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theses

One of the key components to a feasible and sustainable nuclear fuel cycle is a viable set of waste forms for the radionuclides and fission products. Of particular concern are the highly volatile radionuclides such as iodine – 129. The current proposed technology for the removal of iodine from reprocessing plant off-gas is to pass it through a bed of silver solid sorbents (i.e. silver zeolite) to form chemisorbed AgI and/or bubble the off-gas into a caustic solution. At this point in time, this iodine capture media is being stored on waste treatment facilities awaiting a viable waste form solution for storage into a geological repository. Iodine is not amenable to conventional borosilicate vitrification routes because of its low solubility in some glass chemistries and, more importantly, its high volatility at typical glass processing temperatures (1000 - 1100°C). Therefore, considerable effort is being made to develop alternative waste forms for iodine. However, most proposed waste forms to date have not achieved sufficient maturity and satisfactory properties (specifically: ease of processing, high waste loading, and high chemical durability) to be considered as promising technologies. Apatite-based ceramic waste forms have been considered as potential candidates for immobilization of radioactive iodine for a long time. In particular, lead vanadate based apatite minerals [i.e. Pb10(VO4)6I2] have been suggested as promising waste forms in this regard mainly due to their ability to accommodate iodine in their crystal structure. Although promising, the major challenges associated with these minerals are either high processing temperatures along with complicated synthesis routes or poor chemical durability. While the concerns pertaining to poor chemical durability of these minerals have been addressed by partial substitution of vanadate (VO4)3- by phosphate (PO4)3- ions [Pb10(VO4)6-x(PO4)xI2], as investigated in literature, the high processing temperatures and complicated synthesis routes of these minerals still pose a big challenge for the continuous mass production of these waste forms. In this study, we present the first reported instance of wet-chemical synthesis of the well-known apatite, Pb10(VO4)6I2. By being able to utilize wet-chemical synthesis, the ability to scale up the production of an iodine-containing apatite to an industrial level is made possible and thus the vast amount of iodine waste media being stored on nuclear waste treatment facilities internationally can be addressed. Several factors paramount to the successful synthesis of the material will be presented, including the choice of precursors, order of synthesis steps, pH level, and temperature of the solution. Aside from this, solid solutions studies will be presented substituting calcium for lead and phosphate for vanadate. These studies help set a baseline for future apatite compositional studies, so that its characteristics can be optimized for the application of geological repositories with the use of our wet-chemical synthesis method.

ETD_7798

M.S.

Includes bibliographical references

by Charles En Hua Cao

doi:10.7282/T3CF9SJ3

ETD graduate

The author owns the copyright to this work.