Simulation of the dynamic beyond-design-basis regime in an extraction–scrubbing unit consisting of mixer–settlers or centrifugal contactors in reprocessing spent nuclear fuel from nuclear power plants and of impurex process

The possibilities of calculating several successive perturbations without time restriction caused by reaching the steady state and of taking into account the uranium extraction efficiency in the steady state of the cascade operation in adjacent steps of the uranium concentration front were introduced into the algorithm for calculating the transient extraction regime in the Purex process. The results of long-term experiments concerning transient regime of the head mixer–settler unit of the RT-1 complex were reconsidered as applied to reprocessing of spent nuclear fuel (SNF) from WWER (water-cooled water-moderated energy reactor), and good agreement of the experimental data and calculation results for beyond-design-basis regime was reached using the model of ideal mixing in a settling chamber without corrections for the efficiency of the step. The effect of significant extraction of Pu at limiting loading of the solvent with uranium was confirmed. Consideration of this effect allowed adequate description of the Pu accumulation in the uranium-free zone of the extraction–scrubbing unit. The adequacy of the ideal mixing model for a unit of centrifugal contactors was demonstrated as applied to reprocessing of SNF from fast reactors, although the difference between the calculation results obtained using the ideal mixing and plug-flow models becomes considerably smaller. A separate experiment has shown that the Pu accumulation in the uranium-free zone does not occur in this case because of higher acidity of the aqueous phase and higher process temperature, and the Pu distribution between the contactor steps correlates with the U distribution. Comparison of the result of the calculation using the corrected model with the results of step-by-step sampling demonstrated the possibility of complete recovery of Pu(IV) in the beyonddesign- basis (with respect to U) regime at lower values of the HNO3 concentration and temperature than it was assumed previously on the basis of published data.