Studies of Intensified Small-scale Processes for Liquid-Liquid Separations in Spent Nuclear Fuel Reprocessing

Abstract

The main contribution of the thesis is to study and develop small-scale processes for ionic liquid-based extractions that can intensify the liquid-liquid separations in the spent nuclear fuel reprocessing cycle. The industrial application of small scale processes requires that their hydrodynamics and mass transfer behaviour are well characterised and predicted. In addition, modelling methodologies are proposed to evaluate the applicability of the small scale extractors in reprocessing the large volumes of nuclear waste used in industrial scale. The first part of the work involves the study of the hydrodynamic behaviour of two-phase (ionic liquid-aqueous) flows. Flow pattern formations within channels have been identified for a wide range of operating conditions and were found to be strongly affected by channel size and material, fluid properties, and flow rates. The main patterns observed were plug flow, annular flow, and drop flow. Subsequently, the work focused on the investigation of the plug flow which has been found to enhance mass transfer because of circulation patterns that develop within the phases. Plug flow was thoroughly investigated in various channel sizes of different material mainly for TBP/ionic liquid (30% v/v) mixtures-nitric acid solutions, relevant to spent nuclear fuel reprocessing. Several hydrodynamic characteristics, such as plug length, plug velocity, film thickness, and pressure drop have been investigated for different ionic liquids, channel sizes, and phase flow rates. Results have been compared with literature, and new (or modified) correlations have been proposed for estimating the plug length, film thickness, and pressure drop. Furthermore, circulation patterns and mixing characteristics within aqueous plugs were investigated by means of μ-PIV (micro Particle Image Velocimetry). The mixing within a plug was locally quantified by the non-dimensional circulation time and the results were correlated with the mass transfer performance. Mixing within the plug was found to be affected by several parameters, but the most decisive one was the size of the channel; mixing was enhanced by decreasing the channel diameter. The last stage of the experimental part of this research involves studies of the extraction of dioxouranium(VI) ions from nitric acid solutions into TBP/IL mixtures (30%, v/v), relevant to spent nuclear fuel reprocessing in channels with sizes ranging from 0.5 to 2 mm ID. The effects of ionic liquid type, initial nitric acid concentration, and residence time on the extraction performance of the contactor were studied. Experimental mass transfer coefficients were compared against predictive models derived from the literature and good agreement was found with those for liquid-liquid contactors. Experimental results were also compared with extraction units already in operation in spent nuclear reprocessing plants. It was found that comparable amount of spent nuclear fuel (1045 tonnes per year) can be reprocessed and extraction of dioxouranium(VI) >99% can be achieved in 4 stages (cycles) with approximately 400 assemblies (one assembly consists of 6 channels of 2 mm internal diameter and 285 cm length). Finally, a numerical finite element model for the hydrodynamics and mass transfer was developed, and the results were compared with the experimental findings. The model used experimental data for the geometric characteristics of the plug flow and predicted reasonably well the experimentally measured extraction efficiencies (with a 11.3 % mean relative error).