Patent Number: 046559990
Section: claims

1. A control rod for a nuclear reactor comprising a central supporting member which extends in an axial direction of said control rod; sheaths which are arranged in the shape of a cross in a radial direction and which are mounted on said central supporting member; a plurality of neutron absorbing rods which are received in said each sheath and which are arrayed in a row in the radial direction; a lower supporting member which is disposed under said sheaths and which supports said neutron absorbing rods; at least one neutron absorbing hafnium sheet which is located between an inner wall of said each sheath and said row of neutron absorbing rods so as to absorb some of neutrons directed toward said neutron absorbing rods; and an upper supporting member which is mounted on said sheaths and said central supporting members. 2. A control rod for a nuclear reactor as defined in claim 1, wherein said each neutron absorbing rod is composed of cladding and boron carbide powder packed therein. 3. A control rod for a nuclear reactor as defined in claim 1, wherein said hafnium sheet is thicker in end parts said row of the neutron absorbing rods than in a central part thereof. 4. A control rod for a nuclear reactor comprising: a plurality of neutron absorbing rods arranged in a row, each neutron absorbing rod containing a cladding and boron carbide powder packed in said cladding;  sheath means extending in the direction of said row for containing said row of neutron absorbing rods therein, said sheath means having a wall surface spaced from said row of said neutron absorbing rods contained therein; and  hafnium sheet means extending at least in the direction of said row and disposed in a gap between at least a portion of the wall surface of said sheath means and at least a portion of said row of said neutron absorbing rods, said sheath means containing said hafnium sheet means and said row of neutron absorbing rods therein.  a plurality of sheaths each elongated axially and extending radially, said sheaths being arranged so as to be cruciform in a plane perpendicular to the axial direction;  a plurality of neutron absorbing rods inserted in each of said plurality of sheaths so that said neutron absorbing rods within said each sheath are in a row extending in the radial direction, each of said neutron absorbing rods being constituted of a cladding and boron carbide powder packed in said cladding;  a plurality of thin hafnium sheets, a respective sheet being disposed in a gap defined between a respective sheath and one of the opposite sides of said neutron absorbing rods in said row so as to absorb part of neutrons to be directed to each of said neutron absorbing rods, said thin hafnium sheets being independent of said sheaths and said neutron absorbing rods and having substantially the same width in the radial direction as one of said neutron absorbing rod rows;  a lower supporting member, connected to lower portions of said sheaths, for supporting said plurality of neutron absorbing rods and said plurality of thin hafnium sheets; and  an upper supporting member, mounted on upper portions of said plurality of sheaths, for supporting said plurality of sheaths. 5. A control rod for a nuclear reactor as defined in claim 4, wherein said hafnium sheet means includes a pair of thin hafnium sheet members arranged to have oppositely facing surface portions spaced from one another with said row of neutron absorbing rods being interposed between said oppositely facing surface portions of said pair of hafnium sheet members so that one sheet member faces on side of said row of neutron absorbing rods. 6. A control rod for a nuclear reactor as defined in claim 5, wherein said pair of hafnium sheets members are independent of said sheath means and absorb a portion of neutrons directed toward said neutron absorbing rods. 7. A control rod for a nuclear reactor as defined in claim 5, wherein said pair of hafnium sheet members are joined at one end so as to provide a U-shape. 8. A control rod for a nuclear reactor as defined in claim 5, further including a round solid rod of hafnium disposed at one end of said row of neutron absorbing rods for absorbing neutrons. 9. A control rod for a nuclear reactor as defined in claim 5, wherein each of said hafnium sheet members extending in the direction of said row has a smaller thickness in a central part thereof than in the end parts thereof. 10. A control rod for a nuclear reactor as defined in claim 9, wherein said pair of hafnium sheet members are joind at one end so as to provide a U-shape. 11. A control rod for a nuclear reactor as defined in claim 5, wherein said thin hafnium sheet members have a length of about 1/4 of a length of said neutron absorbing rods, said thin hafnium sheet members being arranged so as to face only a top end part of said row of neutron absorbing rods. 12. A control rod for a nuclear reactor comprising: 13. A control rod for a nuclear reactor as defined in claim 12, wherein said hafnium sheets have substantially the same length as one of said neutron absorbing rods. 14. A control rod for a nuclear reactor as defined in claim 12, wherein each of said thin hafnium sheets is connected to said upper supporting member so as to be removable with said upper supporting member. 15. A control rod for a nuclear reactor as defined in claim 12, further including a round solid rod of hafnium disposed at a radial outermost end of said neutron absorbing rod row. 16. A control rod for a nuclear reactor as defined in claim 12, wherein each of said thin hafnium sheets is 0.3 mm thick.