Abstract: For the system Th(NO3)4-UO2(NO3)2-HNO3-H2O/30%TBP in kerosene,mathematical models of Th,U and HNO3 extraction equilibrium in 0. 42-3 mol/l HNO3 are developed using the experimental data. Using these models,an optimal solvent extraction process for reprocessing of Th-U fuel is obtained by computer programme for optimization of solvent extraction process. Cascade extraction experiments containing Th, U and fission products in laboratory scale are performed using the optimal conditions calculated. The calculated results are in good agreement with the experimental data. The recovery of uranium and thorium are >99. 9% ,and good decontamination (>103) of fission products from Th-U fuel are obtained.