Patent Number: 054426650
Section: summary

FIELD OF THE INVENTION This invention relates to monitoring of operating conditions inside a water-moderated nuclear reactor. In particular, the invention relates to monitoring of flow-induced vibrations in the internal components of a nuclear reactor. BACKGROUND OF THE INVENTION Flow-induced vibrations (FIVs) of the internal components of a nuclear reactor may cause fatigue-initiated cracking and/or component failure of those components. Because of the safety hazards associated with component failure in a nuclear reactor, it is necessary that the state or condition of internal components susceptible to vibration-induced damage be carefully monitored. A conventional technique for monitoring and determining flow-induced vibrations is to attach vibration sensing instruments to the reactor internal components. Each vibration sensor is attached to a separate 1/16-inch-diameter mineral insulated (MI) signal cable to form a sensor string. To protect the MI signal cable from mechanical damage during installation and from the reactor environment during operation, protective conduits are used to encase the MI signal cables, each conduit encasing multiple cables. For example, such protective conduits can comprise 0.375-inch-diameter stainless steel tubing which is bracketed to components inside the reactor pressure vessel. Signal cables from the vibration sensors are routed within the reactor pressure vessel to a pressure boundary seal located in the vessel top head, through the drywell and to a data acquisition system in the reactor building. One type of internal reactor component which is monitored to determine flow-induced vibrations therein is the in-core monitor housing. Core power is monitored by neutron flux monitors located within in-core monitor instruments supported by in-core monitor support assemblies. Each in-core monitor support assembly includes an instrumentation guide tube and an in-core monitor housing. The vibration sensor is conventionally mounted on the outside diameter of the in-core monitor housing. Upon completion of FIV testing, the instruments, MI cables and protective conduits are removed to the extent possible. The respective accessibility of each item of equipment determines the extent to which removal is possible. To date, it has not been possible to remove the instruments, MI cables or conduits attached to components, such as the in-core monitor housing, which are located in the reactor lower plenum region. i.e., below the core plate. Thus there is a need for a method and device to facilitate removal of all FIV instruments, cables, conduits, and associated equipment after completion of FIV testing. This includes sensors, conduits and equipment located in inaccessible regions below the core plate which current methods and technology cannot remove. SUMMARY OF THE INVENTION The present invention is a method and an apparatus for facilitating installation and removal of FIV instrumentation and associated equipment from inside the reactor pressure vessel. The FIV instrumentation of concern to the invention consists of two strain gauges which are attached to the upper portion of the in-core monitor housings within the reactor lower plenum region. In accordance with the teaching of the present invention, the strain gauges are positioned on the inside diameter of the in-core monitor housing instead of on the outside diameter, as has been the past practice. Relocating these instruments to the inside diameter of the in-core monitor housing permits the MI signal cables to be routed to outside the reactor pressure vessel through a flange insert located between the in-core monitor instrument flange and the in-core monitor housing flange. This design makes the FIV instrumentation and associated equipment accessible from below the reactor pressure vessel for removal and installation. The MI signal cables are very small in diameter and require support and protection from damage during the insertion of the local power range monitor (LPRM) instrument. This protection and support is provided by two concentric thin-walled tubes between which the MI signal cables are routed. These support tubes are attached to each other by spot welding or other suitable means to increase the assembly stiffness. Further, the support tubes are attached to the flange insert, making an easily handled assembly of the instruments, cables, and pressure boundary penetration. Thus complete removal is easily achieved by unbolting the in-core monitor instrument flange and extracting the FIV instrumentation flange insert with the attached MI signal cables and support tubes. The strain gauge instrumentation device in accordance with the invention comprises a pair of vibration sensors and associated MI signal cables. The vibration sensors are attached to an inside diameter of the in-core monitor housing. The signal cables extend into the exterior of the reactor pressure vessel through a brazed or welded penetration in the flange insert. A protective assembly minimizes the damage inflicted on the signal cables during insertion operations. The signal cables are helically wound in an annular space between a pair of concentric thin-walled tubes and then passed through and brazed or welded to respective cable penetration bores formed in the body of a flange insert. The flange insert supports the concentric tube assembly and is sandwiched between the in-core monitor housing flange and the in-core monitor instrument flange of a conventional in-core monitor assembly.