Method for monitoring boron dilution during a reactor outage

Abstract

A method for monitoring changes in the boron concentration in the coolant of a reactor during a nuclear plant outage that applies temperature compensation to the source range detector output. The method then monitors the compensated output signal to identify changes in the detector count rate above a preselected value.

Claims

1. A method for determining a change in boron concentrations in a reactor coolant system as a result of a reactivity change during a nuclear plant outage comprising the steps of: monitoring an output signal representative of a count rate of a source range neutron detector positioned outside of a reactor vessel within proximity of a core of a reactor, as a function of time, during a plant outage; monitoring a temperature of a coolant within the reactor coolant system as a function of time; generating a compensation signal which is a function of the monitored temperature, that when combined with the count rate output signal compensates the count rate output signal for substantially any change in the count rate output signal resulting from a change in the temperature of the coolant; applying the compensation signal to the count rate output signal to obtain a compensated count rate output signal; and identifying a preselected increase in the compensated count rate output signal as an indicia of a change in boron concentration.

2. The method of claim 1 wherein the compensation signal is defined as a Downcomer Temperature Attenuation Factor (DTAF) given by the expression:
DTAF(T.sub.1)=e.sup.−(p(T.sup.i.sup.)−p(T.sup.R.sup.))R; Where the value of R is a function of distance between the source range detector and the reactor vessel and the effective macroscopic neutron removal cross section between the source range detector and the fuel assemblies on the core periphery.

3. The method of claim 2 wherein a deviation in the compensated count rate output signal (ΔC.sub.c(t)) from a selected reference ΔC (ΔC.sub.R) is an indication that a reactivity change is happening or has happened and the value of ΔC.sub.c(t) is given by the expression:
ΔC.sub.c(t)DTAF(t)−C.sub.R.

4. The method of claim 3 wherein the application of error propagation techniques to determine the expected random fluctuations in ΔC.sub.c from one monitored count rate output signal set to the next allows the expected range of random fluctuation in ΔC.sub.c (ΔCE) to be expressed substantially as: $\Delta C_E\left(t\right)\in 0\pm 2{\sigma }_{\mathrm{CR}}\left[1+\right]\frac{1}{\mathrm{DTAF}\left(t\right)};$ Where the value of σ.sub.CR is the measured mean deviation of significant population of source range count rate measurements obtained in an interval around time t; the value of C(t) is the mean value of the data used to determine σ.sub.CR ; and the number of count rate measurements used to determine σ.sub.CR is an operator addressable constant that is a function of the desired maximum value of σ.sub.CR needed to obtain a desired reactivity change detection sensitivity.

5. The method of claim 4 including the step of determining if ΔC(t) is outside an expected range of ΔC provided by the equation $\Delta C_E\left(t\right)\in 0\pm 2{\sigma }_{\mathrm{CR}}\left[1+\right]\frac{1}{\mathrm{DTAF}\left(t\right)};$ and identifying that a reactivity change is occurring or has occurred.

6. The method of claim 5 wherein a selected number of consecutive samples of ΔC(t) are determined with a given fraction of the samples being outside the expected range of ΔC before identifying that a reactivity change is occurring.

7. The method of claim 6 wherein the selected number of consecutive samples is approximately ten.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a schematic representation of a primary side of a nuclear power generating system; and

(3) FIG. 2 is a graphical plot of the count rate ratio versus reactor coolant system cold leg temperature for the expected count rate and the measured count rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(4) FIG. 1 illustrates the primary side of a nuclear electric power generating plant 10 in which a nuclear steam supply system 12 supplies steam for driving a turbine-generator (not shown) to produce electric power. The nuclear steam supply system 12 has a pressurized water reactor 14 which includes a reactor core 16 housed within a pressure vessel 18. Fission reactions within the reactor core 16 generate heat, which is absorbed by a reactor coolant, like water, which is passed through the core. The heated coolant is circulated through hot leg piping 20 to a steam generator 22. Reactor coolant is returned to the reactor 14 from the steam generator 22 by a reactor coolant pump 24 through the cold leg piping 26. Typically, a pressurized water reactor has at least two and often three or four steam generators 22 each supplied with heated coolant through a hot leg 20, forming with the cold leg 26 and the reactor coolant pump 24, a primary loop. Each primary loop supplies steam to the turbine-generator. Two such loops are shown in FIG. 1.

(5) Coolant returned to the reactor 14 flows downward through an annular downcomer and then upward through the core 16. The reactivity of the core and therefore the power output of the reactor 14 is controlled on a short term basis by control rods, which may be selectively inserted into the core. Long term reactivity is regulated through control of the concentration of a neutron moderator such as boron dissolved in the coolant. Regulation of the boron concentration effects reactivity uniformly throughout the core as the coolant circulates through the entire core. On the other hand, the control rods effect local reactivity and therefore, result in an asymmetry of the axial and radial power distribution within the core 16. Conditions within the core 16 are monitored by several different sensor systems. These include an excore detector system 28 which measures neutron flux escaping from the reactor 14. The excore detectors 28 includes source range detectors used when the reactor is shut down, intermediate range detectors used during startup and shutdown and power range detectors used when the reactor is above approximately five percent power. In-core detectors are also typically employed during power operation.

(6) It is known to those skilled in the art that changes in the source range detector count rate (ΔC) can be identified by corresponding changes in the core K.sub.eff. In an ideal case the change in reactivity between a reference K.sub.eff (K.sub.R) and another condition value of K.sub.eff (K.sub.1) and time t is typically expressed as:

(7) $\begin{array}{cc}{C}_1\left(t\right)-C_R=\Delta C\left(t\right)=C_0\left[\frac{K_1\left(t\right)-K_R}{1-K_1\left(t\right)}\right]& \left(4\right)\end{array}$
It has also been shown that changes in reactor coolant system temperature produces changes in the measured source range detector response due to the change in density (p) of the water inside the reactor vessel. The measured source range count rate at any time and corresponding reactor coolant system temperature (T.sub.1) may be corrected to account for changes from a reference temperature (T.sub.R) by applying a correction factor defined as the downcomer, temperature attenuation factor (DTAF) given by the expression:
DTAF(T.sub.1)=e.sup.−(p(T.sup.i.sup.)−p(T.sup.R.sup.))R.  (1)
The monitored reactor coolant system temperature may be taken from the cold leg, the hot leg or an average of the two. The value of R is a function of distance between the source range detector and the reactor vessel and the effective macroscopic neutron removal cross section between the source range detector and the fuel assemblies on the core periphery. R is determined either empirically from count rate measurements taken at different temperatures while holding core reactivity constant, or analytically using standard neutron transport methods. If the temperature inside the reactor vessel is changed with no corresponding significant change in core reactivity, the application of the DTAF to the measured count rate during the temperature change will serve to keep the corrected ΔC(t) essentially constant. The correction ensures that the reactor operators do not confuse a change in source range count rate caused by the reactor coolant system temperature change with a count rate change caused by reactivity changes such as those that would be seen if the reactor coolant system boron concentration was being changed.

(8) Deviation of the corrected measured value of ΔC(t) (ΔC.sub.C(t)) from a selected reference ΔC(ΔC.sub.R) is now an indication that a reactivity change is happening or has happened. This unexpected change in count rate is shown graphically as a function of reactor coolant system temperature in FIG. 2. The value of ΔC.sub.C(t) is given by the expression:
ΔC.sub.c(t)DTAF(t)−C.sub.R.  (2)
The process measurement and inherent random nature of measured source range detector signals will induce fluctuations in the measured value of ΔC at each time step, i.e., sampling These fluctuations will make the use of ΔC(t) for detecting small reactivity changes problematic. Application of error propagation techniques to determine the expected random fluctuations in ΔC.sub.C from one count rate measurement set to the next allows the expected range of random fluctuation in ΔC.sub.C(ΔC.sub.E) to be determined at a 95% confidence level by the expression:

(9) $\begin{array}{cc}\Delta C_E\left(t\right)\in 0\pm 2{\sigma }_{\mathrm{CR}}\left[1+\right]\frac{1}{\mathrm{DTAF}\left(t\right)}.& \left(3\right)\end{array}$
The value of σ.sub.RC is the measured mean deviation of a significant population of source range count rate measures obtained in an interval around time t. The value of C(t) is the mean value of the data used to determine σ.sub.CR. The number of count rate measurements used to determine σ.sub.CR is an operator addressable constant that is a function of the desired maximum value of σ.sub.CR needed to obtain a desired reactivity change detection sensitivity. If the measured ΔC.sub.C(t) is outside the expective range of ΔC provided by equation three, the operator can conclude that a reactivity change is occurring. In order to avoid false positive or negative indications, the use of a requirement for a number of consecutive cycles outside or inside the expected range is required before the status is set for display to the operator.

(10) The steps of the preferred embodiment of the methodology of the invention claimed hereafter is as follows:

(11) (a) obtain a set of source range detector count rate measurements;

(12) (b) compute the mean value of the set;

(13) (c) compute the mean deviation of the data set;

(14) (d) repeat steps (a), (b) & (c) until a target mean deviation value is obtained;

(15) (e) input the mean deviation value from step (d) as a reference value (C.sub.R), into an alarm system for identifying unacceptable changes in the boron concentrations;

(16) (f) obtain a new data set of source range detector measurements until the mean deviation of the new data set is no larger than the mean value obtained at step (d);

(17) (g) compute the mean value of the data set used to complete step (f);

(18) (h) compute the value of DTAF to be applied to the mean value from step (g) using the mean temperatures corresponding to the reference count rates and the count rates used to calculate the mean value from step (g);

(19) (i) multiply the DTAF from step (h) times the mean count rate from step (g);

(20) (j) subtract C.sub.R from the value obtained at step (i);

(21) (k) determine whether the difference calculated in step (j) is outside an expected deviation range provided in equation three;

(22) (l) if the difference from step (j) is inside the expected range, repeat steps (f) through (k) approximately ten or more times (if the difference is outside the expected range proceed to step 13);

(23) (m) if at least nine of the measured difference values are within the expected range, set the reactivity change status output to a no alarm status;

(24) (n) repeat steps (f) through (l) approximately ten or more times;

(25) (o) if at least nine of the values obtained from step (13) are outside the expected range, set the reactivity change status output to “yes;” and

(26) (p) repeat steps (f) through (o) until the source range detectors are de-energized.

(27) A new value of C.sub.R will be obtained and inputted into the system following the completion of all plant outage reactivity changes. It should be appreciated that the number of additional data collected and analyzed that is specified in steps (l) and (n) is a user adjustable input. Similarly, the number of measured difference values that have to be within or outside the expected range to set the reactivity status of the system set forth in steps (m) and (o) will depend upon the desired accuracy of the result and is user adjustable input.

(28) 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 embodiments 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 appended claims and any and all equivalents thereof.