Patent Number: 041750020
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of this invention will be explained by referring to the accompanying drawings. In FIG. 6 a pellet 18 is obtained by compression-molding powdered ceramic nuclear fuel, such as powdered uranium dioxide, at a molding density-theoretical density ratio of 51% and sintering it for 2 hours at 1,700.degree. C. in a stream of hydrogen. A pair of bottomed holes 21, 22 are centrally provided one at each end of the pellet 18. The opposite bottomed holes 21 and 22 are convergent toward their bottoms to provide U-shaped cross-sections. A ratio d/l of the diameter d of the opening to the depth l of the holes 21 and 22 is smaller than unity. The holes 21 and 22 of the pellet 18 do not communicate with each other. Pellets 18 having, for example, a diameter D of 12.4 mm, a length L of 12.5 mm, a hole diameter d of 2 mm, a hole length l of 3 mm and a hole inclination of 1/5 were received, as shown in FIG. 1, into a clad casing 1, and both the ends of the clad casing 1 were sealed to assemble a nuclear fuel rod. Irradiation tests were conducted under such a condition that PCMI tends to occur. It has been found that the extent to which the clad casing 1 is locally deformed under the PCMI condition is very small and that the axial elongation is smaller than that of a conventional counterpart. Likewise, pellets whose d/l ratios were greater than unity was manufactured and a nuclear rod was assembled using these pellets. However, no comparable effect was founded in comparison with those pellets whose d/l ratio were smaller than unity. Unlike a pellet 16 shown in FIG. 5, the pellet as shown in FIG. 6 has its opposite holes 21 and 22 bottomed, preventing a drop of fragments as occurs due to the destruction of the pellet during irradiation. As a result, the temperature distribution was made uniform. Pellets having no such holes 21 and 22 were manufactured under the same condition. In this case, a very narrow crack appeared on the end surfaces of the pellet and a local deformation of the clad casing 1 was developed under such a condition that PCMI tends to occur. Although the nuclear fuel element as shown in FIG. 6 has been explained, this invention is not restricted thereto. The nuclear fuel element may have shapes as shown in FIGS. 7 and 8. A pellet shown in FIG. 7 has, like a pellet shown in FIG. 3, shallow plate-like dimples 24 and 25 which are provided one at each end. A pellet shown in FIG. 8 has, like a pellet 13 shown in FIG. 4, its peripheral end edges chamfered to provide bevel surfaces 26 and 27, respectively. It has been found that the pellets shown in FIGS. 7 and 8 involves a decreased PCMI in comparison with the pellet shown in FIG. 6. A better result can be obtained if the shoulder of the bottomed holes 21 and 22 of the pellet shown in FIG. 7 is curvilinearly formed.