The design and synthesis of selective adsorbents for nuclear fission product removal using continuous chromatography: A new concept in nuclear reprocessing and waste management

Abstract

The current nuclear fuel reprocessing technique -PUREX -has remained in operation, unchallenged for over 50 years. The underlying reason for this is that development of alternative processes have not achieved the required criteria. The PUREX process hashowever, some inherent challenges that hinder its efficiency for separating uranium and plutonium from other radionuclides in spent nuclear fuel liquors which are discussed within this thesis. In addition, the PUREX process produces High Level Waste (HLW)which contains both relatively short lived beta/gamma emitters (cesium and strontium) and longer lived minor actinides (neptunium and americium) which ultimately present waste disposal challenges. An Alternative Reprocessing Technique (ART) is described for efficient separation of radionuclides from spent fuel dissolver liquors which could potentially eliminate High Level Waste and enhance the efficiency –or even replace -the PUREX process. A highly selective phosphate (ammonium molybdophosphate, AMP) has been shown to remove cesium for nitric acid liquors but not containing greater concentrations of uranium and/or plutonium. This research is split into 3 main parts; [1] optimisation of the synthesis of AMP/PAN (polyacrylonitrile) to maximise ion exchange performance; [2] focus on synthesis and development of mixed metal phosphates/polyacrylonitrile composites for strontium adsorption and [3] scoping the capabilities of a Simulated Moving Bed Chromatography technique to be applied to ART for the downstream extraction of fission products from spent fuel dissolver liquors following the removal of cesium.

Previously studies of cesium and strontium adsorbents for their extraction from HLW were reviewed. These adsorbents are evaluated for their ion exchange performance as a function of selectivity, stability, kinetics and capacity. The outstanding material for cesium extraction from spent fuel dissolver liquors is currently AMP as it displays a high affinity for cesium with high capacities and rapid ion exchange kinetics whilst exhibiting high thermal and radiolytic stability. It can also be encapsulated into a polyacrylonitrile support for synthesis of spherical beads that are more applicable to an industrial process. The preparative routes were varied in order to produce optimised composite spherical beads that had superior selectivity, uptake and kinetics compared with previously prepared material. There is a wide library of selective strontium adsorbent materials but do not possess all the characteristics required for applications in nuclear reprocessing. This study was therefore designed to address the preparation and development of novel highly selective materials that could be used in ART and to evaluate Simulated Moving Bed Chromatography for radionuclide extraction. A selection of mixed metal phosphate powders was provided, untested for their strontium uptake capabilities. The powders were encapsulated in PAN to mimic the AMP/PAN composite in the hope they would display comparatively uptake capacities and kinetics for strontium. Initial results of AMP/PAN synthesis optimisation revealed that beads of 1-2mm in diameter exhibited limited uptake in 1 M and 3 M HNO3with a cesium concentration of 5 mM. Smaller beads <1 mm in diameter produced using anew bead preparative technique displayed an increase in cesium uptake by almost 10 fold from a similar concentration cesium solution. In addition, the selectivity of cesium was also high in the presence of a large excess of ceriumions. The uptake of cesium when the oxidation state of cerium was increased to 4+, was noticeably lower but reasons for this are inconclusive as of yet. The results were compared to previously produced AMP/PAN composites. The modified AMP/PAN beads showed evidence of more internal voids, but less ordered channels and greater surface area. These characteristics resulted in a significant, greater cesium uptake. A series of untested mixed metal phosphate powders synthesised via a fast flow synthesis were selected for their uptake capabilities for strontium. Most of the adsorbents exhibited rapid uptake kinetics with near complete adsorption of strontium in pH values of ~5. They were subsequently encapsulated into a polyacrylonitrile matrix to produce spherical beads ~0.5 –1mm in diameter and re-tested for their ion exchange performance. Kinetics results were constant with the starting powders but capacities and Kd values decreased. Their stability in acidic media was also limited. Various commercial Purolite resins were investigated using an isocratic SMBC technique using a multi-ion feed solution (Zr, Mo, Ce). Initial results were compared to previous single column work performed with the same resins. The elution of the different ions displayed some very distinctive differences to single column results due to the different underlying fundamentals of SMB. Ce was used in high excess as a uranium simulant and presented competitive adsorption behaviours. These preliminary studies were used to gain an initial insight to its potential use in spent fuel nuclear reprocessing.