Patent Number: 042657083
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the upper portion of a nuclear reactor fuel assembly 10 engaged with the fuel alignment plate 12 during typical nuclear reactor operating conditions. The fuel assembly 10 includes a plurality of guide tubes 14 to which are attached fuel spacer grids 16 which form a matrix to support a plurality of fuel elements 18. The guide tubes 14 typically extend a distance of approximately 13 feet from the fuel alignment plate 12 to the fuel assembly lower end fitting (not shown). The guide tubes 14 have giude posts 20 welded to their upper ends and are rigidly connected to a perforated flow plate 22. A spider-shaped holddown plate 24 having one lobe associated with each guide post 20 is located below the alignment plate 12 and is vertically movable relative to the guide post in order to tansmit a downward force from the alignment plate 12 through the holddown springs 26 to the guide tubes 14 whereby the assemby is held down against the upward flow of coolant over the fuel elements. During the course of their lifetimes within a reactor, most assemblies 10 will have control rods 28 located within the guide tubes 14. The control rods 28 are typically about 15 feet long and are rigidly held at their upper ends (not shown) and reciprocated vertically within the guide tubes 14. The control rod 28 is protected from the highly turbulent coolant flow that interacts with the fuel elements 18 below the alignment plate 12, and from the strong cross-flows existing in the plenum region 30 above the alignment plate 12. This protection is afforded by the guide tube 14, the post 20, the alignment plate 12, and shrouds 34 in the plenum region 30. Although not shown, the alignment plate 12 has a plurality of flow passages for directing the coolant flow from the fuel assemblies 10 into the plenum region 30. A continuous flow of coolant must be maintained within the guide tube 14 to provide cooling to the control rods 28. Because the control rods 28 are so elongated, each rod is unlikely to be exactly centered within its respective guide tube 14 and therefore, especially when the rod is in the withdrawn position shown in FIG. 1, the rod will be eccentric relative to the guide post exit 36. It is believed that such eccentricity produces a pattern of axial vortices 38, with axes generally vertical, and diffuser vortices 40, with axes generally in a horizontal plane, as schematically represented. The structure associated with the control rod 28 as it exits the guide posts 20 can be generally described as a center rod eccentrically disposed within a rather abrupt diffuser region represented generally at 42. It should be appreciated that depending on the particular nuclear reactor, the exact structure defining the diffuser region 42 and the diffuser cross section can be quite different. During reactor operation, most control rods 28 are maintained in the withdrawn position so that the control rod tip 44 is continuously located, depending on the particular reactor, at a fixed elevation approximately 1 to 2 feet from the guide post exit 36. Inspection of fuel assemblies 10 removed from operating reactors shows severe fretting on the inside of the guide tube 14 at precisely the elevation corresponding to the control rod tip 44 in the withdrawn position. Analyses were made and tests outside the reactor were performed in order to identify the mechanism causing the guide tube wear. Although the source of wear has not been completely explained analytically it was found that the vibrations causing the control rod interaction with the guide tubes 14 are apparently self-excited and predominantly at the natural frequency of the control rod (about 4H for a typical control rod). These vibrations are believed to be the result of guide tube flow effects caused by driving forces related to the periodic interaction near the guide post exit 36 of the axial vortices 38 with the diffuser vortices 40, as described above. A variety of proposed improvements were tested in a flow loop wherein the dimensions and flow rates were similar to typical reactor operating conditions. Most of the tested guide posts had very little effect in reducing the vibration of the control rod in the guide tube 14. The present invention was effective in reducing control rod vibration. An invention described in another patent application field on even date herewith entitled "Parallel Flow Collar for Reducing Vibration of a Rod Within a Diffuser", by F. Bevilacqua, and assigned to the same assignee as the present invention, was, however, most effective. FIGS. 2 and 3 show the preferred embodiment of the improved guide post 20B having a uniform inner diameter that is the same as the inner diameter of the guide tube 14 except that at the upper end of the post 20B the inner diameter is reduced to form a flow restriction 46. In this embodiment of the invention the tube and post inner diameter are 0.900 inches for receiving a control rod 28 having an outer diameter of 0.816 inches. The flow restriction 46 has an inner diameter of 0.860 inches. Generally, the flow restriction is chosen to be as narrow as will permit acceptable control rod scram time. Immediately below the restriction 46 there are provided a plurality of bypass channels 48 which divert most of the coolant away from the rod 28. This reduces the velocity of the coolant exiting the guide post 20B at the diffuser mouth 36 immediately adjacent to the control rod 28. Although the flow exiting the post at 36 may produce the diffuser vortices 40 (since the rod is still eccentric relative to the exit cross section), the bypass flow through the channels 48 enters the diffuser region 42 spaced away from the control rod 28. In addition, the axial vortices 38 are dissipated or at least distributed in a manner that minimizes their periodic interaction with the diffuser vortices 40 Since the exact mechanisms causing the control rod vibration are not thoroughly understood, the foregoing explanation cannot be analytically demonstrated. It is believed, however, that at least four discrete flow channels are preferred. The illustrated embodiment has eight flow channels symmetrically located about the upper portion of the guide post, each channel being upwardly oriented at approximately 45 degrees to the horizontal. This orientation minimizes the pressure drop in the guide tube. Preferably, the flow channels are located such that the channel exits 50 are in the enlarged head 52 of the guide post 20B in order that the diverted flow exits as far as possible from the post exit 36. As an example of the improvement provided by the preferred embodiment of the invention, the results of comparative flow test on a guide tube 14, guide post 14, diffuser region 42 and shroud 34 equivalent to the structure shown in FIG. 1 will be discussed. In the tests the guide tube 14 inside diameter was 0.900 inches and the control rod outside diameter was 0.816 inches. The control rod 28 was 14 feet long and fixedly suspended at its top. The mass of the control rod 28 was equivalent to a stainless steel clad column of B.sub.4 C pellets. The rod tip 44 was located 21 inches below the guide post exit 36. The standard prior art post 20A was similar to that depicted in FIG. 1 and had an inside diameter of 0.900 inches. Accelerometer probes were connected to the midspan of the control rod 28. At the typical operating volumetric flow rate of 9 gallons per minute (4500 lbs. per hour) flow through the guide tube and standard post 20A, the rod response was 0.23 g's. Since the guide tube 14 in the test model was made of plexiglass, the control rod tip 44 could be observed vibrating against the guide tube 14 inner wall. The test was repeated with the same flow conditions using the improved guide post 20B shown in FIGS. 1, 2 and 3. Eight flow channels at angles of 45 degrees were provided. The dimensions indicated by lower case letters in FIGS. 2 and 3 were as follows: a=1.375 inches PA1 b=0.600 inches PA1 c=0.200 inches PA1 d=0.25 inches The variation in the angle of the transition from the restriction 46 to the post exit 36 was six degrees, but this parameter was found to have little effect on the control rod vibration. With the improved guide post 20B, the acceleration response at the rod midspan dropped down to 0.17 g's. There was a visible oscillation of the control rod but the tip 44 did not touch the guide tube 14. Although the present invention was not as effective in reducing control rod vibrations as was the invention claimed in the above-mentioned related application, the present invention is easy to implement in fuel assemblies that have already been built. It is also an inexpensive way to provide a safety margin in new assemblies where the prior art guide posts are barely acceptable.