2].A spent fuel should be treated prior to a molten salt processing.High temperature reaction of the spent fuel under oxidation or reduction conditions could supply oxides proper to a molten salt process.Therefore,it is of importance to understand cesium behavior during the oxidation or reduction so that radioactivity and heat load distribution can be estimated throughout the pyroprocessing.In this study,the cesium oxide complex reaction equilibrium was calculated up to 1400 oC under oxidation or reduction gas conditions.Every species are assumed to be in their standard states because currently there is no reliable thermodynamic model describing complex solid solutions at high temperatures.The calculation results showed that some cesium oxide complexes such as Cs2UO4 would be decomposed into two oxides and substantial amount of produced cesium oxide would be removed from the solid oxides under reducing gas flow (4% H2) at high temperatures.It is expected that the results of this work will give an insight of a dry pretreatment of a spent fuel for the pyroprocessing with respect to the radioactivity and heat load monitoring.