The Chemical Basis for Separating Recycling Materials by Hydro-Processes

Abstract

This article describes the PUREX process which is the dominant technology for the reprocessing of spent nuclear fuel. The PUREX process is a hydro-metallurgical process based on solvent extraction (SX) between an aqueous nitric acid and an organic phase which comprises the extractant tri-butyl phosphate (TBP) and a paraffinic diluent, such as odourless kerosene (OK). TBP is very effective in selectively extracting uranium and plutonium from dissolved spent fuel solutions although some minor interferences from a few fission products (e.g., technetium, zirconium, ruthenium) must be dealt with in the separation process flowsheet. The PUREX process chemistry of the minor actinide neptunium is particularly complicated and its control requires specific measures. There are a number of process upsets that must be avoided in implementing the PUREX process. Of these, one of the most important is avoiding conditions that lead to “third phase” formation where the organic phase can split into two separate layers. This article covers the preparation of spent fuel by dissolution in nitric acid, the basic chemistry of the PUREX solvent extraction process and its industrial application by reference to two exemplar reprocessing plants. The specific focus is placed on the chemical basis for the separation process rather than its engineering. Finally, the chapter concludes with a short perspective on future directions for the PUREX process.