In order to obtain the exact information of atomic number density in UO₂(NO₃)₂ · 6H₂O–HNO₃–H₂O ternary mixture, the theoretical estimation of excess volume which is one of excess properties of non-ideal solution is discussed in this study. The calculation model based on the Stokes-Robinson application of the Brunauer-Emmett-Teller (BET) adsorption isotherm and activity equations of water nitric acid, and uranyl nitrate hexahydrate (UNH) which were used to calculate distribution coefficients in a solvent extraction system has one adjustable parameter and two BET constants. The BET constants have been determined by asymptotic method on the BET adsorption isotherm as a function of water activity in the ternary system. As results of the study, the observed volume of mixture agrees with the volume predicted by the simulation within a maximum relative deviation of 0.78% and the excess volume is also estimated as a function of total molality.