Comparison of two powder conditioning methods to improve UO2 powder flowability for press die filling

Abstract

In the nuclear industry, mixed oxide fuel pellets are manufactured by powder metallurgy process. This work aims at improving the physical and flow properties of nuclear powders in order to obtain a reproducible die filling and produce homogeneous and dense fuel pellets after sintering. The studied powder was a grinded mixture of UO2 powder and grinded sintered pellets scraps (recycled). Two powder conditioning methods were tested and compared: a V-blender and an oscillating and rotating OscilloWitt-Lab sieve mill associated to a Turbula mixer. The first conditioning method consisted in three main steps realized in a V-blender: a deagglomeration step with an intensifier bar, a stirring step and a lubrication step. The second conditioning method also consisted in three main steps: a sieving step with an OscilloWitt-Lab, followed by a stirring step and a lubrication step in a Turbula mixer. Different intensifier bar speeds on the V-blender and different sieve mesh sizes on the OscilloWitt-Lab were studied. Particle size, morphology, mechanical properties and flow properties were characterized after each step. The V-blender only slightly improved the flowability compared to the initial powder. The intensifier bar speed had a negligible impact on the physical characteristics and the flowability of the powder. The OscilloWitt-Lab associated to the Turbula mixer allowed a marked improvement of the powder flow properties compared to the initial powder. The sieve mesh size of the OscilloWitt-Lab had a significant impact on the powder physical and flow characteristics. Indeed, using the 250 μm mesh of the OscilloWitt-Lab with stirring and lubrication resulted in the most favorable powder properties. Therefore, it seems to be the most promising conditioning method to obtain reproducible die filling as well as dense and homogeneous fuel pellets.