Patent Number: 041475917
Section: description

DETAILED DESCRIPTION OF THE INVENTION Hereunder, this invention will be described in detail in connection with embodiments. FIG. 5 shows a cross section of a fuel assembly according to this invention. In this embodiment, the increase of the wall thickness of a cladding tube at the upper end portion of each outermost fuel pin is achieved by mounting a ring spacer on said portion. In the figure, numeral 206 designates a ring spacer, numeral 203 a fuel pin, numeral 205 a wire spacer, and numeral 201 a wrapper tube. FIG. 6 is a front view of the outermost fuel pin within the fuel assembly. Owing to the ring spacer attached to the top or the upper end portion of the fuel pin, the sectional deformation of the fuel pin attributed to an external load is prevented. Simultaneously therewith, the direct contact between the cladding tube of the fuel pin and the wrapper tube wall at the upper end portion of the outermost fuel pin as illustrated in FIG. 4 is avoided, so that the fretting corrosion of the cladding tube is prevented. Further, the outermost fuel pins have their upper ends brought into contact with the wrapper tube through the ring spacers on all sides of the wrapper tube wall, so that the leaning of the fuel pin bundle into one direction can be lessened. The ring spacer is mounted on a gas plenum portion at the top of the fuel pin, not on a heat generating portion charged with fuel pellets as corresponds to the central part of the fuel pin, so that the width of temperature rise in the cladding tube at the top of the fuel pin is small. When the same material is employed for the ring spacer and the cladding tube, any compressive load on the cladding tube as otherwise caused by the difference of the expansions of both the members does not arise. Moreover, since the ring spacers are provided on only the outermost pins, their influence as a flow resistance to the coolant flowing within the fuel assembly is little as a whole. According to the present embodiment, the fuel assembly of this invention can be readily fabricated in such a way that the fuel pin with the ring spacer attached to the top portion thereof is used as the outermost fuel pin. On the other hand, the fuel assembly of this invention can also be performed in such a way that the cladding tube itself is thickened into the salient form at a ring spacer position in FIG. 6 and that the resultant fuel pin is arranged in the peripheral region of the assembly. As set forth above, according to the fuel assembly of this invention, the wall thickness of the cladding tube at the top or upper end portion of the outermost fuel pin is increased as compared with that at the remaining portion thereof whereby in case of the bowing deformation of the fuel pin, the fuel pin bundle within the fuel assembly is restrained from leaning, and simultaneously, the section of the cladding tube is prevented from being deformed by the external load acting on the fuel pin and the fretting corrosion of the cladding tube at the top of the fuel pin is avoided, so that the damage of the fuel pin can be prevented.