Process development for the manufacture of plutonium containing wastes and residues

• Main Author: Squire, Jonathan
• Other Authors: Hyatt, Neil C.
• Published: University of Sheffield 2013
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.570178

This thesis explores the process development and scale-up challenges for the immobilisation of plutonium containing wastes and residues. The first section details the difficulties encountered with utilising an attrition mill operated in the dry mode to prepare zirconolite samples for consolidation by hot isostatic processing. The failure to remove the milling additives from the milled powders led to porous and heterogeneous products with no correlation between the candidate wasteform properties and milling conditions. Similar attrition milling conditions were examined with a zirconia candidate wasteform. Pellets, prepared by cold isostatic and cold uniaxial pressing, showed a clear correlation between the increased milling aggressiveness and the density and microstructure of the products. Cold isostatic pressing produced pellets with the highest density. The effect of precursor particle size, by utilising manufacturer size defined precursors, showed that homogenous and dense zirconolite products could be manufactured by HIP. The experiment confirmed that the failure to remove the lubricant from the HIP cans led to low quality products. Further work examined the effect that different precursor treatments had on the ability of zirconolite precursors to be planetary milled. The powders were pressed into pellets and sintered in either air or nitrogen. The air sintered pellets were all highly porous but homogenous whilst the nitrogen sintered pellets were heterogeneous. The difference in sintering behaviour was determined to be the result of a eutectic point between the iron and titanium oxides. Brief studies examined how the density and homogeneity of zirconolite samples improve with increased milling durations. Another experiment attempted to demonstrate a fine precursor digestion method that did not work. Modified attrition mill internals are also considered. Finally, the impact of this work on the design of a candidate bulk plutonium immobilisation plant is discussed as well as the on going experimentation with uranium active samples.