Patent Number: 
Section: claims

1. A high-temperature gas reactor control rod comprising:first and second control rod elements each having a neutron absorber between an outer cylinder and an inner cylinder that form a double cylindrical tubular shape along a longitudinal direction of each control rod element;first and second columnar support members respectively disposed in the inner cylinder of the first and second control rod elements, each of the first and second columnar support members axially extending in each control rod element in its entire length of the longitudinal direction to allow the first and second columnar support members to be vertically aligned and connected, thereby joining the first and second control rod elements in the longitudinal direction; anda joint structure provided to at least one of the first and second columnar support members and configured to connect a lower end of the first columnar support member and an upper end of the second columnar support member,wherein each of the first and second control rod elements has a neutron absorber supporting flange extending toward the inner cylinder, the neutron absorber supporting flange being formed on each support member, each support member supporting the load of the neutron absorber from below by the neutron absorber supporting flange,each support member has a side wall assembly and a hollow portion, the side wall assembly having a plurality of flat plates which define and form the hollow portion inside each support member, the hollow portion extending in the longitudinal direction,the plurality of flat plates are made of composite materials which are selected from the group consisting of C/C composite materials and SiC/SiC composite materials, andthe joint structure comprises a shaft and a loop-shaped joining belt, the shaft being inserted through the loop-shaped joining belt and two holes formed in the side wall assembly in each of the first and second support members to connect the lower end of the first columnar support member and the upper end of the second columnar support member. 2. The high-temperature gas reactor control rod according to claim 1, wherein the joining belt forms a flat belt shape. 3. The high-temperature gas reactor control rod according to claim 1, wherein a gap is provided between the control rod elements. 4. The high-temperature gas reactor control rod according to claim 1, wherein the joint structure is provided respectively at both upper and lower ends of the first or second support member, the shaft provided at the upper end and the shaft provided at the lower end are disposed so as to extend in different directions in a planar view, and the width of the joining belt is configured to be smaller than the axial length of the shaft in the hollow portion. 5. The high-temperature gas reactor control rod according to claim 1, wherein the diameter of the shaft is smaller than the diameter of the two holes formed in the side wall assembly so as to form a clearance for alleviating stress. 6. The high-temperature gas reactor control rod according to claim 1, wherein an internal hollow portion width of the joining belt along a direction perpendicular to the axis of the shaft is larger than the diameter of the shaft so as to form a clearance for alleviating stress. 7. The high-temperature gas reactor control rod according to claim 1, wherein the joint structure comprises a first shaft, a second shaft, and a loop-shaped joining belt, the first shaft is inserted through the loop-shaped joining belt and holes formed at a position near the lower end of the side wall assembly of the first support member in the first control rod element, and the second shaft is inserted through the loop-shaped joining belt and holes formed at a position near the upper end of the side wall assembly of the second support member in the second control rod element. 8. The high-temperature gas reactor control rod according to claim 1, wherein a length of the shaft (E), a distance between outer ends of the two holes (D), and a space between each outer end of the two holes and an inner surface of the inner cylinder (S) meet the following relationship (1):S+D≦E≦2S+D  (1). 9. The high-temperature gas reactor control rod according to claim 1, further comprising a neutron absorber supporting flange extending toward the inner cylinder, the neutron absorber supporting flange formed at the lower end of the first support member or in the vicinity thereof. 10. The high-temperature gas reactor control rod according to claim 1, wherein the support members have a polygonal cross-sectional shape, and the polygon is an even number polygon. 11. The high-temperature gas reactor control rod according to claim 10, wherein the support members in the polygonal cross-sectional shape are formed by combining flat plate-shaped parts made of a carbonaceous material with each other. 12. The high-temperature gas reactor control rod according to claim 11, wherein the neutron absorber supporting flange is integrally formed with the flat plate-shaped part made of a carbonaceous material by forming the neutron absorber supporting flange together with the flat plate-shaped part as one solid piece, so as to protrude in the shape of a rectangular piece. 13. The high-temperature gas reactor control rod according to claim 1, wherein the neutron absorber supporting flange is integrally formed with the flat plate-shaped part made of either one of a C/C composite material and a SiC/SiC composite material by forming the neutron absorber supporting flange together with the flat plate-shaped part as one solid piece, so as to protrude in the shape of a rectangular piece. 14. The high-temperature gas reactor control rod according to claim 1, wherein the joint structure is configured to join the first and second control rod elements by bolt connection. 15. The high-temperature gas reactor control rod according to claim 11, wherein the neutron absorber supporting flange is integrally formed with the flat plate-shaped part made of a carbonaceous material by forming the neutron absorber supporting flange separately from the flat plate-shaped part as different pieces to join the neutron absorber supporting flange to the flat plate-shaped part, so as to protrude in a shape of a rectangular piece. 16. The high-temperature gas reactor control rod according to claim 1, wherein the neutron absorber supporting flange is integrally formed with the flat plate-shaped part made of either one of a C/C composite material and a SiC/SiC composite material by forming the neutron absorber supporting flange separately from the flat plate-shaped part as different pieces to join the neutron absorber supporting flange to the flat plate-shaped part, so as to protrude in a shape of a rectangular piece.