Patent Number: 052176797
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is generally related to nuclear reactor fuel assemblies and in particular to a thimble tube sleeve for reducing thimble tube wear. 2. General Background Operating experience with typical unmodified and early model Westinghouse and Framatome/EDF nuclear reactors has shown that a high maintenance and wear problem exists in the guide path for the nuclear instrument thimble tubes which contain nuclear instruments and leads. The thimble tubes are guided into the inside diameter of the fuel assembly instrument sheath (tube) through guide paths located inside column structures that are integral parts of the reactor lower internals. The wear problem on the thimble tubes is attributable to the high velocity coolant flow along the length of the thimble tubes. The high velocity flow causes the thimble tubes to vibrate. The vibrating contact of thimble tubes against the lower core support plate, through which the thimble tubes pass, causes rapid and excessive wear of the thimble tubes. A known approach to alleviate the problem is the addition of sleeves anywhere from 30 to over 50 inches in length to house the thimble tubes above and below the core support plate. Although this has been somewhat effective in mitigating the flow-induced vibration, there still exists a need for a more effective means of further reducing or preventing flow-induced vibration of the thimble tubes. SUMMARY OF THE INVENTION The present invention addresses the above need in a straightforward manner. What is provided is an apparatus for restricting the motion of a thimble tube inside a nuclear reactor. A cylindrical sleeve open at each end has upper, middle, and lower sections. The middle section is formed from two materials joined together that have different coefficients of thermal expansion. This allows insertion or removal of the thimble tube during cold conditions. During hot conditions in the reactor the inner material constricts radially against the thimble tube inside the sleeve. The lower section is provided with a labyrinth or baffle that serves to reduce the velocity of the coolant flow through the sleeve. The upper section of the sleeve extends above the lower core support plate in the reactor.