Analysis of 235U Enrichment by Chemical Exchange in U(IV)‐U(VI) System on Anionite

Abstract

A theoretical study about ²³⁵U enrichment by the chemical exchange method in U(IV)‐U(VI) system on anion‐exchange resins is presented. The ²³⁵U isotope concentration profiles along the band were numerically calculated using an accurate mathematical model and simulations were carried out for the situation of product and waste withdrawal and feed supply. By means of numerical simulation, an estimation of the migration time, necessary for a desired enrichment degree, was obtained. The required migration distance, the production of uranium 3 at.% ²³⁵U per year and the plant configuration are calculated for different operating conditions. An analysis of the process scale for various experimental conditions is also presented.     

Partitioning of U(VI) and Eu(III) between Acidic Aqueous Al(NO3)3 and Tributyl Phosphate in n‐Dodecane

Abstract

The preferred approach to removing Al from Hanford tank sludges, based on aqueous alkaline leaching, often does not achieve complete success. Previous laboratory investigations on the treatment of Hanford tank sludge simulant samples indicate that an acidic scrub can enhance the dissolution of Al from various sludge matrices. If acidic leaching was deployed to enhance removal of tank waste residues, the resulting acidic Al(NO₃)₃ leachate solution could contain measurable amounts of solubilized transuranic elements and so would demand treatment prior to disposal. In this study, a liquid‐liquid extraction system for the decontamination of the HNO₃/Al(NO₃)₃ aqueous leachate by contact with 60% v/v tributyl phosphate (TBP)/n‐dodecane organic solution has been examined. The partitioning of U and Eu between the TBP phase and solutions of varying [HNO₃] and [Al(NO₃)₃] containing small amounts of Cr or ascorbic acid have been investigated._. The results indicate that >99% of both species could be removed from the aqueous phase using such a process.     

New Multi‐Dentate Ion‐Selective AXAD‐16‐MOPPA Polymer for the Preconcentration and Sequential Separation of U(VI), Th(IV) from Rare Earth Matrix

Abstract

A new class of multi‐dentate ligand anchored polymeric resin has been synthesized by grafting Amberlite XAD‐16 with [2‐(1‐Methyl‐3‐oxo‐2‐phenyl‐2, 3‐dihydro‐1H‐pyrazol‐4‐ylcarbamoyl)‐ethyl]‐phosphinic acid (AXAD‐16‐MOPPA). The modification steps involved during the grafting process are characterized by FT‐IR spectroscopy, ³¹P and ¹³C‐CPMAS (cross‐polarized magic angle spin) NMR spectroscopy, CHNPS elemental analysis and thermogravimetric analysis. The influence of various physio‐chemical parameters on the quantitative extraction of metal ions by the resin phase are studied and optimized by both static and dynamic methods. The developed grafted polymer shows greater selectivity for actinide ions like U(VI) and Th(IV) when compared to the lanthanides with greater distribution ratio values in highly acidic matrices. However, the lanthanides compete for the active sites in near neutral conditions. But the sorbed actinide ions and lanthanide elements can be separated by the sequential elution methodology. Moreover, the polymer exhibits faster metal ion phase exchange kinetics, where with a high sample flow rate of 25 mL min^?1 quantitative analyte sorption is achievable during the extraction chromatographic column operation for all the analytes. It also offers good ion‐selectivity and greater preconcentration factor values of 400 for U(VI) and 333 for Th(IV) in 4M HNO₃ conditions. The resin shows very high sorption capacity values of 1.45 mmol g^?1 for U(VI), 1.39 mmol g^?1 for Th(IV), and 1.31 mmol g^?1 for La(III) at near neutral conditions. Finally, the developed grafted resin has been successfully applied in extracting Th(IV) from matrix monazite sand which comprises large rare earth matrix, U(VI) from seawater and also U(VI) and Th(IV) from simulated nuclear spent fuel mixtures. The analytical data obtained from triplicate measurements are within 3.5% rsd, reflecting the reproducibility and reliability of the developed method.     

Interpretation of Third Phase Formation in the Th(IV)–HNO3, TBP–n‐Octane System with Baxter's “Sticky Spheres” Model

Abstract

Small‐angle neutron scattering (SANS) data for the tri‐n‐butylphosphate (TBP)–n‐octane, HNO₃–Th(NO₃)₄ solvent extraction system, obtained under a variety of experimental conditions, have been interpreted using two different models. The particle growth model led to unrealistic results. The Baxter model for hard‐spheres with surface adhesion, on the other hand, was more successful. According to this model, the increase in scattering intensity in the low Q range observed when increasing amounts of Th(NO₃)₄ are extracted into the organic phase, has been interpreted as arising from interactions between small reverse micelles containing three TBP molecules. Upon extraction of Th(NO₃)₄, the micelles interact through attractive forces between their polar cores with a potential energy of up to about 2 k_BT. The intermicellar attraction, under suitable conditions, leads to third phase formation. Upon phase splitting, most of the solutes of the original organic phase separate in a continuous phase containing interspersed layers of n‐octane.