Patent Number: 051204916
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 illustrates the prior art. Thimble tube 10 is positioned inside guide tube 12 which penetrates reactor vessel wall 14 at its lower end. Guide tube 12 and reactor vessel wall 14 are sealed together in a manner known in the industry to prevent loss of coolant and pressure from the reactor. The lower end of guide tube 12 (not shown) is attached to a sealed table that allows insertion of thimble tube 10 without loss of coolant or pressure and such apparatus is known in the industry. Thimble tube 10 has no openings to allow coolant to flow thereinto and forms and is part of the reactor coolant system pressure boundary. Coolant flowing upwardly through guide tube 12 enters through flow holes 16,18 respectively provided in lower core plate 20 and the lower portion of guide tube 12. The lower portion of guide tube 12 is provided with outwardly flared edges 22 to help direct coolant into guide tube 12. A neutron detector (not shown) is inserted through the lower end of thimble tube 10 and directed to a position in thimble tube 10 to take readings of core reactivity during reactor operations. For ease of illustration, only one guide tube and thimble tube are shown and it should be understood that there are multiple guide tubes and thimble tubes in a fuel assembly adapted to receive neutron detectors. The flow of coolant through guide tube 12 in conjunction with the lack of support of thimble tube 10 results in vibration and wear of thimble tube 10. Thimble tube 24 of the invention, seen in FIG. 2 and 3, is similar to thimble tube 10 in that it is an elongated hollow tube having one end closed and adapted to receive a neutron detector. The portion of thimble tube 24 that extends into the reactor, beginning slightly below reactor vessel wall 14, is formed from a bimetallic construction. As indicated by the vertical and horizontal lines in FIG. 2 the bimetallic section of thimble tube 24 is formed from a series of alternating sections 26,28 attached to each other to form thimble tube 24 in an alternating fashion along the width and length of thimble tube 24. Sections 26,28 are materials that have different coefficients of thermal expansion. As seen in the preferred embodiment and top sectional view of FIG. 3, each section 26,28 is semicircular in cross section and attached to its corresponding section having a different coefficient of thermal expansion at lines 30. At temperatures encountered during assembly or when the reactor is below normal operating temperature, the portion of thimble tube 24 in guide tube 12 is straight as in FIG. 1. This allows easy insertion and removal of thimble tube 24. At reactor operating temperatures the different coefficients of thermal expansion of sections 26,28 cause thimble tube 24 to warp into a wavy or spiral shape along its length inside guide tube 12. This provides continuous or multiple points of support to minimize coolant flow induced vibration, wear, and cracking of thimble tube 24 and guide tube 12. Sections 26,28 are formed from metals suitable for use inside a nuclear reactor and that have sufficiently different coefficients of thermal expansion and in the preferred embodiment are formed from inconel and stainless steel. The structure of thimble tube 24 is not limited to semicircular sections as illustrated. The different metals may be joined together in a spiral fashion as shown in FIG. 4. Also, a thimble tube formed from a single material may have a material having a different coefficient of thermal expansion deposited on a portion of the tube surface to achieve a similar bending effect at operating temperatures. Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.