Patent Number: 
Section: claims

1. An apparatus comprising:a pressure vessel;a nuclear reactor core comprising fissile material disposed in the pressure vessel;a core basket disposed in the pressure vessel and containing the nuclear reactor core;an incore instrumentextending in a guide tube of the nuclear reactor core andhaving a cable extending out of a bottom of the nuclear reactor core and then making a 180° turn inside the pressure vessel; anda bottom support element located inside the pressure vessel,attached to a bottom of the core basket, andincluding a routing tube that routes the cable of the incore instrument through the 180° turn. 2. The apparatus of claim 1 wherein the bottom support element includes flow openings to allow flow of primary coolant water through the bottom support element and into the bottom of the nuclear reactor core. 3. The apparatus of claim 1 wherein the core basket includes a lower core plate and the bottom support element is attached to the lower core plate. 4. The apparatus of claim 1 wherein the bottom support element is attached to the core basket without welds. 5. The apparatus of claim 1 wherein the bottom support element does not include any welds. 6. The apparatus of claim 1 wherein the bottom support element has an egg crate grid structure. 7. The apparatus of claim 1 wherein the bottom support element does not contact the pressure vessel. 8. The apparatus of claim 1 wherein the core basket is suspended within the pressure vessel and the bottom support element is attached to the bottom of the core basket without contacting the pressure vessel. 9. The apparatus of claim 8 wherein the bottom support element has structural strength sufficient to support the core basket and the nuclear reactor core in the event that the core basket suspension fails. 10. The apparatus of claim 1 wherein the cable of the incore instrument extends upward after the 180° turn to a vessel penetration located above the nuclear reactor core and below a top of the pressure vessel. 11. The apparatus of claim 1 wherein:the pressure vessel includes an upper vessel portion and a lower vessel portion, the nuclear reactor core and core basket being disposed in the lower vessel portion, the cable of the incore instrument extending upward after the 180° turn to a vessel penetration through the lower vessel portion that is located above the nuclear reactor core. 12. The apparatus of claim 1 wherein:the pressure vessel includes an upper vessel portion, a lower vessel portion, and a mid-flange joining the upper and lower vessel portions, the nuclear reactor core and core basket being disposed in the lower vessel portion, the cable of the incore instrument extending upward after the 180° turn to a vessel penetration located in the mid-flange or in the lower vessel portion. 13. The apparatus of claim 12 wherein the core basket is suspended from the mid-flange. 14. A method performed in conjunction with a nuclear reactor includinga pressure vessel,a nuclear reactor core comprising fissile material disposed in the pressure vessel,a core basket disposed in the pressure vessel and containing the nuclear reactor core,an incore instrumenthaving a sensor disposed in a guide tube of the nuclear reactor core andhaving a cable extending out of a bottom of the nuclear reactor core and then making a 180° turn inside the pressure vessel, anda bottom support element located inside the pressure vessel,attached to a bottom of the core basket, andincluding a routing tube that routes the cable of the incore instrument through the 180° turn,the method comprising:retracting the cable to move the sensorout of the guide tube of the nuclear reactor core andinto the routing tube of the bottom support element;performing maintenance on the nuclear reactor, including replacing the guide tube with a different guide tube; andre-inserting the cable to move the sensorout of the routing tube of the bottom support element andinto the different guide tube. 15. The method of claim 14 wherein the retracting and the re-inserting are blind operations in which the position of the incore instrument is not visibly observable. 16. The method of claim 15 wherein the re-inserting includes monitoring a signal from the incore instrument to determine placement of the incore instrument in the reactor core.