Development of the FPEX II solvent system for simultaneous extraction of cesium and strontium

The simultaneous extraction of cesium and strontium for the purpose of recycling and consolidation of civilian nuclear fuel poses a difficult challenge. After initial cooling, the majority of radioactivity and heat produced in used reactor fuel is due to a select few of the many different isotopes present. Strontium and cesium represent the major radiotoxicity and heat source in used reactor fuel and their separation from other components would increase the capacity of long-term geological storage space. Selective partitioning of used reactor fuel currently uses a series of selective solvent/solvent extraction systems. The current flowsheet incorporates five different stages of solvent extraction for strategically grouping the targeted isotopes. The second stage of the flowsheet named the Fission Product Extraction (FPEX), has been demonstrated in a lab environment to remove cesium and strontium using calix[4]arene-bis-(tert-octylbenzo-crown-6) (BOBCalixC6), 4,4',(5')-di-(t-butyldicyclo-hexano)-18-crown-6 (DtBuCH18C6), trioctylamine (TOA), and 1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol (Cs7-SB). However, 3rd phase formation occurs at concentrations above 2.5M HNO3, making FPEX less efficient and technically difficult to use in large scale operations.;The object of this research was to improve the existing FPEX by developing a better solvent system. Twenty compounds were investigated as solvent modifiers and the distribution of 85Sr and 137Cs measured as a function of modifier concentration and aqueous acidity. The results of the survey identified a mixture of aliphatic 8-carbon chain primary alcohols, Exxal(TM)8, and n-octanol as more robust modifiers. These primary alcohols enabled the FPEX solvent to withstand acid concentrations to 6M HNO3 when used at a concentration greater than 0.75M. A third possible alkyl phenoxy modifier Cs5-SB, was also identified. This modifier also eliminated 3rd phase formations up to 5M HNO 3 when used above 0.75M, and allowed lower concentrations of the extractants to be used. Finally a new calix compound, with higher solubility in the modified solvent system provided enhanced extraction and stripping efficiency for cesium over the original extractant BOBCalixC6.;The distribution data for Sr and Cs in these new solvents were modeled using the program SXFIT. The formation constants for the important species in the organic phase were calculated. Modeling showed the crown used for Sr extraction, also extracts HNO3 and Cs. This interaction increased as acid concentrations exceeded 3M HNO3, and was integral to the modeling fit. A pentamer formation of n-octanol also proved to be vital in the extraction mechanism.;This study identified three modifiers effective up to 5M HNO3 with no third phase formations. The Cs5-SB modifier allowed lower concentrations of extractants without a decrease in efficiency. Successful modeling of the three solvent systems suggested the identity of the major primary organic species. With the replacement of the modifier and calix, it was possible to demonstrate a substantial improvement over the previous FPEX solvent system. The new solvent system eliminated 3rd phases up to 5M HNO3, and provided a viable alternative to the original solvent composition.