Patent Number: 044514280
Section: claims

1. A method of producing a control rod comprising the steps of: producing by a zone melting process a second neutron absorbing material causing an (n, .UPSILON.) reaction to take place by continuously reducing the ratio of a material of large neutron absorption cross section to a material of small neutron absorption cross section in going from at least a predetermined position along a major portion of said second neutron absorbing material toward one end thereof; and  arranging said second neutron absorbing material in an end portion of the control rod at which the control rod is inserted in a reactor core, said second neutron absorbing material being located nearer to said inserting end of the control rod than a plurality of poison tubes having a charge of a first neutron absorbing material causing an (n, .alpha.) reaction to take place filled therein and sealed, said second neutron absorbing material extending in the insertion direction of the control rod and having a volume which exhibits substantially no decrease along the extent of said second neutron absorbing material in the insertion direction and said first and second neutron absorbing materials have substantially the same neutron absorbing capacity in an area between the poison tubes and the second neutron absorbing material. 2. A method according to claim 1, wherein said second neutron absorbing material is in the form of neutron absorbing plates extending in the insertion direction of the control rod. 3. A method according to claim 2, wherein the step of arranging includes arranging said neutron absorbing plates to contact an end portion of at least a portion of said plurality of poison tubes with said neutron absorbing plates extending forwardly therefrom, said neutron absorbing plates and said poison tubes having substantially the same absorption cross section in the region of contact therebetween. 4. A method according to claim 1, wherein said second neutron absorbing material is an all solid solution type alloy. 5. A control rod for a nuclear reactor comprising an absorber rod means extending in the direction of insertion of the control rod in a reactor core, said absorber rod means being charged with a material having a large neutron absorption cross section for causing an (n, .alpha.) reaction to take place upon absorbing neutrons, and neutron absorbing plates extending in the insertion direction of the control rod and being arranged at a forward end portion of the control rod, said neutron absorbing plates being formed of an alloy of a material of a large neutron absorption cross section and a material of a small neutron absorption cross section for causing an (n, .UPSILON.) reaction to take place upon absorbing neutrons, the mixing ratio of said material of small neutron absorption cross section to said material of large neutron absorption cross section in said alloy being continuously varied in the insertion direction from at least a predetermined position along the extent of a major portion of said neutron absorbing plates and becoming increasingly higher toward the forward end of the control rod, said neutron absorbing plates having a volume which exhibits substantially no decrease along the extent of said neutron absorbing plates in the insertion direction and said material charged in said absorber rod means and said material of said neutron absorbing plate have substantially the same neutron absorbing capacity in an area between the absorber rod means and the neutron absorbing plate. 6. A control rod according to claim 5, wherein said neutron absorbing plates contact an end portion of at least a portion of said absorber rod means and extend forwardly therefrom, said neutron absorbing plates and said absorber rod means having substantially the same absorption cross section in the region of contact therebetween. 7. A control rod according to claim 5 or 6, wherein the material with which said absorber rod means is charged is boron carbide and the alloy forming said neutron absorbing plates is an Hf-Zr alloy. 8. A control rod according to claim 7, wherein said absorber rod means comprises a plurality of poison tubes. 9. A control rod according to claim 5, wherein the variation in the mixing ratio of the materials of said alloy forming said neutron absorbing plates is obtained by a zone melting process. 10. A control rod according to claim 9, wherein said alloy forming said neutron absorbing plates is an all solid solution type alloy of an Hf-Zr alloy. 11. A method according to claim 1, wherein the ratio is continuously reduced in a linear manner. 12. A method according to claim 1, wherein the volume of said second neutron absorbing material is substantially constant along the extent thereof. 13. A method according to claim 1, wherein the volume of said second neutron absorbing material increases along the extent thereof in the insertion direction. 14. An apparatus according to claim 5, wherein the mixing ratio continuously varies in a linear manner. 15. A control rod according to claim 5, wherein the volume of said neutron absorbing plates is substantially constant along the extent thereof. 16. A control rod according to claim 5, wherein the volume of said neutron absorbing plates increases along the extent thereof in the insertion direction.