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 at least one rotation-preventing protrusion provided on a circumferential outer surface of the inner-extension tube such that the rotation-preventing protrusion is disposed within the coupling through hole of the flow plate;a circumferential inner surface of the coupling through hole of the flow plate includes a first part disposed at a position corresponding to the rotation-preventing protrusion of the inner-extension tube and a second part; andthe rotation-preventing protrusion 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, wherein the rotation-preventing protrusion has a planar surface, and the first part is formed as a planar surface. 3. The combination as set forth in claim 1 further comprising:a first coupling protrusion provided on a circumferential outer surface of the inner-extension tube head;a first coupling groove formed in a circumferential inner surface of the inner-extension tube so that the first coupling protrusion is inserted into the first coupling groove;a second coupling protrusion provided on the circumferential outer surface of the inner-extension tube; anda second coupling groove formed in a circumferential inner surface of the outer guide post so that the second coupling protrusion is inserted into the second coupling groove. 4. The combination as set forth in claim 3, wherein the inner-extension tube has at least one longitudinal slit extending a predetermined length from an upper end of the inner-extension tube.