Patent Number: 
Section: claims

1. A nuclear reactor fuel assembly comprising:a top nozzle;a bottom nozzle;a plurality of elongated control rod guide thimbles respectively attached at a first end to the top nozzle and at a second end to the bottom nozzle;a plurality of elongated fuel rods supporting fissile material there within, the fuel rods extending parallel to the control rod guide thimbles, between the top nozzle and the bottom nozzle;a plurality of transverse grids arranged in a spaced tandem array between the top nozzle and the bottom nozzle, the grids respectively forming a lattice to latterly support the fuel rods in a spaced orderly array, the grids being attached to and supported axially by the control rod guide thimbles;at least one elongated instrumentation tube extending and captured between the top nozzle and the bottom nozzle, the control rod guide thimbles, fuel rods and the instrumentation tube having parallel axes extending along their elongated dimension, the instrumentation tube being adapted to receive an in-core instrumentation that extends along a substantial axial length of the instrumentation tube, the in-core instrumentation remaining fixed during reactor operation; andan instrumentation tube insert that extends within and substantially along the elongated dimension and over the substantial axial length of the instrumentation tube, the insert having an inside narrow-most diameter at a plurality of axial locations along the length of the insert that is adapted to closely approximate the outside diameter of the in-core instrumentation, the plurality of axial locations extending over the substantial axial length, so as to maintain the in-core instrumentation centered in the instrumentation tube when inserted, the inside narrow-most diameter being supported a fixed distance from an inside diameter of the instrumentation tube at spaced segmented locations along an interior and substantially along the elongated dimension of the instrumentation tube, the insert comprising a spiral spring and being adapted to center the in-core instrumentation over the substantial axial length of the instrumentation tube. 2. The nuclear fuel assembly of claim 1 wherein an outside diameter of the spring closely matches the inside diameter of the instrumentation tube and an inside diameter of the spring closely matches the outside diameter of the in-core instrumentation. 3. The nuclear fuel assembly of claim 1 wherein the spring has a first and second end and a central axial region and has a close pitch at the first and second end and a larger pitch in the central axial region sized to preclude snagging of the in-core instrument. 4. The nuclear fuel assembly of claim 3 wherein the spring has an approximately one inch (2.54 cm) pitch in the central axial region. 5. The nuclear fuel assembly of claim 1 wherein the insert further comprises an instrument thimble tube having an outside diameter that closely approximates an inside diameter of the spring and the instrument thimble tube having an inside diameter that closely approximates the outside diameter of the in-core instrumentation. 6. The nuclear fuel assembly of claim 5 wherein the instrument thimble tube has an end that flairs, radially outward to retain the spring. 7. A nuclear reactor fuel assembly comprising;a top nozzle;a bottom nozzle;a plurality of elongated control rod guide thimbles respectively attached at a first end to the top nozzle and at a second end to the bottom nozzle;a plurality of elongated fuel rods supporting fissile material there within, the fuel rods extending parallel to the control rod guide thimbles, between the top nozzle and the bottom nozzle;a plurality of transverse grids arranged in a spaced tandem array between the top nozzle and the bottom nozzle, the grids respectively forming a lattice to latterly support the fuel rods in a spaced orderly array, the grids being attached to and supported axially by the control rod guide thimbles;at least one elongated instrumentation tube extending and captured between the top nozzle and the bottom nozzle, the control rod guide thimbles, fuel rods and the instrumentation tube having parallel axes extending along their elongated dimension, the instrumentation tube being adapted to receive an in-core instrumentation that extends along a substantial axial length of the instrumentation tube, the in-core instrumentation remaining fixed during reactor operation; andan instrumentation tube insert that extends within and substantially along the elongated dimension and over the substantial axial length of the instrumentation tube, the insert having an inside narrow-most diameter at a plurality of axial locations along the length of the insert that is adapted to closely approximate the outside diameter of the in-core instrumentation, the plurality of axial locations extending over the substantial axial length, so as to maintain the in-core instrumentation centered in the instrumentation tube when inserted, the inside narrow-most diameter being supported a fixed distance from an inside diameter of the instrumentation tube at spaced segmented locations along an interior and substantially along the elongated dimension of the instrumentation tube, the insert being adapted to center the in-core instrumentation and wherein the insert has an oval cross section at a plurality of locations along its axial dimension wherein a major outside diameter of the oval cross section approximates the inside diameter of the instrumentation tube and wherein a minor inside diameter of the oval cross section substantially approximates the outside diameter of the in-core instrumentation. 8. The nuclear fuel assembly of claim 7 wherein the major diameter of the oval cross section is rotated relative to the axis of the instrumentation tube at different elevations along the axis of the instrumentation tube. 9. The nuclear fuel assembly of claim 8 wherein the rotation is 90 degrees between adjacent oval cross sections. 10. The nuclear fuel assembly of claim 1 wherein inside narrow-most diameter of the insert is approximately equal to or greater than 0.266 in. (.676 cm). 11. The nuclear fuel assembly of claim 7 wherein the inside narrow-most diameter of the insert is approximately equal to or greater than 0.266 in. (.676 cm).