Abstract: In order to prepare the dense uranium dioxide（UO₂） kernel for high temperature gas-cooled reactors（HTGR） fuel,the conventional sol-gel processes are optimized and modified.The wet process of modified external gelation of uranium（EGU） is introduced,which includes the dissolution of U₃O₈,i.e.the preparation of acid-deficiency uranyl nitrate,preparation of broth solution,casting and gelation of broth solution and aging,washing and drying of the gelled spheres,and etc.The mechanism of wet process of EGU is also discussed.With the optimized wet process,the ammonium diuranate（ADU） microspheres with good sphericity,uniform diameter and perfect porous structure are prepared.After the subsequent treatments of dry processes such as calcination,reduction and sintering,the eligible dense UO₂ kernels for HTGR fuel are manufactured.