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Received for review June 29, 1978 Accepted December 18, 1978
Presented in part at the 174th National Meeting of the American Chemical Society,Division of Fuel Chemistry, Chicago, Ill., Aug 29-Sept 2, 1977.
An Analysis of the Transient and Steady-State Operation of a Countercurrent Liquid-Liquid Solvent Extraction Process William S. Groenier,' Robert H. Rainey, and Sarah B. Watson Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
The computer program SEPHIS has been developed to aid in determining optimum solvent extraction conditions for the reprocessing of nuclear power reactor fuels by the Purex method. The possible applications to inventory control for nuclear material safeguards, nuclear criticality analysis, and process analysis and control are of special interest. The method is also applicable to other countercurrent liquid-liquid solvent extraction processes that are characterized by rapid chemical kinetics, that may involve multiple solutes, and that are performed in conventional contacting equipment.
Scope The development of methods for processing spent nuclear power reactor fuels is difficult and costly because of required material containment and radiation shielding. Testing to establish optimum values for operating parameters for solvent extraction treatments of fuel solutions can be minimized through the use of computer modeling of the process. A satisfactory model also enables the process designer to simulate and analyze a variety of nonroutine process modes quickly and economically. Conclusions and Significance A method for calculating the time-dependent behavior of countercurrent solvent extraction processes has been developed and applied to a proposed flowsheet for the recovery of valuable constituents in spent nuclear power reactor fuels. This effort led to the development of a computer program, SEPHIS, which employs a combination of approximate mathematical equilibrium expressions and the transient, stagewise process calculational method to allow the prediction of stage and product stream concentrations with accuracy and reliability. The applications of SEPHIS to process analysis and control, nuclear criticality analysis, and inventory control for nuclear material safeguards purposes are particularly interesting. Introduction Since only a few percent of the fissionable content of nuclear power reactor fuels is consumed during power generation, economics dictate that recycle of the unused
* Correspondence concerning this paper should be addressed t o W. S. Groenier, Bldg. 7601, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, Tenn. 37830. 0019-7882/79/1118-0385$01.00/0
portion is desirable. Fuel recycle involves the separation of unused uranium and plutonium from fission products and the subsequent purification and refabrication into new fuel elements. The basic separation process is the Purex process which was developed in the 1950's a t the Oak Ridge National Laboratory. It involves the selective extraction of uranium and plutonium nitrates from the nitric acid solution produced from dissolution of spent fuel into an organic solution of tri-n-butyl phosphate (TBP) in a hydrocarbon diluent. Uranium and plutonium are subsequently separated by changing the valence state of plutonium to a value where the plutonium has a different extractability than uranium. The partitioning occurs in a second solvent extraction contactor to produce an aqueous solution of plutonium nitrate. Finally, the organic solution is stripped in a third contactor to recover uranium. Additional cycles of extraction and stripping may be used to further purify the heavy-metal nitrate solutions. The comparatively high plutonium content (i.e., -20%) of breeder reactor fuels presents criticality design problems in the development of fuel recycle processes. To circumvent such problems in solvent extraction equipment, a "dilute-Purex" flowsheet utilizing 15 vol % T B P in a hydrocarbon diluent has been suggested. [The reprocessing of light-water-moderated power reactor (LWR) fuels normally involves the use of 30 vol 7'0 TBP.] However, at ordinary temperatures (25 to 30 "C), solvent loadings
