The mean activity coefficients of HNO3 in mixed electrolyte solution HNO3-UO2(NO3)2-H2O with different concentrations (the total ionic strongth up to 6.14mol/kg)were measured at 25°C by use of ion selective electrodes(H^+ ISE and NO3^- ISE) the binary and ternary interaction parameters (^S0(0), ^S0(1),Ψ) in the pitzer formalism (includding ^E0 and the relation of ^s0-ionic strength) were evaluated. the activities of H2O in these systems are predicted by the Pitzer formalism. the mean activity coefficients of HNO3 and UO2(NO3) for these systems are calculated by the pitzer formalism , the McKay -perring method and the harned equation. respectively.

Key words: WATER, TERNARY SYSTEM, NITRIC ACID, ION SELECTIVE ELECTRODE, NITRATE, OXIDE, URANIUM COMPOUNDS, ACTIVITY COEFFICIENT, ELECTROLYTE SOLUTION