Patent Number: 048775754
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is well known that the concept of reactivity is most readily applicable in a nuclear reactor core under the condition that the fractional rate of change of neutron population is identically the same in all regions of the core or, equivalently, that the shape of the neutron flux distribution is static. The present invention instead of accepting a single signal believed to be representative of neutron flux level in the core and passing the information in that signal through a single "solver" of the point kinetics equations, accepts two or more such input signals and processes the information carried by the respective signals through identical, but independent, "solvers" of the point kinetics equations. The output reactivity values generated by the several "solvers" are then compared and when all of the independent reactivity values agree within a preset tolerance, the average of all the independent values is accepted as a valid measure of the instantaneous core reactivity. A minimum acceptable set of input signals to be processed by the reactivity computer of the present invention consists of the signals generated by the top section and the bottom section of a single long ion chamber, provided that only symmetric perturbations in the radial neutron flux distribution occur as the measurement progresses. Signals generated by one or more movable incore detectors or by suitable fixed incore detectors temporarily or permanently positioned in appropriate locations within the reactor core would be highly desirable supplements to this minimum acceptable set of input signals. In the event that non-symmetric perturbations to the core neutron flux distribution are involved in the measurements being performed such as would result from a single control rod withdrawal or insertion or an N-1 worth test, signals from all available long ion chamber sections would constitute a minimum acceptable input signal set. The present invention also requires that boron dilution between rod movements be done at a constant rate for maintaining a very nearly just critical configuration in the reactor at zero power level. The present invention also accepts the instantaneous reactivity values determined by the methodology mentioned above to be valid and continuously, within the unavoidable discrete time step character of digital processing, generates a linear fit in time to the current set of acceptable instantaneous reactivity values and subjects the fit so generated to a statistical test of acceptability. When enough valid data have been collected and processed, as indicated by acceptance on statistical grounds of the fit of the reactivity value variation versus time, a set of values of current time, reactivity and rate of change of reactivity, i.e., slope of the fit, is recorded. Using this set of values, the fit of reactivity versus time is extrapolated back in time t the time of the most recent earlier set of values and the change in reactivity associated with the most recent control bank movement is evaluated and reported to the user. When a sufficient number of acceptable reactivity values have been produced or when an acceptable fit to the current reactivity variation has been obtained, a signal, which may be in the form of an activated indicator light, can be transmitted to the reactor operator to inform him that data collection and processing at the current rod position has been completed and that he may adjust control bank position and continue on with the measurement process. A simplified block diagram of the system of the present invention is illustrated in FIG. 2. At least two neutron detectors 20 detect the neutron flux produced in the reactor core and produce electrical current signals proportional to the flux detected. The current signals are processed by a conventional signal conditioning unit 22 and supplied to a reactivity computer 24 as analog values. The reactivity computer 24 can include the same conventional digital reactivity computer that determines reactivity from a single channel neutron detector. A suitable pair of single channel digital reactivity computer that can be connected to a personal computer, such as an IBM PC-AT, to form the reactivity computer 24 for a two neutron detector system can be obtained from Westinghouse. The reactivity computer 24 determines the reactivity in the core for different regions by averaging typically ten samples for each detector taken over a period of approximately 10 milliseconds, solves the known point kinetics equations, and displays these instantaneous reactivities on a strip chart recorder type display 26. When the reactivities produced by the plural neutron detectors remain coincident for a long enough time to satisfy a statistical test of "goodness" such as The Standard Error of Estimate Test, the reactivity computer 24 signals an operator via display 28 that the currently measured reactivity is valid or correct, thereby allowing the operator to move the control rods by the next step increment. After rod movement, the reactivity computer 24 determines the control rod worth based on the coincident reactivity values and provides the operator with a differential control rod worth for the previous control rod movement. FIG. 3 illustrates a typical reactivity trace produced by upper and lower neutron detectors provided exterior to the nuclear reactor core. The substantially vertical line 40 indicates the reactivity change that occurs during control rod movement from, for example, the 180 step position to the 190 step position. Once the control rods have been moved the reactivity trace 42 produced by the lower neutron detector. diverges from the trace 44 produced by the upper detector. During the period of reactivity change between control rod movements the reactivity is increasing because of the reduction of neutron absorbers (boron) in the coolant. During a period of constant dilution the traces of the detectors should be straight, and before the operator is allowed to move the control rods a second time, the rate of reactivity change must stabilize into substantially a straight line. The portions 46 in FIG. 3 indicate diverging reactivities measured by the upper and lower neutron detectors after the reactivity traces have crossed. The regions 46, where the reactivity traces 42 and 44 are not coincident and do not provide a substantially straight trace, are inappropriate for determining core reactivity and thus control rod worth. The regions 48 identified in FIG. 3 are regions where core reactivity is valid because the traces are coincident and straight. This general area of FIG. 3 is illustrated substantially enlarged in FIG. 4. The present invention compares the instantaneous reactivities of the lower 42 and upper 44 detectors looking for the first point of coincidence 50 that can be identified. Coincidence occurs when the reactivity values are within 0.5% of the largest expected step reactivity change during measurements. After coincidence begins, the present invention requires that coincidence be maintained for a time period T sufficient to allow a statistical fit to be made. When coincidence has continued for the time period T, the operator can be signaled indicating that control rod movement can be performed. A least squares fit to the coincidence reactivity 52 values occurring during the time period T is performed to produce a straight line 54 and the corresponding slope intercept form equation for the straight line 54. The computer then determines the intersection 56 of the straight line 54 and vertical line 58. The vertical line 58 passes through the point where reactivity equals zero during control rod movement. Control rod worth is then determined by the reactivity difference between point 56 and a point 60 where the previous fitted straight line intersects the vertical line 58. The control rod worth for the control rod movement associated with vertical line 58 is then provided to the operator on display 28. The process described above is illustrated by the flowchart of FIG. 5. A suitable language for the corresponding digital computer program is PASCAL. The process starts 70 by setting 72 and 74. a first fit flag to off and an operator signal flag to on. The first fit flag is set when a new measurement cycle is started. The on operator flag tells the operator the control rods can be moved. A control rod movement by the operator causes the reactivity and the flux distribution in the core to change abruptly. After setting the flag, the reactivity computer 24 takes 10 samples 76 of the flux values produced by each neutron detector along with other parameters necessary to determine reactivity such as time and control rod position. If the control rods were not moved in the last time step 78, the computer 24 averages the values, calculates 80 the reactivities associated with each flux detector, stores the calculated reactivity values and outputs the values to the strip chart recorder. If the control rods were moved in the last time step, the first fit flag is set 82 and the operator signal flag is turned 84 off. A commonly used reference for calculating reactivity,,as in step 80, is A. F. Henry's paper. "The Application of Reactor Kinetics to the Analysis of Experiments", Nuclear Science and Engineering: 3, 52-70 (1958). In this paper Henry derives the well known reactor kinetics equations which appear as equation 6 in the paper. Equation 6 requires that the conditions of equation 5 there,in be satisfied before the simplification of equation 6 will be valid. Equation 5 essentially requires that the reactor settle down to a steady rate of change state before reactivity can be measured. This settling down period is typically from 2 to 4 minutes after control rod movement. When the settling down has occurred, the coincidence between detector reactivity values begins. It is conventional to make measurements of core reactivity under conditions in which the source term Q of the Henry equations is a negligible contributor to the neutron balance and in a situation where the time derivative term dT(t)/dt very quickly becomes of negligible consequence after a perturbation in the core properties. With these two simplifications equation 6 of the Henry paper will produce: ##EQU1## and subsequently ##EQU2## It is of course impractical to measure the actual value of the amplitude function T(t) of a large nuclear reactor. Hence, the assumption is routinely made that the response signal from a suitably located neutron detector is proportional to the value of the amplitude function at any given time. Thus, EQU T(t)=.mu.DR(t) (6) where DR(t) is the magnitude of the detector response signal and the equations actually solved are ##EQU3## where EQU C.sub.i (t)=.LAMBDA..mu.C.sub.i (t) (9) In this final form the equation set is commonly referred to as the "point kinetics" equations. Continuous, online evaluation of the reactivity of a large nuclear reactor core and of changes in the reactivity resulting from externally produced changes in core properties can be accomplished readily by solving the set of simultaneous linear and differential equations discussed above. Once the reactivities are determined by the point kinetics equations, a determination is made 86 concerning whether the reactivities are equal, that is, the reactivities are considered equal when they are all within a preselected variance of each other. This is done for all reactivities produced, by comparing each reactivity with all other determined reactivities if more than two neutron detectors are used in the comparisons. If the reactivities are not equal 86, the process cycles back to sample the measurement parameters again. If the reactivities are within the preselected variance, the average reactivity is produced 88. The average is then used with the previously stored average reactivities to perform a least squares fit 90 versus time. If the rate of change is approximately zero 92 the process is stopped 94. If the rate of change is not approximately equal to zero, a statistical check 96 of the fit of the values using the Standard Error of Estimate Test is performed 96. If the fit is not acceptable 98, a new set of parameters is obtained and the process is repeated. If the fit is acceptable, the fit parameters, core conditions and time are stored 100. If the first fit flag is not on 102, the fit variables are initialized 104 and a new set of parameters are sampled. This allows the process to produce a new fit if the operator does not promptly move the control rods. If a new fit is obtained it is used for future differential worth calculations. If the fit flag is on, the intersections and differences are determined 106 in accordance with the procedures described with respect of FIG. 4. The differences are then used to compute 108 the differential rod worth which is output to the operator after which the operator signal flag is turned on 110. The first fit flag is also turned on 112 after which the process returns to obtain new sample parameters. Once the operator has the last control rod worth for the insertion steps, the operator compares these values to a chart produced by the core designer for the latest fuel loading. The comparison will show whether the design values are accurate. It is possible, though not desirable for safety reasons, to allow the reactivity computer 24 to initiate control rod movement when the period for coincidence has been detected, rather than wait for the operator to move the rods. This would further improve the efficiency of the control rod worth test. It is also possible to vary the time period required for coincidence before reactivity values are validated by setting tighter or less restrictive statistical criteria of "goodness". A further improvement in the present invention would be to calculate boron balance (or worth) and measure reactor temperature, and provide these values as outputs along with reactivity and control rod worth. The many features and advantages of the 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.