Patent Number: 050248014
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention, when used online on a nominally continuous basis, will provide both frequent periodic updates of certain of the contents of a one dimensional (axial) analytical model and as--required adjustments of certain other contents of the model, so that the analytical core model can serve the dual functions of supplying to a graphic system the necessary data that permits the generation of graphic displays regarding current core conditions and trends of immediate use to the reactor operator and of providing a reliable basis for intializing sequences of analytical predictions of expected core response to anticipated plant maneuvers when requested by plant personnel or by dedicated automatic control systems. In addition, the present invention will defeat the tendency mentioned earlier of the analytically updated axial power distributions generated by a core response predictor to progressively deviate from the true core average axial power distribution. Constraining the analytically calculated axial power distribution to closely approximate the true core average axial power distribution insures that the calculated axial distributions of iodine, xenon, (promethium and samarium, if explicitly represented) and long term burn-up will also closely approximate the corresponding existing core average distributions. Specifically, the present invention, on a continuous, automatic and periodic basis, determines a "measured" value of incore axial offset synthesized from conventional plant instrumentation response signals as a basis for adjusting the A.sub.2 coefficient in the analytical representation of the axial distribution of transverse buckling values in the analytical core model to force a match between the axial offset of the calculated axial power distribution and the measured incore axial offset value. The present invention also synthesizes a measured value of incore axial pinch from plant instrumentation response signals and the measured axial pinch value is used in like manner to adjust the A.sub.3 coefficient in the representation of the axial distribution of transverse buckling values in the analytical core model to force a match between the axial pinch of the calculated axial power distribution and the measured incore axial pinch value. The present invention, in addition to measuring the core power level, control bank position and cold leg temperature signals that are currently supplied to existing core predictors, measures also at a minimum, the signals from the top and bottom detectors of at least one conventional excore power range neutron detector channel. The signal from at least one specified core exit thermocouple is also added to this input signal set. The selection of the thermocouple is described in U.S. Pat. No. 4,774,050 and a second thermocouple can also be selected and used for verification. If one or more so-called "multi-section" incore neutron detectors are in service at a particular installation or one or more strings of fixed incore detectors with at least three detectors per string is/are in service, the signals from the individual excore detector sections or fixed incore detectors are appropriate substitutes for the combination of two section incore detector signals and the core exit thermocouple signal. The two detector signals, together with the control bank position signal, along with the other variables supplied to the model, are used to synthesize the measured axial offset value (AO) directly using the equation below: ##EQU2## where B.sub.1 -B.sub.3 are axial offset expansion coefficients that are obtainable by a person of ordinary skill in the art by using a least squares fit calibration against a set of transient flux maps, DR.sub.t and DR.sub.b are the signals from the top and bottom sections of the incore detector, Q is core thermal power level conventionally provided to prediction models and bp is controlling bank position. In the least squares fit calibration mentioned above a series of flux maps, taken over a period of a few days once each quarter where the core is put into a transient axial oscillation state and the flux maps, are taken at various values of axial offset and least squares fit to obtain the coefficients. The axial pinch is synthesized by adding a term representative of local coolant enthalpy rise in the peripheral region of the core seen by the incore detectors and using a correlation of the form: ##EQU3## where C.sub.1 -C.sub.4 are axial pinch expansion coefficients obtainable by a least squares fit calibration against a series of transient flux maps as mentioned above and .DELTA.h is the local enthalpy rise derived from the core exit thermocouple signal and from a cold leg temperature signal as set forth in U. S. Pat. No. 4,774,050. If one or more "multi-section" incore detectors or one or more strings of fixed incore detectors are available, the measured axial offset value and the corresponding measured axial pinch value can be synthesized using: EQU AO.sub.meas. =F.sub.AO (I.sub.top, I.sub.upper middle, I.sub.lower middle, I.sub.bottom, h.sub.D) (5) EQU AP.sub.meas. =F.sub.AP (I.sub.top, I.sub.upper middle, I.sub.lower middle, I.sub.bottom, h.sub.D) (6) where the I's represent the currents generated by, for example, the individual sections of "multi-section" incore detector and the functions are simple numerical correlations between detector values and the actual offset or pinch determined at these values determinable by a nuclear engineer familiar with the detector system and the reactor core structure. The present invention on a prespecified periodic basis (every five minutes, for example) determines the values of time, power level, control bank position, cold leg temperature, RCS pressure (optionally), axial offset and axial pinch. The time, power level, control bank position, cold leg temperature and pressure (if provided) are used to update the model data file that contains the description of the current core model parameters. From the model, calculated core axial power distribution values of axial offset and axial pinch are extracted. The calculated values of axial offset and axial pinch are compared to the "measured" values of axial offset and axial pinch. If the respective values are in agreement within a prespecified tolerance, that is, if the current deviations or the time integral deviations, for a more accurate tracking function, are below a predetermined threshold, the updated model core description is stored and the updating process is suspended until the next scheduled update time. If the value of either calculated axial offset or calculated axial pinch fails to match the corresponding measured value within the specified tolerable error, adjustment, in a manner as described in U.S. Pat. No. 4,711,753, of the values of the A.sub.2 and A.sub.3 expansion coefficients in the analytical representation of the axial distribution of transverse buckling values are made to obtain acceptable agreement between the calculated and measured values of axial offset and axial pinch. When satisfactory agreement is obtained, the resulting analytical core description is stored and the updating process is suspended until the next scheduled update time. As illustrated in FIG. 1 the model adjustment system 10 obtains the core thermal power from a reactor control system 12 and control bank position information from a rod control system 14. The thermocouple system 16 connected to thermocouples 18, positioned at core fuel assembly exits, along with cold leg temperature obtained from the reactor control system 12 allow determination of the enthalpy rise while neutron detector signals are provided by a reactor protection system 20 connected to incore detectors 22 or from an incore fixed detector system 24 connected to incore detector strings 26. The systems 10, 12, 14, 16, 20 and 24 are normally provided as software modules in the plant computer. The relationship of the model adjustment system 10 to other software modules is illustrated in FIG. 2. Monthly a full scope calibrator 40 obtains the equilibrium power, axial iodine, xenon, promethium, samarium and long-term burn up distributions from the model file 42 along with an input boron concentration and iteratively, performs a conventional 1-D diffusion theory calculation of axial power shape with an equilibrium xenon distribution, compares the AO, AP, AQ, AR components of the calculated power distribution with the corresponding components of the average axial power distribution derived from a conventional equilibrium flux map and adjusts the A.sub.1 -A.sub.5 expansion coefficients of equation (2) until the calculated critical power shape of the model 42 closely matches the measured power shape in a manner as described in U.S. Pat. No. 4,711,753. Thereafter, a conventional front end data processor 44 obtains the plant instrumentation data previously discussed and supplies such to the adjuster module 10. This module or tracker model adjuster system 10 of the present invention substantially continuously, automatically and online adjusts the analytical core model 42 concurrently with an update of the model 42 by a model update system 46. With the substantially continuously adjusted analytical core model a conventional core response predictor 48 can predict the response of the nuclear reactor to contemplated changes entered by the user 50 by using the axial iodine, xenon, promethium, samarium, long term burn-up and transverse buckling distributions stored in the file as a starting point for a prediction in response to a user specified maneuver. In addition, the model can be used by a conventional graphics display generator 52 such as described in U.S. Pat. No. 4,642,213 to produce a display for the user 50 on a graphics monitor 54. An example of a possible sequence of execution of the steps necessary in the present invention to automatically, without user intervention, continuously update the analytical core model initiation parameters is illustrated in FIG. 3. At the beginning of a monthly update cycle 60 a flux map is conventionally obtained, boron concentration in the reactor cooling system is determined and the calibration as described in U.S. Pat. No. 4,711,753 is executed to calibrate 62 the model or data file to particularly adjust the A.sub.1 -A.sub.5 coefficients. It is also possible to obtain the A.sub.1 -A.sub.5 coefficients periodically from reactor design calculations as a much less desirable alternative. If the model has been calibrated or after a determination that the monthly calibration is not necessary is made the current state of the core is progressively read 64 as core operations proceed. This involves obtaining the current time, power level, rod positions, inlet temperature, pressure, detector readings and core exit thermocouple readings from the core instrumentation sensors. At this step the core model 42 is then read in from its storage location and the core model is updated 66 using a conventional depletion calculation using the time, power level, rod positions inlet temperature and pressure. Next the current analytical axial offset and axial pinch values are calculated using conventional neutronics equations such as the one dimensional diffusion theory algorithms, the one dimensional nodal algorithms or the one dimensional neutron transport algorithms, such as is found in the full scope calibrator 40 or core response predictor 48, by performing a conventional criticality search. Next the current actual axial offset and pinch values 70 are estimated using equations 3 and 4 using the detector readings, thermocouple reading, rod positions, inlet temperature and power level where enthalpy rise .DELTA.h is calculated as set forth in U.S. Pat. No. 4,774,050. The actual values determined using equations 3 and 4 are compared 72 to the analytical values. If the absolute value of the difference between the calculated analytical axial offset and measured actual axial offset is less than or equal to a predetermined value n, for example 0.5%, and the absolute value of the difference between calculated analytical axial pinch and measured actual axial pinch is less than or equal to a predetermined value m which could be the same 0.5%, the differences are acceptable and the model need not be adjusted. As a more accurate alternative the absolute values discussed above should be integrated over time prior to performing the comparison with the predetermined constants n and m, respectively, and this will provide a more accurate determination of the total drift or cumulative drift in the core from the previously established model initialization values. That is, the increase or decrease in the integrals of the axial offset and axial pinch deviations that have accumulated since the last readjustment of the A.sub.2 and A.sub.3 parameters should be tested against the preset limits. If the comparison indicates that the limits have not been exceeded, the system stores 74 the model or data file and waits 76 for a predetermined period of time, that is, waits until it is time for another periodic update cycle. As indicated by the dashed box 75, the system could also display the adjusted and updated model by providing the model to the display generator 52. If deviation is significant, that is, not acceptable, the values of the A.sub.2 and A.sub.3 coefficients in the buckling equations are adjusted 78. With these adjusted coefficient values the neutronic equations are again used to determine 80 new calculated analytical values for axial offset and axial pinch. The cycle of comparing 72, adjusting 78 and calculating 80 are cyclically executed until the A.sub.2 and A.sub.3 coefficients are compensated for the drift using a standard method, called over-compensation in control theory, to produce non-zero opposite sign deviation values. This type of compensation requires that the magnitude of the deviation be offset by a deviation in the opposite direction of somewhat less than the magnitude of the deviation. For example, if the deviation is calculated as 0.5% in the positive direction, the compensation criteria require that the compensated result deviate in the negative direction for a value of for example 0.25%. The reason for overcompensating is that the errors that accumulate in the iodine and xenon axial distributions are, in affect, time integrals of axial power distribution errors; and, hence, by over compensating the system is, in effect, burning out the iodine and xenon errors. The time constants of promethium and samarium are relatively large compared to those of iodine and xenon and so their errors are relatively insensitive to the variation in power distribution errors. When compensated the model is stored 74, a time out 76 occurs waiting for a new cycle to begin or a display 75 followed by the time out 76. In addition to insuring that predictions of core response to anticipated plant operating maneuvers start from realistic current initial conditions, several other benefits from the availability and use of the present invention are provided. Since the present invention is in operation continuously and on-line in a particular installation, it is possible to generate an on-going log of the values of the monitored operating parameters supplied to the system and of changes in the values of the A.sub.2 and the A.sub.3 expansion coefficients. Such a recording step could be provided after step 74. This log could then be analyzed off-line either manually or automatically to detect correlations between changes in operating conditions and adjustments in the A.sub.2 and A.sub.3 parameter values. Any correlations identified would point directly to deficiencies in the conventional computational neutronics algorithms. Such deficiencies could then be corrected by an analyst with the result that the analytical algorithms would become progressively more closely attuned to reality. A calculated value of the critical reactor coolant system boron concentration is available as a by-product of the criticality search because boron concentration is recalculated as the update and adjustment proceeds and this allows the A.sub.1 coefficient to also be adjusted when reliable values of reactor core coolant system boron concentration are obtained. Since the core model is routinely adjusted to match the calculated boron concentration value to the measured value secured during monthly flux mapping operations and since the model can be refined to match the calculated boron concentration value to a measured value whenever suitable measured values are obtained, a reliable, frequently updated estimate of current boron concentration can be displayed for the operator's use. Further, systematic deviations of calculated boron concentration from measured values can point to analytical core model deficiencies. Since the analytical core model will have been checked against the plant just prior to any trip that may occur and since the present invention can update the model (although it can not verify it) when the core is in a subcritical condition, the present invention can be modified to do estimated critical condition and shutdown margin estimates with minimum interaction, except for output of results, with the user. The many features and advantages of this invention are apparent from the detailed specification and thus it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope thereof. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.