Patent Number: 
Section: claims

1. A combination of a guide thimble and a top nozzle for a nuclear fuel assembly, the combination comprising:a guide thimble, anda top nozzle coupled to the guide thimble, the top nozzle comprising:a flow plate located above the guide thimble, with a coupling through hole formed through the flow plate;an outer guide post coupled at a lower end thereof to the coupling through hole of the flow plate;an inner-extension tube disposed in the outer guide post in such a way that a lower end of the inner-extension tube passes through the coupling through hole of the flow plate; andan inner-extension tube head coupled to an upper end of the inner-extension tube and to an upper end of the outer guide post,wherein:the inner-extension tube includes a rim disposed within the coupling through hole of the flow plate;the rim has a circumferential outer surface which includes a first region and a second region;a circumferential inner surface of the coupling through hole of the flow plate includes a first part disposed at a position corresponding to the first region of the rim of the inner-extension tube and a second part; andthe first region and the first part, collectively, prevent the inner-extension tube from rotating when the inner-extension tube head is rotated. 2. The combination as set forth in claim 1, whereinthe first region is formed as a planar surface; andthe first part is formed as a planar surface.