Counter current experiments in a single cycle solvent extraction system were carried out with centrifugal contactors to study flow rate reductions in extraction solvent and stripping solution with a dissolver solution of irradiated mixed oxide (MOX) fuel. Uranium and Pu leaked to the raffinate with a 15% decrease in the extraction solvent flow rate, which brought about an accumulation of U and Pu in the co-decontamination section. The discharge of other actinide elements and fission products (FPs) in the raffinate was constant when the extraction solvent reduced by 15%. On the other hand, a 15% decrease in the stripping solution flow rate did not affect the stripping of U and Pu so greatly. The polymerization of Pu was not observed in the counter current experiments, and the time taken to change 2% of Pu(IV) to Pu polymers in the stripping section was evaluated. Simulation results based on these experiments showed that a high HNO3 concentration in the feed solution was advantageous for Np extraction, because Np oxidation progressed in the extraction section. (c) 2010 Elsevier Ltd. All rights reserved.