Patent Number: 
Section: claims

1. A control rod, comprising:a tie-rod;a handle mounted to an upper end portion of said tie-rod;a connector plate mounted to a lower end portion of said tie-rod;four sheaths having a U-shaped cross-section, welded intermittently to said tie-rod at a plurality of locations of a weld portion in the axial direction of said tie-rod, extending in four directions from said tie-rod, said sheaths having an upper end in the axial direction welded to said handle and a lower end in the axial direction welded to said connector plate; anda neutron absorbing member disposed inside each of said sheaths,wherein an upper end of said weld portion located at an uppermost position in the axial direction of said tie-rod among the plurality of said weld portions between said tie-rod and said sheath in the axial direction is disposed at a position within a range between 0.8 and 13% of total axial length Ls of said sheath below an upper end of said sheath. 2. The control rod according to claim 1,wherein said upper end of said weld portion located at said uppermost position is disposed at a position within a range between 10 to 13% of said total axial length Ls below said upper end of said sheath. 3. A control rod, comprising:a tie-rod;a handle mounted to an upper end portion of said tie-rod;a connector plate mounted to a lower end portion of said tie-rod;four sheaths having a U-shaped cross-section and a plurality of projecting portions, which are welded at a weld portion to said tie-rod, protruding in a direction perpendicular to an axial core of said tie-rod being formed intermittently on a tie-rod-side end portion in an axial direction of said tie-rod, extending in four directions from said tie-rod, said sheaths having an upper end in the axial direction welded to said handle and a lower end in the axial direction welded to said connector plate; anda neutron absorbing member disposed in each of said sheaths,wherein an upper end of said weld portion located at an uppermost position in the axial direction of said tie-rod among said weld portions between said tie-rod and said plurality of projecting portions is disposed at a position within a range between 0.8 and 13% of a total axial length Ls of said sheath below the upper end of said sheath. 4. The control rod according to claim 3,wherein said upper end of said weld portion located at said uppermost position is disposed at a position within a range between 10 and 13% of said total axial length Ls below said upper end of said sheath. 5. The control rod according to claim 1,wherein said neutron absorbing member is a hafnium member. 6. The control rod according to claim 4,wherein said neutron absorbing member is a hafnium member. 7. The control rod according to claim 1, wherein the sheaths are made of stainless steel and have a plurality of apertures therein, and wherein the neutron absorbing member is disposed inside each of said sheaths so as to form a gap between said neutron absorbing member and said sheath. 8. The control rod according to claim 3, wherein the sheaths are made of stainless steel and have a plurality of apertures therein, and wherein the neutron absorbing member is disposed inside each of said sheaths so as to form a gap between said neutron absorbing member and said sheath.