Patent Number: 
Section: claims

1. A reactivity control device for storing nuclear fuel, the device comprising:a top tube sheet;an array comprising a plurality of vertically elongated neutron absorber rods fixedly attached to the top tube sheet, the absorber rods arranged parallel to each other; anda floating guide plate slideably mounted on the absorber rods for upward and downward movement along the absorber rods, the floating guide plate movable between a lower position proximate to bottom ends of the absorber rods and an upper position abuttingly engaging the top tube sheet. 2. The reactivity control device according to claim 1, wherein the top tube sheet and floating guide plate have the same shape in top plan view. 3. The reactivity control device according to claim 1, wherein the floating guide plate has a substantially rectilinear shape in top plan view including a plurality of straight lateral peripheral edges and arcuately rounded corners formed between adjoining peripheral edges. 4. The reactivity control device according to claim 3, wherein the top tube sheet has a substantially rectilinear shape in top plan view including a plurality of straight lateral peripheral edges and arcuately rounded corners formed between adjoining peripheral edges. 5. The reactivity control device according to claim 1, wherein the floating guide plate includes guide holes which slideably receive the absorber rods therethrough and allow the plate to slide upwards and downwards along a length of the absorber rods. 6. The reactivity control device according to claim 1, wherein the top tube sheet and floating guide plate each include a plurality of flow apertures, each flow aperture in the top tube sheet being concentrically aligned with a corresponding flow aperture in the floating guide plate. 7. The reactivity control device according to claim 1, wherein the absorber rods each comprise a solid ductile neutron-absorbing material or a hollow ductile tube containing a neutron absorber material. 8. The reactivity control device according to claim 1, wherein the absorber rods each include a fixed top end mounted to the top tube sheet and a free bottom end. 9. The reactivity control device according to claim 8, wherein the free bottom end of each absorber rod is tapered. 10. The reactivity control device according to claim 1, further comprising at least one end stop formed on at least one absorber rod proximate to a bottom end of the at least one absorber rod, the at least one and stop engaging and preventing the floating guide plate from sliding off of the absorber rod array. 11. The reactivity control device according to claim 10, wherein the end stop is formed by a stepped shoulder formed by a reduced diameter portion of the at least one absorber rod that engages and captures the floating guide plate. 12. The reactivity control device according to claim 1, further comprising a lifting coupling element mounted on the top tube sheet for filling the reactivity control device, the coupling element configured for engagement by a lifting tool. 13. A reactivity control system for storing nuclear fuel, the system comprising:a nuclear fuel assembly comprising a bottom nozzle box, a top nozzle box, a plurality of fuel rods extending vertically between the nozzle boxes, and a plurality of guide tubes extending vertically between the nozzle boxes;a reactivity control device comprising a top tube sheet, a plurality of neutron absorber rods fixedly attached to the top tube sheet, and a floating guide plate slideably mounted on the absorber rods for upward and downward movement along the absorber rods, the absorber rods removably insertable into the guide tubes of the fuel assembly;wherein the reactivity control device has a first uninstalled, position prior to insertion of the absorber rods into the fuel assembly in which the floating guide plate is spatially separated from the top tube sheet, and a second installed position after insertion of the absorber rods into the guide tubes of the fuel assembly in which the floating guide plate is abuttingly engaged with the top tube sheet. 14. The reactivity control device according to claim 13, wherein the top nozzle box includes an upwardly open top recess, the top tube sheet and floating guide plate being positioned inside the top recess when the reactivity control device is in the installed position. 15. The reactivity control device according to claim 13, wherein the floating guide plate includes arcuately rounded corners which are positioned inside truncated corner regions of the top nozzle box. 16. The reactivity control device according to claim 15, wherein the truncated corner regions are formed by inward facing angled inner corner surfaces formed adjacent to an upwardly open top recess defined by the top nozzle box. 17. The reactivity control device according to claim 13, wherein each of the guide tubes in the fuel assembly is accessible to the absorber rods of the reactivity control device through penetrations in top nozzle box, the penetrations arranged in a pattern that is the same as a pattern of the absorber rods on the reactivity control device. 18. The reactivity control device according to claim 13, further comprising a spent fuel pool containing water, the fuel assembly submerged in the pool with the reactivity control device inserted in the fuel assembly. 19. The reactivity control device according to claim 13, further comprising a canister including a fuel basket comprising a plurality of open cells, the fuel assembly disposed in one of the cells with the reactivity control device inserted in the fuel assembly. 20. A method for controlling reactivity in a spent nuclear fuel assembly removed from a nuclear reactor core, the method comprising:removing a spent fuel assembly from a nuclear reactor core;positioning a reactivity control device above the spent fuel assembly, the device comprising a top tube sheet, a plurality of absorber rods fixedly attached to the top tube sheet, and a floating guide plate slideably mounted on the absorber rods for upward and downward movement along the absorber rods, the top tube sheet and floating guide plate being spatially separated;aligning each of the absorber rods with a corresponding one of a plurality of guide tubes disposed in the spent fuel assembly;lowering the reactivity control device toward the spent fuel assembly;inserting the absorber rods into the guide tubes;abuttingly engaging firstly the floating guide plate with a top of the fuel assembly;sliding the absorber rods through the floating guide plate while continuing to lower the reactivity control device toward the spent fuel assembly; andabuttingly engaging secondly the top tube sheet with the floating guide plate, wherein the absorber rods are fully inserted in the guide tubes. 21. The method according to claim 20, wherein the aligning step includes angularly rotating the reactivity control device about its centerline until straight peripheral edges of the floating guide plate are oriented parallel to straight peripheral sidewalls of a top nozzle box mounted on the spent fuel assembly. 22. The method according to claim 21, wherein the aligning step includes vertically aligning arcuately shaped corners of the floating guide plate with truncated corner regions of the top nozzle box on the spent fuel assembly. 23. The method according to claim 20, wherein the abuttingly engaging firstly step includes positioning the floating guide plate against a floor plate of a top nozzle box of the fuel assembly that recessed below a top edge of the top nozzle box. 24. The method according to claim 23, wherein the floating guide plate and top tube sheet are recessed below the top edge of the top nozzle box.