Incorporation of iodine into apatite structure: a crystal chemistry approach using Artificial Neural Network
Materials with apatite crystal structure provide a great potential for incorporating the long-lived radioactive iodine isotope (129I) in the form of iodide (I-) from nuclear waste streams. Because of its durability and potentially high iodine content, the apatite waste form can reduce iodine release rate and minimize the waste volume. Crystal structure and composition of apatite was investigated for iodide incorporation into the channel of the structure using Artificial Neural Network. A total of 86 experimentally determined apatite crystal structures of different compositions were compiled from literature, and 46 of them were used to train the networks and 42 were used to test the performance of the trained networks. The results show that the performances of the networks are satisfactory for predictions of unit cell parameters a and c and channel size of the structure. The trained and tested networks were then used to predict unknown compositions of apatite that incorporates iodide. With a crystal chemistry consideration, chemical compositions that lead to matching the size of the structural channel to the size of iodide were then predicted to be able to incorporate iodide in the structural channel. The calculations suggest that combinations of A site cations of Ag+, K+, Sr2+, Pb2+, Ba2+, and Cs+, and X site cations, mostly formed tetrahedron, of Mn5+, As5+, Cr5+, V5+, Mo5+, Si4+, Ge4+, and Re7+ are possible apatite compositions that are able to incorporate iodide. The charge balance of different apatite compositions can be achieved by multiple substitutions at a single site or coupled substitutions at both A and X sites. The results give important clues for designing experiments to synthesize new apatite compositions and also provide a fundamental understanding how iodide is incorporated in the apatite structure. This understanding can provide important insights for apatite waste forms design by optimizing the chemical composition and synthesis procedure.
Wet chemical synthesis of apatite-based waste forms – A novel room temperature method for the immobilization of radioactive iodine
Iodine isotopes make up ∼0.69% of the fission products of 235U slow neutron fission, i.e. ∼360 g iodine per metric ton of 235U. The currently proposed technology for the removal of iodine from reprocessing plant off-gas is to pass it through a caustic scrubber (using ∼1–2 M NaOH) to form NaI, NaIO3 and possibly NaOI, or to pass it through Ag-containing solid sorbents to form chemisorbed AgI. Since iodine is not amenable to conventional vitrification routes because of its poor incorporation into borosilicate glass chemistry and high volatilization at temperatures >500 °C, considerable effort is being made to develop alternative waste forms for the immobilization of iodine at low temperatures (<100 °C). In this paper, a novel wet chemical synthesis method has been described for immobilizing radioactive iodine obtained from caustic scrubber solutions and silver solid sorbents into the apatite, Pb10(VO4)6I2, at room temperature. The uptake of iodine for the synthesized waste form has been shown to be 94.6%. In addition, investigation into the formation of solid solutions in the systems, Pb(10−x)Cax(VO4)6I2 and Pb10(VO4)(6−y)(PO4)yI2, by the proposed method has been discussed. The study provides an easy pathway to design novel waste forms for the immobilization of large volumes of radioactive iodine at room temperature and a methodology to potentially synthesize new compositions of iodide-containing apatites.
The hydration of borosilicate waste glass in liquid water and steam at 200 °C☆
Abstract Simulated borosilicate waste glass was hydrated in steam at 200 °C for times up to 40 days to assess the effect of a very high glass surface area/leachant volume (SA/V) ratio on the reaction. The reactions in steam attained an SA/V in excess of 4000 m−1 due to the limited amount of water that was available to condense on the glass surface. Experiments in liquid water were performed at an SA/V of 40 m−1 for comparison. A solid reaction layer formed on the glass surface in both environments, and the thickness of this layer was used as a measure of the reaction progress. Other secondary phases formed on top of (and within) the layer on the steam-reacted samples after a few days of reaction but not on samples reacted in liquid water. The rate (layer thickness/time) measured in experiments with liquid water slows with time while the reaction in steam is very slow initially but then proceeds at a high rate after secondary phases form. The secondary phases are believed to increase the reaction rate by lowering the solution concentrations of glass species (probably most importantly silicon) which control the reaction affinity. The glass reaction is accelerated in a steam environment relative to liquid environment because, in steam, the small solution volume becomes saturated and precipitates are formed after much less glass has reacted. The experimental technique described allows secondary phases to be generated within short time periods at elevated temperatures in a steam environment. Knowledge of the phases formed is necessary to predict the long-term reaction rate. Precipitates formed on the steam-reacted samples were identified using SEM/EDS analysis and XRD. The EQ3/6 computer code was used to predict secondary phases formed at 200 °C for comparison to the observed phases. Differences in the assemblage predicted by the computer simulation and that produced in the experiments are attributed to the limited data base use by the simulation.