PURPOSE: To obtain a regulating method of density of a nuclear fuel pellet which makes it possible to increase the density of the nuclear fuel pellet advantageously for a stable operation of a plant without relying on a method of controlling molding conditions or sintering conditions and which enables diverse regulation of the density in combination with a method of adding a triuranium octoxide derived from a sinter.

CONSTITUTION: In regard to a nuclear fuel pellet prepared by a method wherein material powder constituted of a substance to which a dioxide of at least one kind of element selected from uranium and plutonium or an oxide of a rare- earth element is added is subjected to molding or oxidation-reduction sintering, a triuranium octoxide of the element formed of the dioxide of the element without being subjected to thermal hysteresis of 1000°C or above is added to the material powder. In addition to the triuranium octoxide, in another way, a triuranium octoxide obtained by roasting a nuclear fuel sinter is added to the material powder. The two kinds of triuranium octoxides can be obtained by roasting the substance to which the dioxide of the aforesaid element or the oxide of the rare-earth element is added, at a temperature of 350 to 800°C.