Patent Number: 052672867
Section: claims

1. A fuel assembly comprising, in a vertically installed state: an outer cylindrical channel box;  an upper tie plate;  a lower tie plate;  a number of fuel rods disposed inside the channel box so as to form coolant flow passage between mutually adjacent fuel rods and having upper and lower end portions supported by the upper and lower tie plates, respectively, each of said fuel rods being packed with a fuel pellet;  a water cross arranged between the fuel rods, an inside of said water cross being divided into a coolant rising passage and a coolant lowering passage; and  a control element guide tube member disposed inside the water cross and extending along an entire axial direction of the water cross, said coolant rising passage having a coolant inlet port formed to a portion above or under a portion at which the fuel rods are supported by the lower tie plate, said control element guide tube member having a coolant outlet port formed at a portion near a portion at which the fuel rods are supported by the lower tie plate so that a coolant introduced into the coolant rising passage flows vertically upwardly, then turns downwardly at a portion near an upper end of the water cross and then flows downwardly in the coolant lowering passage along the outer surface of the control element guide tube member in a direction reverse to the coolant flow in the coolant arising passage, and flows into an inside of the control element guide tube thereof through the coolant outlet port, said control element guide tube member having an upper opening opened outward at an upper end portion of the water cross and a lower end opening through which a control element is inserted into the control element guide tube member,  wherein said coolant outlet port formed to the control element guide tube member is positioned in a vertical alignment between a lower end portion of an effective fuel region defined to the fuel rod and the lower opening end of the control element guide tube member.  an outer cylindrical channel box;  an upper tie plate;  a lower tie plate;  a number of fuel rods disposed inside the channel box so as to form coolant flow passage between mutually adjacent fuel rods and having upper and lower end portions supported by the upper and lower tie plates, respectively, each of said fuel rods being packed with a fuel pellet;  a water rod or a plurality of water rods means arranged between the fuel rods, an inside of said water rod being divided into a coolant rising passage and a coolant lowering passage; and  a control element guide tube member disposed inside the water rod and extending along an entire axial direction of the water rod, said coolant rising passage having a coolant inlet port formed to a portion above or under a portion at which the fuel rods are supported by the lower tie plate, said control element guide tube member having a coolant outlet port formed at a portion near a portion at which the fuel rods are supported by the lower tie plate so that a coolant introduced into the coolant rising passage flows vertically upwardly, then turns downwardly at a portion near an upper end of the water rod and then flows downwardly in the coolant lowering passage along the outer surface of the control element guide tube member in a direction reverse to the coolant flow in the coolant rising passage, and flows into an inside of the control element guide tube thereof through the coolant outlet port, said control element guide tube member having an upper opening opened outward at an upper end portion of the water rod and a lower end opening through which a control element is inserted into the control element guide tube member,  wherein said coolant outlet port formed to the control element guide tube member is positioned in a vertical alignment between a lower end portion of an effective fuel region defined to the fuel rod and the lower opening end of the control element guide tube member.  a plurality of fuel assemblies each comprising, in a vertically installed state:  an outer cylindrical channel box;  an upper tie plate;  a lower tie plate;  a number of fuel rods disposed inside the channel box so as to form coolant flow passage between mutually adjacent fuel rods and having upper and lower end portions supported by the upper and lower tie plates, respectively, each of said fuel rods being packed with a fuel pellet;  a water rod or a plurality of water rods means arranged between the fuel rods, an inside of said water rod means being divided into a coolant rising passage and a coolant lowering passage;  a control element guide tube member disposed inside the water rod means and extending along an entire axial direction of the water rod means, said coolant rising passage having a coolant inlet port formed to a portion above or under a portion at which the fuel rods are supported by the lower tie plate, said control element guide tube member having a coolant outlet port formed at a portion near a portion at which the fuel rods are supported by the lower tie plate so that a coolant introduced into the coolant rising passage flows vertically upwardly, then turns downwardly at a portion near an upper end of the water rod and then flows downwardly in the coolant lowering passage along the outer surface of the control element guide tube member in a direction reverse to the coolant flow in the coolant rising passage, and flows into an inside of the control element guide tube thereof through the coolant outlet port, said control element guide tube member having an upper opening opened outward at an upper end portion of the water cross and a lower end opening through which a control element is inserted into the control element guide sheath member;  a control rod having a cross shaped section to be inserted between the fuel assemblies; and  a plurality of control elements to be used in combination with the control rod and inserted into the control element guide tube member,  wherein a void fraction in the water rod means is controlled by controlling an axial position of the control rod inserted into the reactor core. 2. A fuel assembly according to claim 1, wherein said channel box has substantially a square cross section and said water cross is disposed in the channel box with a crossing central portion arranged at substantially a central portion of the channel box. 3. A fuel assembly according to claim 1, wherein said water cross is provided with blade portions crossing at its central portion, said control element guide tube member is disposed at the crossing portion of the blade portions, upper and lower end portions of said water cross are closed by plugging members and the inside of the water cross is divided into the coolant rising and lowering passages by a sectioning structure formed axially along the control element guide tube member, said sectioning structure having a lower end attached to the lower plugging member and an upper end slightly apart from the upper plugging member. 4. A fuel assembly according to claim 3, wherein said control element guide tube member has an upper end opened at the upper plugging member and a lower end downwardly penetrating the lower plugging member. 5. A fuel assembly according to claim 3, wherein said sectioning structure comprises a sectioning member disposed axially along the control element guide tube member. 6. A fuel assembly according to claim 3, wherein said sectioning structure is formed by narrowing an intermediate portion of each of the blade portions. 7. A fuel assembly according to claim 3, wherein said control element guide tube member is a hollow cylindrical tube and said sectioning structure comprises a hollow cylindrical tube concentrically disposed outside the control element guide tube member with an annular space therebetween. 8. A fuel assembly according to claim 7, wherein the coolant rising passage is formed in each of the blade portions of the water cross and the coolant lowering passage is formed in the annular space. 9. A fuel assembly according to claim 3, wherein said water cross is provided with blade portions crossing at its central portion, said control element guide tube member is disposed in an intermediate portion of each of the blade portions, upper and lower end portions of said water cross are closed by plugging members and the inside of the water cross is divided into the coolant rising and lowering passages by a sectioning structure formed axially along the control element guide tube member, said sectioning structure having a lower end attached to the lower plugging member and an upper end slightly apart from the upper plugging member. 10. A fuel assembly according to claim 9, wherein said sectioning structure comprises a sectioning member disposed horizontally inside and outside the control element guide tube member disposed in each of the blade portions of the water cross. 11. A fuel assembly according to claim 3, wherein a further coolant inlet port is formed to the control element guide tube member at a portion below the lower plugging member. 12. A fuel assembly comprising, in a vertically installed state: 13. A fuel assembly according to claim 12, wherein said water rod comprises one polygonal cylindrical tube arranged centrally in the channel box and the control element guide tube member having a circular cross section is disposed inside the polygonal cylindrical tube. 14. A fuel assembly according to claim 13, wherein the polygonal cylindrical tube has substantially a square cross section. 15. A fuel assembly according to claim 12, wherein said water rod comprises an outer tube constituting an outer appearance of the water rod, an inner tube disposed inside the outer tube, the control element guide tube member disposed inside the inner tube and spacer means for supporting the inner tube to the outer tube and supporting the control element guide tube member to the inner tube. 16. A fuel assembly according to claim 12, wherein said upper and lower end portions of said outer and inner tubes are closed by plugging members and the inside of the water rod is divided into the coolant rising and lowering passages by a sectioning structure formed axially along the control element guide tube member and said sectioning structure is composed of the inner tube, wherein the coolant rising passage is formed by a space between the outer tube and an inner tube and the coolant lowering passage is formed by a space between the inner tube and the control element guide tube member, said inner tube being provided with a conjunction hole at a portion below the upper plugging member so as to connect the coolant rising passage with the coolant lowering passage, said spacer means being provided with opening ports through which the coolant passes. 17. A fuel assembly according to claim 16, wherein said control element guide tube member has an upper end opened above the upper plugging member and a lower end downwardly penetrating the lower plugging member and the lower end of the control element guide tube member has an opening through which the control element is inserted thereinto. 18. A fuel assembly unit comprises a plurality of the fuel assemblies of the structure defined in claim 1. 19. A fuel assembly unit according to claim 18, comprises four fuel assemblies divided by a cross shaped control rod means disposed between four fuel assemblies. 20. A reactor core comprising: 21. A reactor core according to claim 20, wherein said water rod means comprises a water cross having a cross shaped blade portions. 22. A reactor core according to claim 20, wherein said water rod means comprises a cylindrical water rod disposed substantially a central portion of the fuel assembly. 23. A reactor core according to claim 20, wherein said control rod means comprises rod cluster type control rod, which is inserted upwardly into the reactor core.