Patent Number: 
Section: claims

1. A joining structure for joining a top nozzle and a guide thimble, the joining structure comprising:an outer guide post having a hollow cylindrical shape and an external thread formed on an outer circumferential surface of a lower end portion of the outer guide post;an inner-extension tube head comprisingan annular retaining part formed on an upper end portion of the inner-extension tube head and having an outer diameter greater than that of other portion of the inner extension tube head, andan internal thread formed on an inner circumferential surface of a medial portion of the inner-extension tube head;an inner-extension tube body having a hollow cylindrical shape and includinga first external thread formed on an outer circumferential surface of an upper portion of the inner-extension tube body, anda second external thread formed on the outer circumferential surface of a lower end portion of the inner-extension tube body; anda wedge having a hollow cylindrical shape and including at least one stop protrusion formed on a bottom surface of the wedge, the stop protrusion extending a predetermined length in an axial direction of the wedge, the wedge being separated from the inner-extension tube body and the outer guide post,wherein the inner-extension tube body is coaxially inserted into the outer guide post,the inner-extension tube head is mounted on a top of the outer guide post, the first external thread of the inner-extension tube body is engaged with the internal thread of the inner-extension tube head,the second external thread of the inner-extension tube body is engaged with an internal thread of the guide thimble, andthe wedge is mounted on the outer circumferential surface of the inner-extension tube body and disposed between a bottom end of the outer guide post and the guide thimble. 2. The joining structure as set forth in claim 1, wherein the inner-extension tube body further includesat least one rotation-preventing surface formed on the outer circumferential surface of the inner-extension tube body adjacently with the second external thread, the rotation-preventing surface being formed by cutting out a portion of the inner-extension tube body in a circumferential direction to form a planar shape. 3. The joining structure as set forth in claim 1, wherein the inner-extension tube head further includes a threaded-portion introducing part provided on a lower portion of the inner-extension tube head, the threaded-portion introducing part having an inner diameter greater than inner diameters of other portions of the inner-extension tube head, andwherein the inner-extension tube body further includes a threaded-portion guide part provided on an upper end portion of the inner-extension tube body, the threaded-portion guide part having an outer diameter less than outer diameters of other portions of the inner-extension tube. 4. The joining structure as set forth in claim 2, wherein the inner-extension tube head further includes a threaded-portion introducing part provided on a lower portion of the inner-extension tube head, the threaded-portion introducing part having an inner diameter greater than inner diameters of other portions of the inner-extension tube head, andwherein the inner-extension tube body further includes a threaded-portion guide part provided on an upper end portion of the inner-extension tube body, the threaded-portion guide part having an outer diameter less than outer diameters of other portions of the inner-extension tube.