Patent Number: 050154346
Section: description

Referring to FIG. 1, a reactor vessel V is shown with its dome removed and a local power range monitor string S in the process of insertion. Typically, the string S inserts into tube 24. Tube 24 begins at the core plate 12 and extends through the bottom of the reactor vessel V. The portion of the string S within the core extends above the tube 24. An upper portion of the string S registers to the top guide 12. Thus the string S as ultimately held to the core stands vertically upright in measuring exposure to the neutron flux interior of the reactor core. Referring to FIG. 1, a section of a boiling water reactor vessel V is illustrated. The vessel V includes a core shroud 14 surrounding a core 15 controlled by control rods 16. In the portion of the reactor here shown, jet pumps 13 draw water over the top of a top guide 12 downwardly in the interstitial volume between the side of the vessel V and the core shroud 14 through the jet pumps 13 to a plenum P below the core of the reactor. Water then passes upwardly through a core plate 17 into the individual fuel bundles of the core at 15. Water for the jet pumps is extracted at 19, entered at 18 to cause the required forced circulation within the reactor. As here shown, a single instrument guide tube 24 is illustrated for the insertion of a local power range monitor string. It will be understood that insertion begins from above the core. The string is from the top of the core inserted to the bottom of the in-core guide tube. A seal is made at the bottom of the in-core guide tube. The string extends from the top of the core 15 adjacent the top guide 12. Each string includes typically four monitoring sites. These monitoring sites are equally spaced between the top guide 12 and the core plate 17. They are positioned to sample four successive vertical intervals within the reactor. Although the illustration of FIG. 1 only shows one such income guide tube 24, it will be understood that many are in fact used to monitor a typical core 15. For example, it is not uncommon to include forty four (43) such conduits or about 170 discretely wired local power range detectors. The reader will understand that the local power range detectors are combined in varying groups to produce required measurement. Since the combination of such groups is not pertinent to this invention and since the instrumentation for reading such local power range monitors is well known, it will not be further set forth herein. Having set forth the reactor site in which monitoring occurs, a typical monitoring string S in accordance with this invention will now be set forth. Referring to the exploded view of FIG. 2A, one such string is illustrated. The string includes a spring compression portion 44. This spring compression portion fits into the top guide 12. (See FIG. 1) Typically, to cover the active part of the string, there is provided a cover 50. Cover 50 encloses the detectors and coaxial cables, which cables extend from the detectors downward through a seal joint at the bottom of the in-core guide tube and are terminated in an external connector. The string is semirigid in construction. Insertion to in-core guide tube 24 easily occurs. (See FIG. 1) At four discrete elevations, herein denominated A, B, C, and D, groups of conventional local power range detectors and gamma thermometers are placed in a relation which may be side-by-side or vertical. Referring to FIGS. 2B and 2C, a conventional local power range detector M will first be described. Thereafter, the gamma thermometer T will be set forth. Referring to FIG. 3, local power range detector M includes an outer cylindrical cathode 60 and an inner concentric and cylindrical anode 62. Cathode 60 adjacent to anode 62 is provided with a thin coating of fissionable materials 64. Materials 64 are typically combinations of U235 and U234. As is well known in the art, U235 is expended over the life of the monitor M; the U234 breeds replacement U235 thus prolonging the in-service life of the detector M. Typically, anode 62 is mounted by insulating blocks 66 at each end so as to be concentric of the enclosing cathode 60. Preferably, an argon atmosphere 68 is present. Typically a coaxial cable leads from the detector with the center conductor connected to the anode and the outer conductor connected to the cathode. It is the direct current through the cable that provides the real time measurement of thermal neutron flux. In operation, thermal neutrons impact U235 at layer 64. Fission components dissipate into the argon gas 68 and cause electrons to flow to the anode with ions of opposite polarity to the cathode. An overall direct current is induced through the cable 69 which direct current is conventionally read. Since the U235 component of the layer 64 varies with in-service life, calibration is required. Referring to FIG. 4, a typical gamma thermometer is illustrated in a simplistic format. Typically, the gamma thermometer includes an enclosing chamber 72 having a metal mass 74 suspended in a cantilevered fashion from one end of the chamber 72. The mass of metal 74 reaches a temperature which is directly dependent on the gamma ray flux. A reading thermocouple 78 and a reference thermocouple 80 are utilized in a series circuit. Specifically, the temperature differential between the reference thermocouple 80 (typically referenced to a temperature stable interior portion of the core) and the reading thermocouple 78 produce a voltage on paired lines 82, 84 which voltages indicate the gamma flux present which is proportional to reactor power. It will be appreciated that gamma thermometers T are not responsive t rapidly changing reaction within the reactor. Accordingly, it is not possible to use the gamma thermometer T for monitoring short duration flux transients or rapidly changing flux levels. Having set forth the construction of the gamma thermometers and local power range monitors, description of clusters of instruments utilized at the various cluster levels A, B, C, and D can now be set forth. Referring to FIG. 2B a preferred embodiment of the invention is disclosed. The preferred embodiment includes a cover tube 70 having an upward water flow therethrough. In the interior of the tube at 72 a gamma thermometer T1 and a second gamma thermometer T2 are each individually shown. These respective gamma thermometers T1 and T2 are communicated to their respective cables 74, 76. In between the respective gamma thermometers there is located a local power range monitor M. It will be observed that the local power range monitor is separated from both gamma thermometers by a small distance d. This distance is chosen so that the neutron flux is essentially uniform. For example, a 1 inch level of separation may be used. Referring to FIG. 2C an embodiment having a single local power range monitor M and a gamma thermometer T is illustrated. Again insertion has occurred interior of a sheath 70 having water flow in the space 72. Again each of the sensor units is communicated to its own cable. In this case cable 75, 76. Returning to FIG. 2A the discrete parts of the local power range monitor string can be further understood. Simply stated, the local power range monitor string includes a spring compression portion 80 (see spring portion 44). A considerable length of the entire rod 82 fits within the active fuel region. A second length of the monitor string 84 sits below the core and within the reactor vessel. Finally, a portion of the string 86 is outside the vessel. As the unit passes through the vessel, a pressure seal 88 is required. This pressure seal prevents leakage from the pressurized interior of the reactor to the exterior thereof. Instrumentation connects to the wire ends 90 from the respective local power range monitors M or gamma thermometers T. Dependent on the number of gamma thermometers utilized for each level, there will be at least one local power range monitor M with paired connections and two or four connections from the respective gamma thermometers T. The required energy balance measuring the power output of a steady state nuclear reactor is well within the state of the art. Once this is known, measurement of the gamma thermometers can all be correlated to the intensity of the reaction at any given point. Since there is one or two gamma thermometers T adjacent each and every local power range monitor M, it is thereafter possible to calibrate each local power range detector M with the readout of its adjacent gamma thermometer T. It will be observed that gamma thermometers do not include portions thereof which, with increased in-service life, have decreasing effectiveness. This being the case, it will be understood that with reference to any heat balance, there is an expectancy that the output of the gamma thermometers T will remain substantially unchanged. There is therefore an additional check as to the overall operability of the calibration.