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.
Some properties of a lead vanado-iodoapatite Pb10(VO4)6I2
A lead vanado-iodoapatite Pb 10 (VO 4 ) 6 I 2 was synthesized as a potential waste form to immobilize radioactive iodine and some thermal, mechanical and chemical properties were measured. Thermogravimetry-differential thermal analysis (TG-DTA) showed that the apatite was stable up to about 800 K. The thermal conductivity of the hot-pressed sample, with the theoretical density of 82%, increased gradually with increasing temperature from 0.65 W/m.K at room temperature to 0.78 W/m.K at 523 K. The micro-Vickers hardness and Young's modulus were lower than those of a typical glass waste form. The leaching rate of iodine for apatite was two orders of magnitude higher than that of an AgI glass waste form. Despite the high leaching rate (compared to AgI embedded in glass), the high chemical stability up to 800 K and acceptable mechanical properties of this apatite suggest that it will be a good waste form when embedded in a suitable matrix material.
A new method of near field analysis
The problem of the accurate determination of the total electromagnetic field produced by a current distribution on a circular disc type aperture is examined. For comparison of the near and Fresnel region solutions, an exact Maxwellian integral is used as the starting point. Aperture and space coordinates are separated by the use of Bessel function addition theorems, allowing the field to be expressed as a sum of terms containing radial and angular integrals. The radial integrals involve only the disc (aperture) distribution and coordinates while the angular integrals involve only the elevation angle. The distance from origin appears implicitly in the series coefficients. These integrals are suitable for high speed computations and have been evaluated for a number of cases. The results are compared with the small angle and general Fresnel formula values for various observation angles, distances from the aperture, and aperture distributions.