A METHOD FOR PREPARING PARTICLES OF METAL HYDROXIDE GELS BY TREATING A CONCENTRATED AQUEOUS SOLUTION OF AT LEAST ONE HYDROLYZABLE METAL SALT, THE HYDROXIDE OF WHICH IS ONLY SLIGHTLY SOLUBLE IN WATER, SIMULTANEOUSLY WITH AMMONIA AND HEAT. THE SOLID SUBSTANCE THUS OBTAINED IS SEPARATED, WASHED AND HEATED THEREBY CONVERTING THE SOLID SUBSTANCE TO METAL OXIDE PARTICLES. THE HYDROLYZABLE METAL SALT STARTING MATERIALS ARE CONCENTRATED SOLUTIONS ANION-DEFICIENT METAL SALT SOLUTIONS, HAVING A RATIO OF CONJUGATED BASE TO METAL LOWER THAN THAT OF THE NORMAL SALT. WHEN A NITRATE-DEFENDANT URANYL NITRATE SOLUTION IS MIXED WIT AN AMMONIA-RELEASING AGENT, THEN DISPERSED IN A HOT PHASE IMMISCIBLE WITH THE AQUEOUS PHASE, THE RESULTING SOLIDIFICATION PRODUCES MICROSPHERES OF URANYL NITRATE, WHICH ARE USEFUL AS FUEL CELLS IN NUCLEAR REACTORS.