Use of the SEPHIS-MOD4 program for modeling the purex and thorex solvent extraction processes

The SEPHIS-MOD4 computer program simulates solvent extraction in the Purex and Thorex processes. The program employs simple mass balance equations to describe the movement of solutes through the idealized mixer-settler stages which are used in the model. These differential mass-balance equations are integrated by an iterative scheme that allows the user to observe transient changes and the resulting steady state profile in the mixer-settler system. Correlations describe the equilibrium concentrations in the aqueous and organic phases. Kinetic rate equations are used to simulate the reduction of plutonium by hydroxylamine nitrate and uranium (IV). The value of the SEPHIS-MOD4 program lies primarily in its application to the design and testing of solvent extraction flowsheets. Given a set of operating conditions, the program can predict the solute concentration profile at steady state and during the approach to steady state. Changes in the operating conditions can be used to determine the influence of important system variables or to simulate the effects of a process upset. This type of information has been used to evaluate the flowsheets in solvent extraction experiments and to aid in understanding the results of the experiments. Comparisons between experimental results and the calculated results of the program are generally good and demonstrate the accuracy of the program. The program should become a useful tool in planning for the processing of spent nuclear fuel. 11 figures