Patent Number: 053234359
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Reference will now be made in detail to the preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Boiling water reactors use control rods in the reactor vessel for controlling the reaction therein. The control rods are generally contained in a grid-like fashion in a control rod housing with a drive unit for raising the control rods into the assembly of the fuel rods. Located underneath the reactor vessel near the control rod housings are control rod drive housing supports for limiting the travel of a control rod in the event that a control rod housing is ruptured. A top view of the control rod drive housing supports of the prior art and the present invention are shown in FIGS. 1 and 2, respectively. In FIGS. 1 and 2, the positions shown at 10 illustrate the positions which correspond to a control rod drive. The control rod drive housing supports of the prior art as shown in FIG. 1, consist of hanger rods 11 supported by a beam underneath the reactor pressure vessel (not shown) spaced throughout the grid of control rod drive positions. Support bars 12 are bolted between the hanger rods 11. Grid bars 13 are installed on the support bars 12 to support the control rod drive and transfer the load of the control rod drive to the support bars 12. Generally, a pair of grid bars 13 support each control rod drive. Each pair of grid bars is held together by two grid clamps 14 and a bolt (not shown). In FIG. 1, the control drive housing supports of the prior art is shown to the right of line A--A only with the support bars 12 and is shown with the support bars 12 and grid bars 13 to the left of the line A--A. In actual operation, the grid bars 13 would be used throughout the entire housing supports. In order to replace a control rod drive, the grid bars 13 and clamps 14 must be removed. Because the grid bars 13 are interlocking, they must be removed starting from the periphery, resulting in a cumbersome and time consuming process. With reference to FIGS. 2 and 3, the control rod drive housing supports of the present invention will be described. The control rod drive positions and hanger rods are again shown at 10 and 11, respectively. However, the grid bars 13 and the clamps 14 of the prior art are replaced by assemblies that can perform the same function but can be moved to a nonuse position, rather than removed, when changing the control rod drives. As a result, no heavy steel grid bars must be removed every time a control rod drive is being replaced. Thus, changing of the control rod drives is much easier and faster. Moreover, because the time needed to replace a control rod drive is greatly reduced, the length of time a person changing the control rod drive is subject to radiation is correspondingly reduced. In the assembly of the present invention the support bars 12 of the prior art are replaced with support bars 22 placed in rows along opposing sides of the control rod drive position and oriented ninety degrees to the old bar support bars 12. An end of each support bar 22 is supported on a hanger rod. Typically, the support bars 12 of the prior art were one foot long. The length of support bars 22 of the present invention may vary depending on the reactor and the placement of the hangers. Preferably, the length of the support bars 22 is two feet. Each support bar has wing assemblies 25 having four wings 30 disposed thereon for supporting the control rod drives. When the plant is in operation, the wing assembly 25 is oriented in an operating position 32 as shown in FIGS. 1 and 2. One wing 30 of each wing unit 25, when in the operating position 32, supports one quadrant of the control rod drive such that four wings 30 support each control rod drive. When a control rod drive needs to be replaced the wing assembly 25 and wings 30 may be turned to a non-support position 33 as shown in FIG. 2 rather than removed. The wing assemblies 25 are spaced on the support bar to correspond with the control rod drive positions 10. While the number of wing assemblies on each support bar may vary, generally the support bar is a straight piece of carbon steel having three wing assemblies 25 thereon. Further, the wing assemblies 25 are alternately placed on the hanger rods 11 on which the support bar 22 is attached. The relationship of the support bar 22, the wing assembly 25 and the control rod drive is illustrated in FIG. 4. The lower end of a control rod drive is shown at 20. At its lower end the control rod drive 20 consists of a flange 17 and a flange bolt 18. The wing assembly 25 is shown on the support bar 22 which in turn is provided on the hangers 11. The support bar 22 has a hub 27 thereon for receiving the wing assembly 25. The support bar 22 is provided on a hanger rod 11 at its one end through the wing assembly hub 28 and on another hanger rod 11 at its other end through a hanger rod hub 26. The wing assembly 25 is also shown on a hanger rod 11 at 29. At a hanger rod 11, the wing assembly 25 is inserted over a hub 28 on the support bar 22 which has been inserted over the hanger rod 11. FIG. 4 also illustrates another wing assembly 45 which will be described in more detail below. The wing assembly 25 is provided below the control rod drive flange bolt 18 with some clearance to allow for thermal expansion of the control rod drive when the plant is in operation. As a result, a gap 34 exists between the control rod drive flange 17 and flange bolt 18 and the wing assemblies 25. The gap 34 must be sufficient to allow the control rod drive 20 to expand as the system heats up during operation, preferably three quarters of an inch. When the plant is in operation, the control rod drive 20 expands and the gap 34 between the flange 17 and the bolt 18 and the wings 30 is reduced so that the flange bolt 18 and wing are almost touching. Preferably, the gap is reduced to about one quarter of an inch. Notches can be provided between the wing assembly 25 and the support bar hub 27 to provide an locking means for the wing assemblies 25. As a result, the wing assembly 25 cannot move when the plant is in operation and is thereby locked into the operating position 32. When a control rod drive must be replaced or serviced, the operation of the plant is stopped and the plant cools down. As the control rod drives reduce in size and the gap between the flange 17 and flange bolt 18 and the wing assembly 25 increases to its original clearance. At such clearance the wing assembly 25 may be turned to the non-operating position 33 whereby the wing assembly 25 does not interfere with the service and removal of a control rod drive 20. When the plant is not in operation and the system is cool, the wing assemblies 25 moves freely. The relationship of the wing assembly to the support bar 22 is shown in more detail in FIGS. 5A, 5B and 5C. FIG. 5A is a top view of the wing assembly 25 including wings 30. In FIG. 5B, a top view of the support bar 22 and the hubs 26, 27 and 28 are shown. The hub 27 supports the wing assembly 25 between hanger rod positions 11. A side view of the support bar 22 with hubs 26, 27 and 28 and wing assemblies 25 is shown in FIG. 5C. The wing assemblies 25, for purposes of illustrating the relationship of the wing assembly 25 to the support bar 22, are shown above the support bar 22. The hubs 26 and 28 have a hole therethrough sized to fit around the hanger rods 11. The wing assembly 25 has a hole 29 therethrough sized to fit over the hubs 27 and 28. In operation, as shown in FIG. 4, the hanger rods 11 support the support bars 22 which in turn supports the wing assemblies 25. The present invention includes two alternative wing assemblies to cover various situations around the control rod drive. For example, core detectors may be placed throughout the grid of control rod drives. Specifically Start-up Range Monitor/Intermediate Range Monitors ("SRM/IRM") core detector and Local Power Range Monitors ("LPRM") core detector may be provided throughout the grid of control rod drives to monitor the reactor core. While the alternative wing assemblies will be described in connection with a SRM/IRM detector and LPRM detector, the invention may be used with other instruments. Typically, the SRM/IRM and LPRM detectors are placed at the positions shown at 40 and 50, respectively in FIGS. 1, 2 and 3. Due to the nature of these detectors, alternate wing assemblies may be used in conjunction with these detectors. An LPRM detector 40 having a flange 41 is shown in FIG. 4, with a LPRM position wing assembly 45 on a LPRM position support bar 42. The LPRM position wing assembly 45, as shown, is installed around the LPRM detector 40. The LPRM position wing assembly 45 and support bar 42 are also illustrated in FIGS. 6A, 6B and 6C. FIG. 6A is a top view of the LPRM position wing assembly 45 having wings 44. FIG. 6B is a top view of the support bar 42 having hubs 46 and 48 and a throughhole 47. FIG. 6C is a side view of the wing assemblies 25 and 45 and the support bar 42. Like the support bar 22, the support bar 42 has a hub 48 for placement over a hanger rod 11 and for receiving a wing assembly 25 and a hub 46 for placement over a hanger rod 11. Support bar 42, however, is provided with a throughhole 47 rather than a hub. The hole 47 is sized to receive an extension 43 of the LPRM position wing assembly 45. In turn, a hole 49 is provided through the wing assembly 45 which is sized to receive the LPRM detector 40 or another instrument therethrough. In use, the LPRM detector 40 extends through the wing assembly 45 and the support bar 42. The wing assembly 45 rotates on the LPRM detector 40 from a non-support position to a support position and vice versa. Notches may also be provided between the support bar 42 and wing assembly 45 and extension 43 to further retain the wing assembly 45 in a support position during operation of the plant. As a result, the control rod drives can be serviced or replaced without removing the LPRM detectors. An SRM/IRM detector 50 having a flange 51 is shown in FIG. 7 with an SRM/IRM position wing assembly 55 on an SRM/IRM position support bar 52. The SRM/IRM detector has a drive unit 62 and a gear box 63 associated therewith. Because of the shape of the SRM/IRM detector 50 and gear box 63, the detector 50 may interfere with the operation of the previously described wing assembly 25 such that rotation of that assembly may not be possible. As a result, depending on the size of the SRM/IRM unit, it may not be possible to use the wing assembly 25 or 45 because it may not be possible to move the wing assembly to a position which does not interfere with the control rods 20. With such an event a wing assembly 55 having two removable wing blocks 54 may be employed. As shown in FIG. 7, the wing assembly 55 is located between the flange 51 and the gear box 63. The wing assembly 55 is shown in more detail in FIGS. 8A, 8B and 8C. FIG. 8A is a top view of a wing assembly 55 and the support bar 52. The wing assembly 55 has two wing blocks 54 which are bolted to the support bar 52 at 59 when in operation but which can be removed for service of the control rods. For purposes of illustration the wing blocks are shown removed from the hub in FIG. 8A. FIG. 8B illustrates a side view of the support bar 52. The support bar 52 has a hub 58 and 56 for placement over hanger rods 11. As shown in FIGS. 7 and 8B, the hub 58 may be used in a high support position and hub 56 may be used in a low support position. The support bar 52 also has a hole 57 sized to receive the SRM/IRM detector or another instrument therethrough. FIG. 8C illustrates another view of the support bar 52 and the removable wing block 44. The wing blocks 47 are removable to allow for replacement or service of the SRM/IRM detector or the surrounding control rod drives. Alternative support bars for each of the situations described in the above embodiments of the present invention are shown in FIGS. 9, 10 and 11. In the support bar arrangements shown in FIGS. 9, 10 and 11, every other support bar has either an upper or a lower hub. As a result, the total number of support bars used is increased. However, the removal and installation of a specific support is simplified because a maximum of three support bars need be removed in order to access any position. Moreover, for at least half of the positions, only one support bar need be removed. This is to be compared with the main embodiment described above where all the support bars have to be removed from the periphery until the specific bar to be accessed is reached. FIG. 9 illustrates a row of support bars 22 without any detector positions. The support bars 22 alternately have upper hubs 61 and lower hubs 62. FIG. 10 illustrates a row of support bars with LPRM detector positions. The support bars 42 are also shown having alternately, upper hubs 61 and lower hubs 62. FIG. 11 illustrates a row of support bars with SRM/IRM detector positions. Both the SRM/IRM support bars 52 and the support bar 22 are shown. The support bars 52 are also shown with upper hubs 61 and lower hubs 62. Thus, the present invention provides easy access to the detectors as well as the control rod drives. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention only be limited by the claims appended hereto.