PURPOSE: To reduce mechanical interaction between a pellet and a cladded tube by providing low density parts, each having a total volumetric ratio within specified % range and a T.D. density lower by a specified % than other parts, in the vicinity of fringe parts at the opposite ends.

CONSTITUTION: The pellet is formed into a column symmetrical to the axis and low density parts 6, each having T.D. density lower by 0.5-3% than other parts, are formed in the upper and lower end parts, including the fringe parts 1, 2, in parallel with the end faces 3, 4. The total volumetric ratio of the upper and lower low density parts 6 to the entire pellet is set in the range of 5-40%. It is important that the fringe parts at the opposite ends are included in the low density parts 6 and the low density parts including the edge parts have a structure of dish or chamfer. Since the edge part has a low density, flexible contact with a cladding tube is realized thus lightening damage on the cladding tube.