Patent Number: 
Section: description

FIG. 1 illustrates a pressurized water reactor (PWR) 1 which includes an upright cylindrical pressure vessel 3 with a hemispherical bottom 5 and lid 7. A reactor core 9 is suspended within the reactor vessel by a structure which includes an upper support plate 11, a core barrel 13 and a lower support plate 15. The reactor core is made up of a plurality of elongated fuel assemblies 17 each including fissionable material contained within a number of fuel rods (not shown). Clusters of control rods 19 each positioned by a drive mechanism 21 located above the lid 5 are inserted into the fuel assemblies 17 as one means of controlling the reactivity of the fissionable material. Reactor coolant which is circulated by reactor coolant pumps (not shown) enters an inlet nozzle 23, flows down around the core barrel 13, upward through the lower support plate 15 and passes upward through the fuel assemblies 17 where it is heated by the nuclear reactions within the fissionable material. The heated coolant then passes outward through the outlet nozzle 25 for circulation through steam generators (not shown) before returning to the inlet nozzle 23. Typically, a reactor will have two to four loops each having an inlet nozzle and an exit nozzle. The reactor coolant is maintained at a pressure typically about 2,250 psi by the reactor coolant pumps. The hot coolant from the reactor core is passed through the steam generators where it gives up heat. Various parameters of the process are tracked by a plant computer 27. Among the parameters tracked are the inlet temperature of the coolant measured by thermocouples 29 at each of the inlets. As the coolant entering through each of the inlets mixes as it passes downward between the vessel wall and the core barrel, and continues to mix in the lower portion of the vessel, the temperature of the coolant entering each of the fuel assemblies 17 is fairly uniform. Hence, the temperature measured by all of the inlet thermocouples 29 are averaged to provide an average inlet temperature. A number of the fuel assemblies 17, but not all, are provided with exit thermocouples 31 which measure the temperature of the coolant as it leaves the fuel assemblies. Additional measurements include axial power offset which is measured by a strip of excore power detectors 33 extending along the outside of the reactor vessel 3. Numerous other parameters are tracked by the plant computer; however, these are the ones pertinent to an understanding of the present invention. The PWR 1 shown in FIG. 1 is also equipped with a moveable in-core detector system 35, which includes a number of moveable neutron detectors 37, each mounted on a drive cable 39 which is pushed through a thimble guide tube 41 to run the moveable neutron detectors upward through the fuel assemblies in thimbles (not shown). Measurements made by the moveable neutron detector as they pass along the fuel assemblies are used to generate a flux map which is an accurate measure of power distribution within the reactor core 9. As mentioned, these detectors cannot be used on a continuous basis, and therefore other means are needed to determine the power distribution within the reactor core between flux mappings. The exemplary PWR 1 utilizes the BEACON system to continuously monitor core power distribution. The BEACON system is resident in the monitoring processor 43 which can include one or more engineering workstations. As previously discussed, the BEACON system uses the in-core flux map together with a three-dimensional model of the reactor core to continually provide a three-dimensional power distribution within the reactor core. Between flux mappings, the three-dimensional nodal model power is updated for actual conditions by determining the power being developed in the individual fuel assemblies using temperature measurements generated between the inlet thermocouples 29 and exit thermocouples 31. The power developed in the fuel assemblies instrumented with an exit thermocouple 31 is determined by the change in enthalpy of the coolant as it passes through the assembly. As discussed, enthalpy is a function of the temperature rise over the fuel assembly, the pressure of the coolant and certain properties of the coolant. The coolant pressure is a measured quantity and the properties of the coolant are known. The rise in temperature is determined by subtracting the inlet temperature from the temperature measured by the associated exit thermocouple. As will be seen, these measured power values are used to periodically update the three-dimensional analytical nodal model power. The coolant cross flows between assemblies (mixing factors) can vary substantially and need to be updated. In accordance with the invention, the thermocouples are calibrated using data generated during initial power ascension in the current fuel cycle. Thus, the temperatures measured by each of the exit thermocouples 31 are repetitively recorded as power increases during initial power ascension of the reactor. Mixing factors are generated for each thermocouple from the measured power during initial power ascension as a function of the core average power. The mixing factors are applied to subsequent measurements of temperature taken by the thermocouples. In the exemplary embodiment of the invention, these temperature measurements are collected every fifteen minutes during initial power ascension, although it is to be understood that other intervals between measurements can be utilized. For each temperature measurement for each exit thermocouple recorded during initial power ascension, a corresponding predicted assembly power generated using the three-dimensional analytical nodal model is stored with the temperature measurement along with the core average power as measured by, for instance, the excore detector system. These periodic measurements are stored in bins defined by selected ranges of power. For instance, in the exemplary system the bins cover a range of 5% of rated power, although other bin sizes could be utilized. The purpose of utilizing bins is to provide a convenient way of storing the data and analyzing the statistical variation of the data over discrete power ranges. It is common to perform several flux mappings during initial power ascension of the reactor. For instance, the incore detector system 35 can be activated at 30%, 50%, 75% and 100% power. At each flux mapping, the exit thermocouples 31 can be calibrated. Each of the exit thermocouple temperature measurements is converted to a thermocouple power value in the manner discussed above. This thermocouple power value is then divided into the corresponding predicted power stored with the thermocouple temperature reading to generate a mixing factor. The mixing factors calculated for each of the exit thermocouples 31 are fitted to a selected mixing factor function of reactor power. At each of the power level 30, 50, 75 and finally at 100%, additional data is available and used to generate a new fit to the selected mixing factor function. The mixing factor function can range from a constant at lower power levels, to a linear function, at intermediate levels and a polynominal function at higher levels. In the exemplary embodiment, the selected mixing factor function of reactor power is 2nd order above about 90% and 1st order or a constant below about 90% reactor power. For the 2nd order function to be applied, the collected thermocouple data must extend over about a 70% power range. Subsequent to calibration, the thermocouple temperature measurements are used together with the mixing factors to update the three-dimensional analytical nodal model power by determining adjusted values for the predicted power at the corresponding locations. Such calculations are performed in the exemplary embodiment of the invention once each minute. Each time a flux map is generated, the selected mixing factor function as a function of reactor power for each fuel assembly is adjusted. A flux map of the reactor core is used to update the three-dimensional analytical nodal model power to generate an incore measured power distribution which we will call reference. The thermocouple measured temperatures for each exit thermocouple taken at the same conditions at which the flux map was generated is converted to a reference thermocouple power. An associated reference mixing factor is generated for each thermocouple by dividing the assembly reference measured power at the thermocouple location by the reference thermocouple power at the same location. The selected mixing factor function for each thermocouple is then adjusted to pass through the associated thermocouple reference mixing factor at the core power. Such periodic adjustment can be carried out multiple times during initial power ascension. It is also performed at full power and then periodically after initial power ascension whenever a flux map is generated. Standard deviations of the mixing factors for each thermocouple are calculated. This is done by evaluating the difference between the mixing factors calculated directly from the collected data and the mixing factors calculated from the mixing factor functions. The standard deviations for all of the temperature measurements for all of the exit thermocouples are then fitted to a single function of assembly power. The standard deviation information is used by the BEACON system to calculate uncertainties applied to the measured powers. The standard deviation is also used to establish the quality of the thermocouple data which is used to verify the reliability of the individual thermocouples. Thermocouples that deviate too much from the norm can be ignored in future power measurement calculations. A more specific understanding of the invention can be gained from FIGS. 2-5. FIG. 2 illustrates data flow during the plant startup or initial power ascension phase. An UPDATER background process 45 run in the monitoring processor 43 executes and depletes the analytical nodal model represented by the model files 47 approximately every 15 minutes. The UPDATER process 45 has access to plant data 49 such as the sensor information and the analytical nodal model represented by the files 47. The UPDATER process 45 determines the predicted power for each fuel assembly from the analytical nodal model and calculates the measured thermocouple power using the inlet temperatures and the assembly thermocouple measurements. During initial power ascension of the plant, the predicted power and the thermocouple power for every thermocouple location along with the core relative power is written to a startup thermocouple data file 51 every 15 minutes for storage for later processing. This provides thermocouple data as a function of assembly power and core relative power to be used in the analysis. The plant data file 49 contains inlet temperatures, exit thermocouple temperatures, core power level, control rod positions, excore signals, and pressure. The startup thermocouple data file 51 stores the core power level, predicted power and thermocouple power in the instrumented locations. The calibration file 53 includes thermocouple mixing factor functions, coefficients, standard deviation function coefficients, date and time of the calibration and other calibration parameters used in the core monitoring process. Periodically during initial power ascension, such as at 30%, 50%, 75% and 100% power, a flux map measurement is made and a full thermocouple calibration is performed as illustrated in FIG. 3. The BEACON foreground process 55 is the interface used to generate the calibration information. The data required for this phase is the thermocouple information collected during the plant startup and stored in the startup thermocouple data file 51. This collected thermocouple data is used to generate the mixing factors for each thermocouple which are fitted to the selected fitting function. These functions are then adjusted using the flux map data. Additional data required is a set of moveable detector measurement data stored in a flux map file 55, the nodal model consistent with the conditions of the moveable detector measurements provided by the model files 47 and a set of thermocouple data consistent with the moveable detector information. The calibration process analyzes the moveable detector information from the flux map file 57 to generate a reference measured power distribution. This reference measured power distribution is assumed to be truth. Using the inlet temperature and the measured thermocouple temperatures consistent with the time frame that the moveable detector measurements were taken, a second relative assembly power in the assembly can be determined based on the exit thermocouples (thermocouple measured power). The ratio of the power in the assembly from the reference measured power distribution and the thermocouple measurement power is the mixing factor at the relative core power. This reference mixing factor at the relative core power for each fuel assembly is used to modify the previously established mixing factor functions so that the shape passes through this mixing factor just calculated using the reference measured distribution. The flux map reference file 57 includes the moveable detector measurement trace data, the thermocouple temperatures, excore signals, core power level, rod positions, and inlet temperatures all at map conditions. During normal operation, as illustrated in FIG. 4, the UPDATER process 45 and a MONITOR process 59 both access the mixing factor and standard deviation coefficients from the calibration file 53 to determine thermocouple quality factors, measured power distribution and uncertainties applied to the results. The UPDATER process runs every 15 minutes typically and depletes the fuel, burnable absorbers and fission products based on the accumulated core burnup. The thermocouple mixing factors and quality factors are used in the process to determine the core measured power distribution. The MONITOR process runs every minute in between the UPDATER runs and applies the thermocouple mixing factors and quality factors to the measured thermocouple temperatures to determine the changes in the core measured power distribution determined by the UPDATER process. The mixing factors and standard deviation coefficients are a function of core relative power and assembly power, respectively, and are generated for the current operating cycle. Periodically during operation, a calibration process is carried out as illustrated in FIG. 5. The BEACON foreground process 55 is used to generate the calibration information. The data required for this phase is a set of moveable detector measurement data and a set of thermocouple data consistent with the moveable detector information obtained from the flux map file 57 and an analytical nodal model consistent with the conditions of the moveable detector measurements obtained from the model files 47. The calibration process then analyzes the moveable detector information to generate the reference measured power distribution which is assumed to be truth. The relative power in an assembly is determined using the inlet temperature and the measured exit thermocouple temperatures consistent with the time frame that the moveable detector measurements were taken. The ratio of power in the assembly from the reference measured power distribution and the reference thermocouple measurement power is the mixing factor at that relative core power. The mixing factor coefficients from the calibration file 53 are modified so that the shape passes through the just calculated point at that relative power. The modified mixing factor coefficient is saved to the calibration file. The present invention not only provides calibration of the thermocouples utilizing data from the current cycle, thereby avoiding any inaccuracies associated with the prior art technique of utilizing data from the previous fuel cycle, but also provides calibration over the full power range of the reactor rather than just at 100% power. This permits the BEACON system to more accurately determine the power distribution which in turn allows the reactor to be operated closer to various operating limitations. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.