Patent Number: 
Section: claims

1. A method of manufacturing a sintered nuclear fuel pellet having a large grain size, the method comprising:mixing an additive powder and a uranium oxide powder to prepare an additive mixed uranium oxide powder;forming an additive mixed uranium oxide green pellet by using the mixed powder;heating the green pellet to an isothermal sintering temperature in an atmosphere control gas having an oxygen partial pressure corresponding to a minimum oxygen partial pressure of isothermal sintering or less; andchanging a sintering gas atmosphere to perform the isothermal sintering such that an oxygen partial pressure is stepwise increased three times at the isothermal sintering temperature. 2. The method of claim 1, wherein an amount of additive cations with respect to about 1 g of uranium-based cations in the uranium oxide green pellet is about 10-2000 μg. 3. The method of claim 1, wherein the additive powder is a powder mixed with at least one or more of a chromium (Cr)-compound, an aluminum (Al)-compound, and a yttrium (Y)-compound. 4. The method of claim 3, wherein the Cr-compound, the Al-compound, and the Y-compound are at least one or more selected from the group consisting of oxides, nitrates, stearates, chlorides, and hydroxides. 5. The method of claim 1, wherein in the preparing of the additive mixed uranium oxide powder, a UO2-based powder is a UO2 powder or a powder mixed with the UO2 powder and one or more of a PuO2 powder, a Gd2O3 powder, and a ThO2 powder. 6. The method of claim 1, wherein the atmosphere control gas is a hydrogen gas or a mixing gas of a hydrogen gas and at least one or more gases selected from the group consisting of carbon dioxide, water vapor, and inert gases. 7. The method of claim 1, wherein the forming of the green pellets is performed by a step of putting the additive mixed uranium oxide powder in a forming mold and molding at a pressure of about 3-5 tons/cm2. 8. The method of claim 1, wherein the additive mixed uranium oxide powder comprises at least one or more of PuO2 powder, Gd2O3 powder, and ThO2 powder.