A simplified solvent extraction process based on U, Pu and Np co-recovery using tri-n-butylphosphate (TBP) as an extractant has been developed for advanced aqueous reprocessing of fast neutron reactor fuel. The influence of the HNO3 concentration in the feed and scrubbing solutions on the behavior of Np is evaluated experimentally and found to be co-extracted into the TBP with U and Pu. Moreover, the leakage ratio of Np to the raffinate is calculated from the changes in HNO2 concentration in the feed solution. 9.89% of the Np leaked to the raffinate in low HNO3 concentration feed and high HNO3 concentration scrubbing solutions. Almost all the Np in a 4.9 mol/dm(3) HNO3 feed solution is recovered with U and Pu, based on the experimental flow sheet with double scrubbing solutions of 9 and 1 mol/dm(3) HNO3. The experimental results show a large contribution from the HNO3 concentration in the feed solution and at the extraction section to Np(V) oxidation. On the other hand, the calculation results show that high HNO2 concentrations in the feed solution tended to leak Np into the raffinate. The DF of Cs for U, Pu and Np product achieves 10(5) in all runs.