Patent Number: 054426650
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a method and a device for facilitating installation and removal of instrumentation and associated equipment for monitoring flow-induced vibrations in an in-core monitor housing of a boiling water reactor (BWR) during reactor operation. The fluid flow in a boiling water reactor will be generally described with reference to FIG. 1. Feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. A core spray inlet 11 supplies water to a core spray sparger 15 via core spray line 13. The feedwater from feedwater sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and core shroud 18. Core shroud 18 is a stainless steel cylinder which surrounds the core 20 comprising numerous fuel assemblies 22 (only two 2.times.2 arrays of which are depicted in FIG. 1). Each fuel assembly is supported at the top by top guide 19 and at the bottom by core plate 21. Water flowing through downcomer annulus 16 then flows to the core lower plenum 24. The water subsequently enters the fuel assemblies 22 disposed within core 20, wherein a boiling boundary layer (not shown) is established. A mixture of water and steam enters core upper plenum 26 under shroud head 28. Core upper plenum 26 provides standoff between the steam--water mixture exiting core 20 and entering vertical standpipes 30, which are disposed atop shroud head 28 and in fluid communication with core upper plenum 26. The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. The steam passes through steam dryers 34 and enters steam dome 36. The steam is withdrawn from the RPV via steam outlet 38. The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 40 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 42 (only one of which is shown) via recirculation water inlets 44. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The jet pump assemblies are circumferentially distributed around the core shroud 18. The core shroud 18 is supported by RPV shroud support 46, which is welded to shroud 18, and by annular shroud support plate 48, which is welded at its inner diameter to the RPV shroud support 46 and at its outer diameter to RPV 10. Core power is monitored by neutron flux monitors located within in-core monitor instruments 8. Each in-core monitor instrument 8 is supported by an in-core monitor support assembly 2 which includes an instrumentation guide tube ,4 and an in-core monitor housing 6 connected by a weldment W (see FIGS. 2 and 3). As shown in FIG. 2, each in-core monitor housing 6 extends upwardly through bottom head 10a and into the interior of RPV 10. In-core monitoring instruments 8 are inserted into the fuel core 20 from above and through the in-core monitor support assembly 2. Motion of instrument 8 below the core region 20 is guided vertically by instrumentation guide tube 4 (see FIG. 3). The guide tube 4 is an extension of the in-core monitor housing 6 and is welded to the top of the housing after the strain gauges (described below) have been attached to the housing inner diameter. The guide tube extends from the top of housing 6 to the core plate 21 and supports instrument 8 in the=region below the core plate. The instrument 8 extends upward through the core region 20 to the top guide 19. Instrument 8 is supported in the core region by the fuels channels and therefore no guide tube is needed in the core-region. The in-core monitor housing 6 has a flange 6a located external to RPV 10. In a conventional assembly, an in-core monitor instrument flange 54 is bolted to housing flange 6a by a plurality of bolts (not shown) which pass through unthreaded bores 56 in in-core monitor instrument flange 54 and are screwed into threaded bores 58 in housing flange 6a. The housing flange 6a has four threaded bores which are respectively collinear with the four unthreaded bores in in-core monitor instrument flange 54. It is conventional practice to insert in-core monitor instrument 8 into in-core monitor support assembly 2 from the top. The instrument 8 slides down through the central bores of guide tube 4 and housing 6 and then into the central bore of in-core monitor instrument flange 54. The in-core monitor instrument 8 has a conical surface 8a (see FIG. 5) which, in the absence of the present invention, abuts a conical seat 54a formed in the central bore of in-core monitor instrument flange 54. This abutment prevents further downward displacement of in-core monitor instrument 8. An in-core nut 60 is then screwed tight onto a threaded outer circumferential surface of in-core monitor instrument 8 to lock the latter in place. A seal 62 embedded in the bottom end of in-core nut 60 bears against an outer circumferential surface of in-core monitor instrument 8 to seal the annular gap between in-core monitor instrument 8 and in-core nut 60. In accordance with the present invention, a pair of strain gauges 64 and 64' (shown prior to mounting in FIG. 6) are mounted on the inside diameter of the in-core monitor housing 6 at angular positions separated by 90.degree.. Only one strain gage 64 is shown in FIGS. 2 and 3. Each strain gauge is connected via a respective MI signal cable to data acquisition means (not shown) external to RPV 10. After the strain gauges have been installed and the guide tube 4 has been welded to housing 6, the in-core monitor instrument 8 is inserted in guide tube 4 from above. To ensure that the MI signal cables are not damaged during either strain gauge installation or in-core monitor instrument insertion, a novel assembly is used to support and protect the signal cables. A flange insert 66 is sandwiched between housing flange 6a and in-core monitor instrument flange 54 with gaskets 68, 68' therebetween, as shown in FIGS. 2 and 3. Unthreaded bores 70 are provided in alignment with the corresponding bores of housing flange 6a and in-core monitor instrument flange 54 to allow the assembly to be bolted together. The presence of the flange insert 66 means that the in-core monitor instrument flange 54 is displaced downward relative to the housing flange 6a by a distance equal to the thickness of flange insert 66. To maintain the in-core monitor instrument 8 at the same elevation, a sealing insert 72 of height equal to the thickness of flange insert 66 is inserted between the conical surface 8a and the conical seat 54a, as seen in FIGS. 3 and 5. Sealing insert 72 is a metal ring having concentric inner and outer circular cylindrical surfaces and conical end surfaces configured to conform to conical surface 8a and conical seat 54a, respectively. Thus, the in-core monitor instrument 8 will seat on and be supported by the upper conical end surface of sealing ring 72. As best seen in FIG. 5, the strain gauge instrumentation device of the present invention is an assembly comprising the flange insert 66, an outer thin-walled tube 74 and an inner thin-walled tube 76. Preferably, the wall thickness of tubes 74 and 76 is on the order of 4 mils. The diameter of outer tube 74 is greater than the diameter of inner tube 76, with inner tube 76 being concentrically arranged inside outer tube 74. Respective portions of two MI signal cables are helically wound in the annular space 78 between tubes 74 and 76, the helical winding starting at the bottom of the tubes depicted in FIG. 5 and terminating at the top of tubes 74 and 76 as shown in FIG. 6. The length of tubes 74 and 76 will depend on the length of the in-core monitor housing 6 which is to be FIV tested. For a known in-core monitor housing having a length of 181.66 inches, tubes 74 and 76 will extend to a distance of about 170 inches from the radial plane of surface 80 (see FIG. 5). As shown in FIG. 6, the signal cables 82, 82' are routed through the annular space 78 between tubes 74 and 76. The concentric portions of tubes 74 and 76 are spiral spot-welded to increase the stiffness of the assembly. The spot welds 84 are positioned between the signal cables. The signal cables 82 and 82' enter annular space 78 via respective cable penetration bores 86 and 86' formed in flange insert 66 (see FIG. 4) and via respective penetration holes (not shown) formed in outer tube 74. The space between the penetration bore and the signal cable is sealed by welding or brazing to preserve the pressure boundary between the interior and exterior of the RPV. The signal cables 82 and 82' exit annular space 78 at the top of tubes 74 and 76. The strain gauges 64 and 64' are connected to these protruding ends of signal cables 82 and 82'. The lower ends of tubes 74 and 76 are pressed together into an annular groove 88 in flange insert 66 and held there by a tube retaining ring 90 (see FIG. 5). Ring 90 is of the snap-fit variety, having a through gap which enables flexing of the ring to fit through a smaller-diameter ]portion of the central bore of flange insert 66. When the flexed ring is axially aligned with annular groove 88, the ring is released to snap radially outwardly and into an annular groove formed in the pressed ends of tubes 74 and 76. The opposed ends of the ring are then butt-welded together, thereby securely coupling the tubes 74 and 76 to the flange insert. This coupling further increases the stiffness of the tube assembly. The entire assembly can be handled as a single unit, thereby facilitating installation and removal of the strain gauges and MI signal cables incorporated therein. A special insertion technique is used to achieve damage-free installation of the assembly inside the in-core monitor housing 6. This technique utilizes a mandrel 92 as the insertion tool (see FIG. 6). The mandrel is simply a circular cylindrical pipe having a radially outwardly directed flange (not shown) at one end only. The diameter of the cylindrical mandrel body is less than the diameter of inner tube 76 so that the mandrel body can be slid inside the tube assembly. The diameter of the mandrel flange is greater than the inner tube diameter so that the flange can support the flange insert 66. This support facilitates insertion of the assembly inside in-core monitor housing 6. The mandrel has sufficient length that its top end extends above the top end of inner tube 76 when the mandrel and the assembly .are coupled. The two strain gauges attached to the protruding ends of the MI signal cables are then adhered to the outer circumferential surface of the mandrel to temporarily secure the strain gauges during insertion of the assembly into the in-core monitor housing 6. Before the assembly is inserted, gasket 68 must be installed from the upper end and positioned correctly on flange insert 66 as shown. During insertion, the assembly is supported on the flange of the mandrel 92, which is pushed up into the in-core monitor housing 6 by a suitable tool. When the assembly is in its final position, i.e., with flange insert 66 abutting housing flange 6a, a suitable tool is used to hold the flange insert in place temporarily while the mandrel is removed. During sliding out of the mandrel, the strain gauges are separated from the mandrel by the tension applied by the signal cables as the mandrel is pulled out. After the mandrel has been fully removed, the sealing ring 72 is placed in position on in-core monitor instrument flange 54, which is then bolted on, thereby sandwiching flange insert 66 and securing the tube assembly relative to in-core monitor housing 6. The next step in the procedure is to weld the strain gauges to the inside diameter of in-core monitor housing 6 by entering the latter from the top. Then the instrumentation guide tube 4 is welded to the in-core monitor housing 6 (see weld W in FIGS. 2 and 3). The next step in the procedure is to install the in-core monitor instrument 8 from the top. The strain gauges 64, 64', which have been welded to the inside diameter of in-core monitor housing 6, lie outside the path of the in-core monitor instrument 8. The instrument 8 enters the inner tube 76 and slides downward until the conical surface 8a seats on the opposing conical seat formed on the upper end of sealing ring 72. In-core monitor instrument 8 is then locked in place by screwing in-core nut 60 tightly thereon. In this manner, during slidable insertion of the assembly inside the in-core monitor housing 6, the MI signal cables 82 and 82' are protected against damage by the barrier of outer tube 74. Similarly, during slidable insertion of instrument 8 into the assembly, the MI signal cables 82 and 82' are protected against damage by the barrier of inner tube 76. After FIV testing has been completed, the strain gauges and MI signal cables must be removed. This is accomplished by unscrewing in-core nut 60; pulling in-core monitor instrument 8 out via the top of instrumentation guide tube 4; unbolting in-core monitor instrument flange 54 from in-core monitor housing flange 6a; and pulling the assembly out from the bottom of the in-core monitor housing 6,.leaving behind the welded strain gauges 64, 64' which separate from the ends of the respective signal cables 82, 82'. The strain gauges are then removed by reaming the bore of in-core monitor housing 6. Reactor coolant water is prevented from leaking out of the in-core monitor housing 6 by temporarily plugging the guide tube 4 at its upper end. Then the in-core monitor instrument flange 54, in-core monitor instrument 8 and in-core nut 60 can be reinstalled in conventional fashion. The preferred embodiment of the strain gauge instrumentation device in accordance with the invention has been disclosed for the purpose of illustration. Variations and modifications of the disclosed structure which do not depart from the concept of this invention will be readily apparent to engineers skilled in the art of designing equipment for monitoring nuclear reactors. For example, the invention is not limited to devices used with strain gauges or to devices installed in in-core monitor housings. Furthermore, although the signal cable penetration is sealed in accordance with the preferred embodiment by welding or brazing, any other conventional sealing technique can be used, e.g., silver soldering, a compression-type fitting or swaging. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.