Method of manufacturing sintered nuclear fuel material

For sintering nuclear fuel pellets, there has been used a process of applying powderizing treatment to a nuclear fuel oxide comprising UO2 or a mixture of UO2 and U3O8, molding the powder and, further, sintering in H2 or gas mixture atmosphere of H2 and O2. In the present invention, the pellets are manufactured by the following steps for remarkably reducing thermal load during sintering, facilitating the manufacturing step and conducting sintering economically and efficiently in a single sintering surface. After charging and sealing a molding product obtained by molding nuclear fuel starting powder into a vessel, the vessel is heated to sinter in a non-oxidative atomosphere under the state where acidic atmosphere is formed only at the inside of the vessel. After sintering, the product is cooled and, while leaving the vessel open, it is further heated in a reducing atmosphere. Thus, the conventional sintering furnace can be used as it is and, in addition, sintering can be conducted at a relatively low temperature, to moderate the thermal load upon heat treatment, reduce the production cost and save the number of production steps. (K.M.)