Patent Number: 
Section: claims

1. A control rod for nuclear reactors comprising:four wings including neutron absorbing hafnium;a front end structural member which has a cross shape in cross section and includes brackets bonded to leading ends of the wings; anda terminal end structural member which has a cross shape in cross section and includes brackets bonded to tailing ends of the wings, whereinthe four wings are bonded to a wing bonding member including a cross-shaped center shaft so as to form a cross shape in such a manner that the wings are spaced from each other at predetermined intervals in an axial direction,at least the front end structural member and the wing bonding member are made of a zirconium alloy containing hafnium of which the hafnium content is greater than or equal to that of natural compositions,the wings have principal portions including neutron absorbing plates having neutron absorbing portions, each of the wings has an outer surface which is configured to be opposed to a fuel assembly, and each of the wings includes a composite member that includes a hafnium plate or a hafnium-zirconium alloy plate diluted by zirconium and a zirconium plate that is of pure zirconium, the zirconium plate covering the outer surface of each wing that is configured to be opposed to the fuel assembly, andthe neutron-absorbing plates are opposed to each other in such a manner that trap spaces in which reactor water is present are disposed between the neutron absorbing plates, and a thickness of each neutron absorbing plate is substantially uniform in a direction in which the control rod is inserted or withdrawn. 2. The control rod according to claim 1, further comprising tie rods, disposed in the wings, for connecting the front end structural member and the terminal end structural member to each other, wherein the neutron-absorbing plates are mounted in the wings so as to slide from the leading ends toward the tailing ends of the wings or from the tailing ends toward the leading ends of the wings. 3. The control rod according to claim 2, wherein the tie rods are made of hafnium. 4. The control rod according to claim 1, further comprising wing end reinforcing members which are disposed in the trap spaces between the neutron absorbing plates and which slides in the axial direction of the control rod. 5. The control rod according to claim 4, wherein the wing end reinforcing members are made of hafnium. 6. The control rod according to claim 1, wherein each of the neutron absorbing portions has a first portion extending from the leading end of the neutron-absorbing portion and having a length equal to 1/24 to 2/24 of a length of the neutron absorbing portion, a second portion extending from the first portion and having a length equal to a difference obtained by subtracting the length of the first portion from ¼ to ½ of the length of the neutron absorbing portion, and a third portion extending from the tailing end of the neutron absorbing portion, in which the second portion has a width greater than that of the third portion, and an outer end of a leading portion of each wing is aligned with that of a tailing portion of the wing. 7. The control rod according to claim 6, wherein the first portion has a width less than that of the second portion. 8. The control rod according to claim 1, further comprising a hafnium-zircaloy composite material and short narrow hafnium rods, wherein the hafnium-zircaloy composite material is repeatedly mount-folded and valley-folded so as to provide mount-folded and valley-folded portions which are arranged at equal intervals and which extend in parallel to each other, the valley-folded folded portions are brought close to each other so that the folded hafnium-zircaloy composite material has a cross shape in horizontal cross section, and the hafnium rods are arranged in end portions of the wings in form of spacers. 9. The control rod according to claim 8, further comprising a tie cross made of zircaloy, wherein the valley-folded portions partially have longitudinal holes regularly and intermittently arranged in the axial direction and portions of the tie cross are arranged above and below the longitudinal holes so as to maintain the cross shape and improve mechanical strength. 10. The control rod according to claim 1, further comprising short narrow hafnium rods functioning as spacers, wherein the four composite members are bent so as to provide an L-shape, bent portions of the composite members are brought close to each other so as to be directed to a center of a cross shape, and the hafnium rods are attached to end portions of the bent composite members. 11. The control rod according to claim 10, further comprising a tie cross made of zircaloy, wherein the bent portions partially have longitudinal holes regularly and intermittently arranged in the axial direction and portions of the tie cross are arranged above and below the longitudinal holes so as to maintain the cross shape and improve mechanical strength. 12. The control rod according to claim 1, wherein each wing is formed so that two of the composite members are opposed to each other with a space therebetween and spacers for keeping spaces are fixed to both ends of the composite members in an inserting or withdrawing direction and a perpendicular direction, and the four wings are bonded to a tie cross including a cross-shaped center shaft so as to form a cross shape in such a manner that the wings are spaced from each other at predetermined intervals in the axial direction. 13. The control rod according to claim 1, wherein each wing is formed so that one of the composite members is bent so as to provide a U-shape with a space, and a plurality of short spacers are fixed to end portions of the bent composite member located on the side close to a cross-shaped center shaft included in a tie cross, the tie cross is spaced from the wing at a predetermined distance in the axial direction, and the four wings are bonded to each other so as to form a cross shape. 14. The control rod according to claim 1, wherein each wing is formed so that one of the composite members is bent so as to provide a cylindrical shape, both end portions of the bent composite member are bonded to each other to form a cylinder, which is then pressed into a flattened tube, and a plurality of short spacers are fixed to outer end portions and inner portions of the flattened tube, the inner portions being located on the side close to a cross-shaped center shaft, which is included in a tie cross, and the four wings are bonded to form a cross shape so that the tie cross is spaced from the wings at a predetermined distance in the axial direction. 15. The control rod according to claim 1, wherein the wings are fixed with members, located in a vicinity of end portions of the cross-shaped center shaft for preventing the wings from being opened. 16. The control rod according to claim 1, wherein spacers, made of hafnium, disposed in the outer end portions of the wings are short rods and center portions of the short rods are fixed to the composite members. 17. The control rod according to claim 1, whereineach of the zirconium plates has a thickness in the range of approximately 0.2 to 0.5 millimeters, andeach of the composite members has a thickness in the range of approximately 2 to 2.5 millimeters. 18. The control rod according to claim 1, wherein each of the composite members includes another different zirconium plate, the zirconium plate and the another different zirconium plate sandwiching the hafnium plate or the hafnium-zirconium alloy plate. 19. A control rod for nuclear reactors comprising:four wings including neutron absorbing hafnium;a front end structural member which has a cross shape in cross section; anda terminal end structural member which has a cross shape in cross section, whereinthe four wings are bonded to a wing bonding member including a cross-shaped center shaft so as to form a cross shape in such a manner that the wings are spaced from each other at predetermined intervals in an axial direction,at least the front end structural member and the wing bonding member are made of a zirconium alloy containing hafnium of which the hafnium content is greater than or equal to that of natural compositions,the wings have principal portions including neutron absorbing plates having neutron absorbing portions, each of the wings has an outer surface which is configured to be opposed to a fuel assembly, and each of the wings includes a composite member that includes a middle plate, which is a hafnium plate or a hafnium-zirconium alloy plate diluted by zirconium, a first zirconium plate, and a second zirconium plate, the first zirconium plate being bonded to a top surface of the middle plate, and the second zirconium plate being bonded to a bottom surface of the middle plate, the top surface and the bottom surface of the middle plate being covered by the first and second zirconium plates, respectively, without side surfaces of the middle plate being covered by zirconium plates, and the first zirconium plate covering the outer surface of each wing that is configured to be opposed to the fuel assembly, andthe neutron-absorbing plates are opposed to each other in such a manner that trap spaces in which reactor water is present are disposed between the neutron absorbing plates, and a thickness of each neutron absorbing plate is substantially uniform in a direction in which the control rod is inserted or withdrawn.