Influence of hydrogen reduction temperature in a fluidized bed on AUC derived UO₂ powder properties

Abstract

The production of uranium dioxide (UO₂) powders by calcination–reduction of ammonium uranyl carbonate (AUC, (NH₄)₄UO₂(CO₃)₃) is an important step in the UO₂ fuel pellet fabrication process. The nature and quality of the resulting UO₂ depend closely on the initial characteristics of AUC as well as on the applied thermal treatment conditions. The UO₂ powders obtained must meet specific requirements, particularly those related to specific surface area (4.5–7 m²/g) and particle size distribution. The objective of this work is to analyze the effect of the hydrogen reduction temperature on the properties of AUC-derived UO₂ powders using a fluidized-bed furnace. In this study, the reduction temperature of AUC was varied from 400 to 550 °C, and the resulting UO₂ powders were characterized. The O/U ratio, specific surface area, and particle size distribution of the UO₂ powders were determined respectively by spectrophotometry, nitrogen adsorption–desorption (BET method), and laser granulometry. During tests on stabilization of the AUC powder bed, it was observed that the minimum fluidization flow rate ranged between 20 and 30 L/min. Within this range, a proportional relationship was observed between the pressure drop and the fluidization velocity. Beyond 30 L/min, the fluidized bed became stable. As the reduction temperature increased from 400 to 550 °C, the specific surface area decreased from 10.2 to about 7 m²/g (±0.2 m²/g). However, the particle size distribution of the UO₂ powders varied only slightly with temperature. A reduction temperature of 550 °C under hydrogen in a fluidized bed was selected as the optimal condition for the conversion of AUC into UO₂ powder exhibiting good sintering properties for nuclear fuel pellet production (7 m²/g).