Patent Number: 054901847
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is directed to a method and apparatus for determining on-line core wide power output of a pressurized water reactor (PWR) using the excore detector system, and to correction of the same for changes in three-dimensional power distribution in the core and in coolant density. 2. Background of Information The official determination of the reactor thermal power level in a PWR is based on a heat balance across the steam generators (S/G), called a secondary calorimetric measurement. The results of the calorimetric calculation are used to verify that the reactor is operating within the licensed reactor power level limits, and to calibrate the other indications of reactor power level. This calorimetric calculation is performed off line. Other indications of reactor power level, such as excore detector signal levels and RCS loop temperature indication values, are periodically calibrated against the calorimetric and used to provide on-line reactor power level inputs to the reactor control and protection systems. Unfortunately, errors in the reactor thermal power level calculation cause errors in all the other indications of reactor power. Presently, there is no easy way to detect and correct small errors in the reactor thermal power calculation. The major component of the heat balance calculation is the flowrate of feedwater into the S/G. The magnitude of the flowrate is typically determined using flow venturis installed in the main feedwater line for each S/G. The feedwater venturi flow readings are subject to systematic and random error mechanisms, which cause erroneous reactor power level calculation results. The most frequent systemic error, feedwater venturi fouling, causes the calculated reactor power to increase relative to the true reactor power level, necessitating a net reduction in the actual reactor power to maintain the apparent power within the operating limits. The amount of electrical output generated by the plant then decreases, reducing the revenue of the utility. There is a need, therefore, for an improved method and apparatus for on-line measurement of reactor power which is not affected by random errors in the thermal power measurements. There is also a need for a method and apparatus for correcting for errors in the calorimetric calculation of reactor power caused by feedwater venturi fouling or other types of systematic calorimetric error sources. SUMMARY OF THE INVENTION These needs and others have been satisfied by the invention which is directed to a method and apparatus for on-line determination of PWR power using excore detector signals corrected for changes in three-dimensional power distribution in the reactor core and for changes in coolant density. It has been found that feedwater venturi fouling is a phenomena that tends to increase with reactor operating time during a fuel cycle. The thermal power calculations performed early in the cycle tend to be relatively unaffected by venturi fouling effects, and therefore, tend to be quite accurate, allowing the power calibrations of the dependent power indications to be correspondingly accurate. In order to maintain the accuracy of the power calibration developed for the excore detectors in the absence of an accurate thermal power measurement, it is necessary to be able to normalize out the changes in the excore detector signals caused by changes in the core radial and axial power distribution that have occurred since the last accurate thermal power measurement was performed. It is also necessary to correct for the effects on indicated power caused by changes in coolant density that occur when reactor inlet temperature changes. This invention is directed to a method and system capable of allowing the signals from the excore detectors to be used as an independent means of determining the reactor power in an absolute fashion. In fact, the power measurements generated from the excore detectors in accordance with the invention can be used to ascertain the accuracy of and, if necessary, correct the thermal power measurements. In accordance with the present invention, excore detector current measurements are used to generate absolute reactor power by calibrating detector current measurements to the reactor thermal power calculation made at a base time early in the reactor cycle while the thermal reactor power measurement is still accurate. Measurements are also made at the base time of the three-dimensional core power distribution and the core inlet temperature distribution. Present core power measurements are then made by measuring the present excore detector current, the most recent three-dimensional core power distribution and the present core inlet temperature. The present core power is then calculated as the ratio of the present detector current to the detector current at the base time multiplied by the reactor thermal power measurement at the base time. The product is then corrected for changes in three-dimensional power distribution and in core inlet temperature since the measurement of those parameters at the base time. As the typical PWR excore detector system includes a plurality of excore detectors, usually four equally spaced around the reactor vessel, and each includes a top detector section and a bottom detector section, present core power determinations are made for each of the detector sections of each detector with the results averaged to determine present core power. The three-dimensional power distribution can be measured by an incore detector system which may utilize either fixed incore detectors or a moveable incore detector system. In the former case, a three dimensional core power distribution can be continually measured repetitively such as for instance every minute. The three-dimensional power distribution can also be generated by the system described in U.S. Pat. No. 4,774,049 which utilizes inlet temperature and readings from a pattern of thermocouples which measure core exit temperature to calculate on an on-line basis the three-dimensional core power distribution. The invention provides a simplified means for correcting the excore measurement for changes in coolant density. The correction factor is an exponential term in which the difference between the present core inlet temperature and core inlet temperature at the base time is multiplied by a constant. This constant is empirically determined at two different temperatures, preferably during reactor start-up. With the invention, only a single measurement of the thermal reactor power is required. A single calculation is made at a base time when the feedwater venturi used to measure the feedwater flow for the calorimetric calculation is unobstructed and the thermal reactor power calculation is accurate. The invention embraces both the method and apparatus for absolute excore detector reactor power determination.