Patent Number: 
Section: claims

1. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to tips of cruciform arms of the tie rod, each of sheaths having a U-shaped cross section, comprising the steps of: forming the tie rod which is provided with steps for fixing the sheaths at the tips of the cruciform arms;  fitting the tips of each of the sheaths onto the steps of the tie rod; and  fixing each of the sheaths to the tie rod by performing a laser welding with the sheath being fitted onto the tie rod, wherein a surface of the tie rod is irradiated with a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in such a manner that an axial center position of the beam is shifted from an end face position of the step of the tie rod toward an axis center of the tie rod to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof. 2. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to tips of cruciform arms of the tie rod, each of sheaths having a U-shaped cross section, comprising the steps of: forming the tie rod which is provided with steps for fixing the sheaths at the tips of the cruciform arms by a drawing process;  fitting the tips of each of the sheaths onto the steps of the tie rod; and  fitting both tips of the U-shape of each of the sheaths onto the steps of the tie rod; and  fixing each of the sheaths to the tie rod by performing a laser welding with the sheath being fitted onto the tie rod, wherein a surface of the tie rod is irradiated with a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in such a manner as to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof. 3. The method for manufacturing a control rod for boiling water reactor according to  claim 2 , wherein the laser welding is performed in a state set so that an axial center position of the beam is shifted from an end face position of the step of the tie rod at least toward an axis center of the tie rod. claim 2 4. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to a lower end of the handle, comprising the steps of: forming a step for fixing the sheaths on the lower end of the handle;  fitting an upper edge of each of the sheaths onto the step of the handle; and  fixing each of the sheaths to the handle by performing a laser welding with the sheath being fitted onto the handle, wherein a surface of the handle is irradiated with a YAG (Yttrium aluminum garnet) laser beam or a CO 2  laser beam in such a manner that an axial center position of the beam is shifted from an end face position of the step of the handle to a side opposite to the sheath to achieve a continuous weld of at least a part extending along the upper edge of the sheath. 5. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to an upper end of the lower part support member or the velocity limiter, comprising the steps of: forming a step for fixing the sheaths on the upper end of the lower part support member or the velocity limiter;  fitting a lower edge of each of the sheaths onto the step of the lower part support member or the velocity limiter; and  fixing each of the sheaths to the lower part support member or the velocity limiter by performing a laser welding with the sheath being fitted onto the lower part support member or the velocity limiter, wherein a surface of the lower part support member or the velocity limiter is irradiated with a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in such a manner that an axial center position of the beam is shifted from an end face position of the step of the lower part support member or the velocity limiter to a side opposite to the sheath to achieve a continuous weld of at least a part extending along the lower edge of the sheath. 6. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to tips of cruciform arms of the tie rod, each of sheaths having a U-shaped cross section, comprising the steps of: P =0.184+1.11 xc3x97G +0.964 xc3x97A +1.07 xc3x97H xe2x88x921.17 xc3x97D xe2x88x920.11 xc3x97W xe2x88x920.807 xc3x97L  forming the tie rod which is provided with steps for fixing the sheaths at the tips of the cruciform arms;  fitting the tips of each of the sheaths onto the steps of the tie rod; and  fixing each of the sheaths to the tie rod by performing a laser welding with the sheath being fitted onto the tie rod, wherein a surface of the tie rod is irradiated with a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in such a manner that an axial center position of the beam is shifted from an end face position of the step of the tie rod toward an axis center of the tie rod to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof, wherein  the laser welding is performed under a welding condition which allows a parameter P to be in the range of xe2x88x920.5 to 0.5, the parameter P being obtainable from the following equation:  where G represents a gap between an inner surface of the sheath and a bottom surface of the step in a state where the sheath is fitted onto the step;  L represents an overlap of the inner surface of the sheath with the bottom surface of the step;  A represents a distance from the axial center position of the laser beam to a tip of the sheath on the premise that a direction toward the sheath is a positive direction and a direction toward a side opposite to the sheath is a negative direction;  D represents a beam converging diameter of the laser beam;  H represents a heat input generated by the laser welding; and  W represents a supply for one meter of welding length of a fusion promoting welding rod at the time of welding. 7. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to a lower end of the handle, comprising the steps of: P =0.184+1.11 xc3x97G +0.964 xc3x97A +1.07 xc3x97H xe2x88x921.17 xc3x97D xe2x88x920.11 xc3x97W xe2x88x920.807 xc3x97L  forming a step for fixing the sheaths on the lower end of the handle;  fitting an upper edge of each of the sheaths onto the step of the handle; and  fixing each of the sheaths to the handle by performing a laser welding with the sheath being fitted onto the handle, wherein a surface of the handle is irradiated with a YAG (Yttrium aluminum garnet) laser beam or a CO 2  laser beam in such a manner that an axial center position of the beam is shifted from an end face position of the step of the handle to a side opposite to the sheath to achieve a continuous weld of at least a part extending along the upper edge of the sheath, wherein  the laser welding is performed under a welding condition which allows a parameter P to be in the range of xe2x88x920.5 to 0.5, the parameter P being obtainable from the following equation:  where G represents a gap between an inner surface of the sheath and a bottom surface of the step in a state where the sheath is fitted onto the step;  L represents an overlap of the inner surface of the sheath with the bottom surface of the step;  A represents a distance from the axial center position of the laser beam to a tip of the sheath on the premise that a direction toward the sheath is a positive direction and a direction toward a side opposite to the sheath is a negative direction;  D represents a beam converging diameter of the laser beam;  H represents a heat input generated by the laser welding; and  W represents a supply for one meter of welding length of a fusion promoting welding rod at the time of welding. 8. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to an upper end of the lower part support member or the velocity limiter, comprising the steps of: P =0.184+1.11 xc3x97G +0.964 xc3x97A +1.07 xc3x97H xe2x88x921.17 xc3x97D xe2x88x920.11 xc3x97W xe2x88x920.807 xc3x97L  where G represents a gap between an inner surface of the sheath and a bottom surface of the step in a state where the sheath is fitted onto the step; forming a step for fixing the sheaths on the upper end of the lower part support member or the velocity limiter;  fitting a lower edge of each of the sheaths onto the step of the lower part support member or the velocity limiter; and  fixing each of the sheaths to the lower part support member or the velocity limiter by performing a laser welding with the sheath being fitted onto the lower part support member or the velocity limiter, wherein a surface of the lower part support member or the velocity limiter is irradiated with a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in such a manner that an axial center position of the beam is shifted from an end face position of the step of the lower part support member or the velocity limiter to a side opposite to the sheath to achieve a continuous weld of at least a part extending along the lower edge of the sheath, wherein  the laser welding is performed under a welding condition which allows a parameter P to be in the range of xe2x88x920.5 to 0.5, the parameter P being obtainable from the following equation:  L represents an overlap of the inner surface of the sheath with the bottom surface of the step;  A represents a distance from the axial center position of the laser beam to a tip of the sheath on the premise that a direction toward the sheath is a positive direction and a direction toward a side opposite to the sheath is a negative direction;  D represents a beam converging diameter of the laser beam;  H represents a heat input generated by the laser welding; and  W represents a supply for one meter of welding length of a fusion promoting welding rod at the time of welding. 9. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to tips of cruciform arms of the tie rod, each of the sheaths having a U-shaped cross section, said method comprising the steps of: forming the tie rod which is provided with steps for fixing the sheaths at the tips of the cruciform arms;  fitting the tips of each of the sheaths onto the steps of the tie rod; and  welding each of the sheaths being fitted onto the tie rod to the tie rod by irradiating a surface of the tie rod with an axial center position of a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in position shifted from an end face position of the step of the tie rod toward an axis center of the tie rod. 10. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to tips of cruciform arms of the tie rod, each of the sheaths having a U-shaped cross section, said method comprising the steps of: forming the tie rod which is provided with steps for fixing the sheaths at the tips of the cruciform arms by a drawing process;  fitting the tips of each of the sheaths onto the steps of the tie rod;  fitting both of the tips of the U-shape of each of the sheaths onto the steps of the tie rod; and  welding each of the sheaths being fitted onto the tie rod to the tie rod by irradiating a surface of the tie rod with an axial center position of a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in position shifted from an end face position of the step of the tie rod toward an axis center of the tie rod. 11. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to a lower end of the handle, said method comprising the steps of: forming a step for fixing the sheaths on the lower end of the handle;  fitting an upper edge of each of the sheaths onto the step of the handle; and  welding each of the sheaths being fitted onto the handle by irradiating a surface of the handle with an axial center position of a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in position shifted from an end face position of the step of the handle to a side opposite to the sheath. 12. A method for manufacturing a control rod for boiling water reactor, the control rod comprising a tie rod having a cruciform cross section, a handle attached to an axially upper part of the tie rod, a lower part support member or a velocity limiter attached to an axially lower part of the tie rod, and sheaths attached to an upper end of the lower part support member or the velocity limiter, said method comprising the steps of: forming a step for fixing the sheaths on the upper end of the lower part support member or the velocity limiter;  fitting a lower edge of each of the sheaths onto the step of the lower part support member or the velocity limiter; and  welding each of the sheaths being fitted onto the lower part support member or the velocity limiter to the lower part support member or the velocity limiter by irradiating a surface of the lower part support member or the velocity limiter with an axial center position of a YAG (yttrium aluminum garnet) laser beam or a CO 2  laser beam in position shifted from an end face position of the step of the lower part support member or the velocity limiter to a side opposite to the sheath.