Production of Monodisperse, Crack-Free Cerium Oxide Microspheres by Internal Gelation Sol-Gel Methods.

Abstract

Traditional powder processes for fabricating plutonium-238 oxide heat source pellets used in radioisotope thermoelectric generators contaminate facilities and pose a health hazard to workers. These problems have motivated steps to validate an alternative, dust-free technique to produce equivalent pellets. Established internal gelation sol-gel methods for fabricating uranium oxide microspheres have been modified to produce monodisperse spheres of cerium dioxide with diameters less than 200 micron. A two-fluid nozzle designed to accommodate short needles in a chill-able configuration has resulted in the production of sintered cerium dioxide microspheres with an average diameter of 100.28 micron ± 2.8 micron. Final sphere diameter was controlled within the range 65 – 210 micron by adjustments to feed solution and stripping oil feed rates. Sol-gel equipment designed and constructed for this study is capable of producing enough gelling product in one day to yield 10 g of sintered cerium dioxide microspheres. Improvements were made to sphere washing techniques to prevent microsphere cracking upon drying and also reduce impurity levels. Thermogravimetric analysis indicated that spheres washed by the improved method have 4.5 times less volatile mass loss upon heating than spheres washed by a traditional process. Impurity analysis on sintered microspheres has shown carbon impurities below 30 ppm and trace element levels below specified limits for Pu-238 oxide fuels. Experimental results with a lab-scale apparatus demonstrated that 10s of grams of monodisperse cerium oxide microspheres can be produced by internal gelation methods with low impurity levels. These findings provide motivation for additional research to demonstrate dust-free production of plutonium oxide microspheres.