Patent Number: 
Section: claims

1. A coupling structure of a fuel assembly comprising:a latch sleeve having a projecting portion formed at an upper end portion thereof;an upper nozzle having formed therein a latch sleeve installation hole configured to receive the upper end portion of the latch sleeve;a lock key, installed inside the latch sleeve installation hole, comprises an opening portion formed in a bottom surface of the lock key, the opening portion having a shape corresponding to the projecting portion, in which the latch sleeve and the lock key are coupled to each other to form the coupling structure of the fuel assembly by rotating the lock key with the upper end portion of the latch sleeve inserted into the lock key;a flange portion formed at a portion of the latch sleeve arranged below the projecting portion; anda gap portion formed at a portion of the latch sleeve arranged between the projecting portion and the flange portion, whereinthe coupling structure of the fuel assembly comprises a screw coupling means for screw-coupling the lock key to the upper nozzle side with a male screw thread formed on a side surface of the lock key,the flange portion is configured to contact a bottom surface of the upper nozzle,the gap portion is configured to accommodate a receiver portion of the lock key, andthe screw coupling means is a position adjustment means for filling a vertical gap between the receiver portion and the bottom surface of the projecting portion by moving the lock key up by rotating the lock key with the male thread formed on the lock key being screw coupled with the upper nozzle side so that a vertical play is eliminated at a fitting portion between the lock key and the latch sleeve. 2. The coupling structure of a fuel assembly according to claim 1 characterized in that the screw coupling means screw-couples the lock key and the latch sleeve installation hole with a female screw thread formed on an inner surface of the latch sleeve installation hole. 3. The coupling structure of a fuel assembly according to claim 1 characterized in thatthe screw coupling means includes a cylindrical thick-walled pipe installed in the latch sleeve installation hole, the thick-walled pipe having a female screw thread thread formed in an inner surface thereof, andthe screw coupling means screw-couples the lock key and the thick-walled pipe by installing the lock key in the thick-walled pipe. 4. The coupling structure of a fuel assembly according to claim 1, wherein the screw coupling means does not comprise a detachable nut. 5. A coupling method for a coupling structure of a fuel assembly comprising:a latch sleeve having a projecting portion formed at an upper end portion thereof;an upper nozzle having formed therein a latch sleeve installation hole into which the upper end portion of the latch sleeve is inserted, a lock key which is installed inside the latch sleeve installation hole, and which has an opening portion formed in a bottom surface thereof, the opening portion having a shape corresponding to the projecting portion;a screw coupling means for screw-coupling the lock key to the latch sleeve installation hole by a male thread formed on a side surface of the lock key and a female thread formed on an inner surface of the latch sleeve installation hole;a flange portion formed at a portion of the latch sleeve arranged below the projecting portion and configured to contact a lower surface of the upper nozzle;a gap portion formed at a portion of the latch sleeve arranged between the projecting portion and the flange portion; anda receiver portion formed in the opening portion and configured to enter the gap portion, in which the latch sleeve and the lock key are coupled to each other by rotating the lock key with the upper end portion of the latch sleeve being inserted into the lock key so that the projecting portion and the receiver portion are placed over each other,the coupling method comprising:performing position adjustment using the screw coupling means to fill a vertical gap between an upper surface of the receiver portion and a bottom surface of the projecting portion by moving the lock key up by rotating the lock key with the male thread being screw coupled with the female screw so that a vertical play is eliminated at a fitting portion between the lock key and the latch sleeve.