Cryptand Cages for Anion Selective Encapsulation for Nuclear Waste

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The reprocessing of spent fuels and the decommissioning of historical nuclear facilities generates a complex mixture of waste material that needs to rendered safe for long term storage and ultimate disposal. Some of the most difficult waste streams are those that contain oxyanions of toxic and radioactive elements, e.g. pertechnetate, selenite, selenate, chromate, molybdate, iodate, bromate.We propose a cost-effective removal strategy based on a novel selective encapsulation process using self-assembled cryptand cages. A pilot study on common anions (with different charge and morphology) demonstrated a fast recovery, reducing anions concentration from 1000 to <0.1ppm. This research was published in the high-impact journal Angewandte Chemie in 2018, and patent protection is under way. The removal can be engineered to work in extreme environments, highly acidic solutions and high ionic strength, producing stable encapsulation of anions either in solution or as crystalline precipitates. This has an advantage for liquid high-level waste pending vitrification, as the anions will be stabilized within the highly radioactive liquor. Our research vision is to attain controlled removal via immobilization using cryptand cages based on anion charge and morphology. Thus we need to determine the factors controlling the encapsulation and the precipitation: the nature of the cryptand (how changes in its design reflect in encapsulation), the metal center for the complex cage, the conditions of the liquid media (pH, ionic strength), and the kinetics of release. To define this complex relationship, we will employ a multidisciplinary approach that brings together the power of computational screening, experimental synthesis and characterization. This will accelerate the development of sustainable and cost-effective encapsulation strategies, reduce environmental impact and enhance waste form performance.We aim to support our hypothesis and to achieve the following objectives: (1) to synthetize ligands based on those previously reported but modulate the nature of the donor atoms and the size of the cavity, with a range of metal center to determine the factors controlling anion selectivity; (2) to identify the interactions between the cage and the anion to retard kinetic release; (3) to produce atomistic models of target structures using integrated experiments and modelling, and to characterize their stability, morphology, and composition.