Patent Number: 047175290
Section: claims

1. In a nuclear reactor having a reactor core plate with a bore and having an elongated hollow thimble which is longitudinally movable through the bore in the reactor core plate and into a fuel assembly supported by the core plate, the bore having an axis and the fuel assembly having a bottom nozzle with an aperture to receive the thimble, the bottom nozzle having legs which rest on the core plate around the bore and the aperture in the bottom nozzle being spaced apart from the core plate and being positioned along the axis of the bore therein, a thimble guide assembly comprising: an elongated first element having an axis, a top end, and a thimble channel which extends along the axis of the first element to the top end thereof, the first element additionally having means for mounting the first element to the core plate so that the bore of the core plate and the thimble channel of the first element are coaxial and so that the top end of the first element is positioned above the core plate; and  a second element having a top end, a bottom end, an axis running through the top and bottom ends of the second element, and a thimble channel along the axis of the second element, the second element additionally having means at its bottom end for connecting the second element to the top end of the first element so that the thimble channels of the first and second elements are coaxial, the top end of the second element being spaced apart from the nozzle when the first and second elements are connected,  wherein the thimble is longitudinally movable through the first and second elements along the thimble channels thereof.  an elongated first element having an axis, a top end, and a thimble channel which extends along the axis of the first element to the top end thereof, the first element additionally having means for mounting the first element to the core plate so that the bore of the core plate and the thimble channel of the first element are coaxial and so that the top end of the first element is positioned above the core plate; and  a second element having a top end, a bottom end, an axis running through the top and bottom ends of the second element, and a thimble channel along the axis of the second element, the second element additionally having means at its bottom end for connecting the second element to the top end of the first element so that the thimble channels of the first and second elements are coaxial, the top end of the second element being spaced apart from the nozzle when the first and second elements are connected,  wherein the thimble is longitudinally movable through the first and second elements along the thimble channels thereof. 2. The thimble guide assembly of claim 1, wherein the second element has a length that is selected so that the top end of the second element is less than about an inch (2.54 cm) below the aperture in the bottom nozzle. 3. The thimble guide assembly of claim 2, wherein the second element has a length that is selected so that the top end of the second element is less than about half an inch (1.27 cm) below the aperture in the bottom nozzle. 4. The thimble guide assembly of claim 3, wherein the second element has a length that is selected so that the top end of the second element is about a quarter inch below (0.64 cm) the aperture in the bottom nozzle. 5. The thimble guide assembly of claim 1, wherein the bore in the core plate has a region that is threaded, and wherein the means for mounting the first element to the core plate comprises a first portion of said first element having threads to engage the threaded region of the core plate. 6. The thimble guide assembly of claim 5, wherein the first element additionally has, at the top end thereof, a second portion with threads, and wherein the means for connecting the second element to the first element comprises a threaded region at the bottom end of the second element to engage the threads of the second portion of the first element. 7. The thimble guide assembly of claim 6, wherein the threads of the first portion of the first element are right-handed and the threads of the second portion of the first element are left-handed. 8. The thimble guide assembly of claim 6, wherein the threads of the first portion of the first element are left-handed and the threads of the second portion of the first element are right-handed. 9. The thimble guide assembly of claim 6, wherein each of said first and second elements have surfaces that are configured to provide tool-engaging regions so that the elements can be rotated with tools. 10. The thimble guide assembly of claim 9, wherein the tool-engaging regions are recessed wrench-engaging regions. 11. The thimble guide assembly of claim 9, wherein the tool-engaging regions are protruding wrench-engaging regions. 12. The thimble guide assembly of claim 1, wherein the first element additionally comprises a lower sleeve portion which extends into the bore of the core plate, the lower sleeve portion having an outer wall. 13. The thimble guide assembly of claim 12, wherein the outer wall of the lower sleeve portion has at least one protruding shoulder which contacts the core plate, the outer wall of the lower sleeve portion being spaced apart from the core plate except at the at least one protruding shoulder. 14. The thimble guide assembly of claim 13, wherein the bore of the core plate communicates with a bottom mounted instrumentation column that is affixed to the core plate, and the lower sleeve portion extends into the bottom mounted instrumentation column. 15. The thimble guide assembly of claim 1, wherein the first element has an upper annular groove, and further comprising upper spring clip means affixed to the second element for engaging the upper annular groove. 16. The thimble guide assembly of claim 15, wherein the first element additionally has a lower annular groove, and further comprising lower spring clip means affixed to the core plate for engaging the lower annular groove. 17. The thimble guide assembly of claim 1, wherein the thimble channel of the second element is cylindrical and the top of the second element comprises a flat annular lip that extends radially from the periphery of the cylinder. 18. In combination with a nuclear reactor having a core plate with a bore, a thimble guide assembly for accommodating an elongated hollow thimble which is longitudinally movable through the bore in the core plate and into a fuel assembly supported by the core plate, the bore having an axis and the fuel assembly having a bottom nozzle with an aperture to receive the thimble, the bottom nozzle having legs which rest on the core plate around the bore and the aperture in the bottom nozzle being spaced apart from the core plate and being positioned along the axis of the bore therein, comprising: