Production of high quality ammonium uranyl carbonate from “uranyl nitrate + carbonate” precursor solution

Abstract

The purpose of this study is to investigate production of high quality ammonium uranyl carbonate (AUC) from “uranyl nitrate + ammonium bicarbonate”, or “uranyl nitrate + sodium carbonate” precursor solutions, by controlled injection of ammonium carbonate solution which could be applicable in material testing reactor (MTR) fuel production plant for recycling of rejected uranium oxide powder. The experimental observations revealed: at pHs higher than 6, precipitation proceeds with formation of no intermediate, thus ensuing a better morphology and size distribution of the AUC products. The results of scanning electron microscopy, X-ray diffraction, particle size analysis, and uranium content analysis showed that precipitation of AUC in the sodium carbonate medium was slow and incomplete (with a yield around 40%); and quality of the precipitate and morphology of its particles could not meet the required criteria. In contrast, ammonium bicarbonate medium resulted in the AUC crystals with suitable morphology and appreciable particle size distribution. In the latter case, precipitation was fairly well modelled by simple “diffusion-reaction” model. Application of the supersaturated ammonium bicarbonate solution for precipitation resulted in the following advantages over the sub-saturated solution: 1) higher yield of uranium precipitation, 2) smaller size of AUC particles (dmean≈86 μm compared to 112 μm for sub-saturated), 3) lower degree of crystals agglomeration, 4) lesser formation of cracks on the surface of U3O8 particles, which are prepared as the products from AUC. The final U3O8 powder, after sintering and milling, appears to be smooth and rounded, and was qualified for MTR fuel plate fabrication.