CA2456284A1 - Method and system for testing in-core flux detectors - Google Patents
Method and system for testing in-core flux detectors Download PDFInfo
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- CA2456284A1 CA2456284A1 CA002456284A CA2456284A CA2456284A1 CA 2456284 A1 CA2456284 A1 CA 2456284A1 CA 002456284 A CA002456284 A CA 002456284A CA 2456284 A CA2456284 A CA 2456284A CA 2456284 A1 CA2456284 A1 CA 2456284A1
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C17/00—Monitoring; Testing ; Maintaining
- G21C17/10—Structural combination of fuel element, control rod, reactor core, or moderator structure with sensitive instruments, e.g. for measuring radioactivity, strain
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C17/00—Monitoring; Testing ; Maintaining
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C17/00—Monitoring; Testing ; Maintaining
- G21C17/10—Structural combination of fuel element, control rod, reactor core, or moderator structure with sensitive instruments, e.g. for measuring radioactivity, strain
- G21C17/108—Measuring reactor flux
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Abstract
A system and method of testing in-core flux detectors that determines an effective prompt fraction for an in-core flux detector based upon the dynamic response of the detector to a power change event within the reactor. Ion chamber detectors may provide a reference signal against which the in-core flux detector signals can be compared. The in-core flux detector response is analyzed over a time interval following the power change event that may be shorter than the time constant for the fastest delayed component of the detector response. The data acquisition system may include multiple subsystems that are isolated from one another through isolation barriers. In particular, a clock signal from one subsystem to another may pass through an isolation barrier.
Description
METHOD AND SYSTEM FOR TESTING IN-CORE FLUX
DETECTORS
FIELD OF THE INVENTION
This invention relates to flux detectors for nuclear reactors and, in particular, to testing in-cure flux detectors.
BACKGROUND OF THE INVENTION
Permanently installed in-core flux detectors (ICFDs) are used in many reactor facilities to prevent local over-power in the reactor core. The ICFDs are part of one or more safety shutdown systems in a nuclear reactor. The ICFDs are designed to detect a sudden increase in flux caused by an accident. Accordingly, the ICFDs must quickly respond to flux increases within a specified time interval in order to meet safety requirements. ICFDs may also be called neutron-over-power (NOP) or regional over-power (ROP) detectors.
ICFDs are self-powered flux detectors that produce a measurable current in the,uA range. The ICFDs are sensitive to both neutron and gamma fields. Under steady state conditions, the generated current is proportional to the local thermal neutron flux. In the case of a transient in neutron flux, the detector's current signal may lag behind the flux change since the ICFD°s current generating processes include slow-responding components, causing signal delays. The dynamic transfer function of the ICFD's flux-to-current conversion includes a large prompt response and a series of smaller exponentially delayed components, with either positive or negative amplitudes, making the ICFD either under-prompt or over-prompt.
The dynamic response of an ICFD can change over time with changes (burn-up) of material and as a result of defects. in order to ensure that the ICFDs can respond quickly enough to meet safety requirements, it is necessary to ensure that the prompt response component is above the minimum allowable limit. Therefore, it is desirable to engage in the periodic testing of the prompt fractions of all safety related ICFDs.
A typical CAIVDU nuclear reactor employs a large number of ICFDs, distributed throughout various redundant safety system channels. For safety reasons, these channels must be kept isolated from each other, so that an anomaly on one channel does not affect the other channels, thereby compromising the whole safety infrastructure. The need to test all ICFDs under similar conditions without compromising the isolation requirements presents a significant challenge.
SUMMARY OF THE INVENTION
The present invention provides a system and method of testing in-core flux defectors that determines an effective prompt fraction for an in-core flux detector based upon the dynamic response of the detector to a power change event within the reactor. The response is analyzed over a time interval following the power change event that is shorter than the time constant for the fastest delayed component.
In one aspect, the present invention provides a method for testing an in-core flux detector (ICFD) in a nuclear reactor. The method includes steps of initiating a step change in the nuclear reactor power, receiving response signals from measurement instruments, including the ICFD, digitizing and recording the response signals, and determining an effective prompt fraction for the ICFD based upon the recorded response signals.
In another aspect, the present invention provides system for testing an in-core flux detector (ICFD) in a nuclear reactor, the nuclear reactor having a mechanism for triggering a step change in the nuclear reactor power. The system includes a data acquisition system for receiving and recording response signals from measurement instruments, including the ICFD, and an analysis module for determining an effective prompt traction for the ICFD
based upon the recorded response signals.
In a further aspect, the present invention provides a computer program product having a computer readable medium tangibly embodying computer executable instructions for testing an in-core flux detector (ICFD) in a nuclear reactor, the nuclear reactor including measurement instruments, including the ICFD, the measurement instruments producing response signals as a result of a step change in the nuclear reactor power. The computer executable instructions include computer executable instructions for receiving and recording the response signals from the measurement instruments, including the ICFD; and computer executable instructions for determining an effective prompt fraction for the ICFD based upon the recorded response signals.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference will now be made, by way of example, to the accompanying drawings which show an embodiment of the present invention, and in which:
Figure 1 shows a block diagram of a data acquisition system for testing in-core flux detectors;
Figure 2 shows a block diagram of a data acquisition subsystem within the data acquisition system from Figure 1; and Figure 3 shows, in flowchart form, a method of testing an in-core flux d etector.
Similar numerals are used in different figures to denote similar components.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The present invention provides a method of determining the effective prompt fraction for an ICFD based upon the measured response of the ICFD
' q. ' to a step change in reactor power.
The prompt fraction for an ICFD is a ratio of the ICFD response to a reactor flux change divided by a reference instantaneous response to the same reactor flux change. The actual response signal produced by an ICFD
in situ contains certain delayed components. Some of these delayed components are attributable to the detector and some are attributable to delayed gamma fields within the reactor. Measurements of the signals generated by an ICFD in response to a step change in reactor power will reflect a delayed response dynamic that is attributable to both the detector characteristics and to external characteristics including delayed gamma fields and physical considerations such as the location of the detector. All of these sources of delay impact the effective prompt fraction determined based upon the measured response.
The reactor power change used to test the promptness of ICFDs is intended to model an accident situation in which 'there is a sudden increase in the local neutron flux. Ideally, the test would involve the introduction of a positive step change in reactor power; however, this is rarely possible for safety reasons. Accordingly, in most embodiments, the reactor power change is a step down in reactor power. In some embodiments, the power change includes a reactor power trip. In many reactors, these power outages are regularly planned and scheduled to occur every two years for maintenance and testing.
The use of a decrease in power instead of an increase in power to test the ICFD promptness necessitates an assumption that the dynamics of the detector response to a step change in reactor power - that is, the resulting change in neutron and gamma field flux - are approximately linear and symmetric in both directions. Gertain delayed components of the neutron or gamma field, such as the delayed gamma field resulting from built-up fission products, may have dynamics that depend upon whether the change in reactor power is positive or negative. To rely upon the symmetry of detector signal promptness a further assumption is made that the fission products build up with the same dynamics after a step-up change in the thermal neutron flux as they decay after a reactor trip.
Byway of example, the Platinum-clad Inconel ICFD has a mixed neutron and gamma sensitivity. Forty percent of its current is induced by the ~ reactor's gamma field interacting with the detector's material via the prompt process of ('y, a ); however, one third of the gamma field is delayed with respect to a change in the neutron flux, resulting in a thirteen percent ~-related delayed component in the detector's current. Although the interaction that takes place in the detector, ('y, a ), is prompt, the delay is still attributed to the detector. The source of the delayed gamma field is the fission products that build up after a long full-power operation. When the reactor is tripped, the neutron and gamma fields go down to zero immediately; however, the slow decay of fission products stored in the fuel and other core components still produce a significant gamma field. The cascade of various beta and gamma decays of fission products have certain dynamics, which partly determines the delayed current of the ICFD signal responding to a reactor trip.
The inclusion of delayed components not attributable to the detector in an effective prompt fraction estimate is justifiable in the sense that in a real accident scenario those non-detector delays will still be applicable to the effective prompt fraction for the detector signal. In other words, the promptness of the detector signal - as opposed to the detector itself - is still influenced by the location of the detector within the core, the length of its lead cable, and the power history of the reactor core.
The effective prompt fraction of an ICFD signal comprises the ratio between the immediate change in the iCFD signal following a reactor change and that of a one-hundred percent prompt reference detector signal measured during the same reactor change. In one embodiment, the reactor change is a reactor trip and the one-hundred percent prompt reference detector signal is obtained from one or more ex-core ion chambers. The ion chambers are responsive to the prompt changes in the neutron flux and are -not affected by delayed gamma flux. In one embodiment, the effective prompt fraction is calculated as the post-trip drop of a normalized 1CFD
signal averaged over a time interval after the trip divided by the average signal drop of the normalized reference ion chamber signal.
After initiation of a trip, there is a short delay of approximately 0.4 seconds before the flux signals start to respond. This initial dead-time is partly attributable to the delay in the trip mechanism reaching the relevant area of the core. For example, where the trip mechanism involves dropping shutoff rods to stop the reactions in the core, there is a finite time for the rods to drop into the calandria and to begin to affect the reactions. Another type of trip mechanism is the injection of liquid poison (Gadolinium) to stop the reactions in the core. Again, it takes a finite amount of time for the poison to be injected into the calandria and to affect the reactions in the local region of the detector. The extent of this delay depends partly on the location of the detector within the core.
Once the flux signals begin to respond, then the neutron flux signals drop to a low level over a time interval of approximately 0.3 to 0.8 seconds.
The time constant of the fastest delayed component is about 3.9 seconds, meaning that there is sufficient separation to estimate the prompt fraction in accordance with the present invention.
In one embodiment, the effective prompt fraction of an lCFD is calculated from the normalized drop of the ICFD signal measured three seconds after the start of the flux drop, relative to the normalized drop of the reference ion chamber measured at three seconds. The signals are normalized between their pre-trip values and the nominal zero-power voltage output of their amplifiers {"zero floor"). These nominal floor values vary from station to station. The actual "zero floor" values may differ from design values due to instrument zero-offset bias of NOP amplifiers (station hardware) and recording channels (data acquisition systems).
The determination of an effective prompt fraction in accordance with the present invention may include two steps. The first is to determine an -uncompensated effective prompt fraction from the measurement data obtained during a reactor trip. The second is to apply correction terms to the uncompensated effective prompt fraction to obtain the effective prompt fraction. In some embodiments, these two steps are not performed in distinctive and consecutive fashion.
The correction terms applied to the measurement-based uncompensated effective prompt fraction comprise modelling-based corrections that typically alter the uncompensated effective prompt fraction by a few percent. The relatively modest impact of the correction terms allows for the use of nominal ICFD parameters in place of actual specific ICFD
parameters and for the use of a simplified time function for flux change.
The correction terms may be used to remove the effects of pre-trip power changes, such as power ramps wherein the reactor power may have been steadily ramped down from full power to sixty percent before the full reactor trip was initiated. They may also be used to remove the effects of the fastest delayed components of the detector current already active and contributing to the ICFD trip response at three seconds, the effects of the zero-offset bias of the data acquisition recording channels, and the effects of the post-trip long-term delayed gamma field emitted by the fission products in delayed beta-decays.
A. Data Acquisition System As noted above, a typical nuclear reactor employs a large number of ICFDs, distributed throughout various redundant safety system channels.
The safety channels each monitor a set of ICFDs and other instrumentation, including ion chambers. The instrumentation is distributed throughout the reactor area, both within the core and external to the core. In order to meet safety requirements, the safety channels need to be kept isolated from one another. If the safety channels were to be interconnected, there is a risk that a fault in one channel could affect other channels, thereby compromising the whole safety system.
Reference is now made to Figure 1, which shows a block diagram of a data acquisition system 10 for testing in-core flux detectors. The data acquisition system 10 is distributed across six safety system channels shown as 12a, 12b, ..., 12f. The six channels correspond to two safety systems having three channels each. The data acquisition system 10 comprises a number of subsystems shown as 14a, 14b, ..., 14f, corresponding to the six safety system channels 12a, 12b, ..., 12f. Each data acquisition subsystem 14 is coupled to an instrument panel 16 that receives signals from a number of instruments; in one embodiment, up to twenty-four instruments. The data acquisition subsystem 14 includes an isolation amplifier 18 for receiving instrumentation signals 30 from the instrument panel 16 in parallel. The isolation amplifier 18 prevents any feedback from the data acquisition system 10 to the instrument panel 16 and outputs isolated instrumentation signals 32.
In one embodiment, the isolation amplifier 18 is a 1:1 amplifier with no filtering.
The data acquisition subsystem 14 also includes a signal conditioning unit 22. The signal conditioning unit 22 receives the isolated instrumentation signals 32 and outputs conditioned signals 34. The signal conditioning unit 22 may perform a number of tasks, including analog amplification, DC offset adjustment, and analog low-pass filtering.
The data acquisition subsystem 14 further includes a computer 20 for receiving the conditioned signals 34, digitizing them and storing them locally.
The data recorded by the computers 20 (shown as 20a, 20b, ..., 20f) during a trip event is later used to determine the effective prompt fraction for one or more ICFDs that are included in the reactor instrumentation. In order to perform this analysis the data for a subject ICFD may be compared with data corresponding to ion chambers, other ICFDs, or other instrumentation. To make a useful comparison, the data needs to have a common reference time.
The computers 20 in the data acquisition system 1U typically each have their own clock, since they operate independently of the other data acquisition subsystems 14. Accordingly a difficulty that arises us in synchronizing the computers 20 without compromising the safety system isolation requirements.
_g_ To address this difficulty, one of the computers 20, such as computer 20a, is designated as a "master" computer and the remainder of the computers 20b, 20c, ..., 20f, as '°slaves", although they are not in a master-slave relationship normally seen in computing wherein the master has some level of control over the slaves. The master computer 20a outputs a clock signal 36 that is fed to the other computers 20b, 20c, ..., 20f. The clock signal 36 passes through an isolation barrier 24 between each computer 20 so as to ensure the computers 20 remain isolated from one another. Each of the slave computers 20b, 20c, ..., 20f receives this clock signal 36 and may timestamp any data recordings or otherwise link the data recordings of the conditioned signals 34 to the reference clock signal 36 from the master computer 20a. Accordingly, the locally stored data at each computer 20 may later be collected and combined for analysis using a common time reference.
In one embodiment, the isolation barrier 24 is an optical isolator. The isolation barrier 24 may include other mechanisms for isolating signals, including a galvanic isolator and others. The isolation barrier 24 should be selected to provide an adequate signal-to-noise ratio and temperature sensitivity. The isolation barrier 24 may provide for double isolation.
In one embodiment, the clock signal 36 includes a clock signal and a time verification signal. The time verification signal may be recorded for later analysis and correction of any errors in the clock signal. In one embodiment, the time verification signal is a triangle wave signal.
Existing CANDU nuclear reactors include a communications system for communicating between rooms within the reactor through headsets. In some embodiments, this existing communication system may be employed to provide the interconnections of components for the data acquisition system 10 shown in Figure 1. In particular, the existing communications system together with the isolation barriers 24 may provide a mechanism for coupling the various data acquisition subsystems 14 to distribute the clock signal 36.
Reference is now made to Figure 2, which shows a block diagram of one of the data acquisition subsystems 14. The signal conditioning unit 22 includes a filter 40, an amplifier 42 and a DC-offset adjuster 44 for each isolated instrumentation signal 32. The filter 40 may include an anti-aliasing low pass filter. The computer 20 includes an analog-to-digital (AID) converter 46 for digitizing the conditioned signals 34. The digitised signals created by the AID
converter 46 are provided to a microprocessor 48, which stores the digital data in a memory 54. The microprocessor 48 co-ordinates and manages the data acquisition subsystem 14, including the sending of control signals 56 to the signal conditioning unit 22 to control the filters 40, amplifiers 42, and DC-offset adjusters 44.
The computer 20 further includes a clock 50, which in some embodiments is a part of the microprocessor 48, and which generates the clock signal 36.
The computer 20 may also include a display 52 and other output and input devices (not shown) to enable a user to interact with the computer 20.
It will be understood by those of ordinary skill in the art that the computer may be a personal computer or other workstation. The computer 20 may also be a microcontroller or other programmable processing device for use in managing a data acquisition process.
In one embodiment, the data acquisition subsystem 14 receives, 20 digitizes, and records data with respect to twenty-four instruments. The data collected has sufficient resolution to provide accurate information regarding the promptness of the ICFDs. In one embodiment, the data acquisition subsystem 14 samples the instrument signals - that is, the AID converter 46 samples the conditioned signals 34 - at 500 Hertz with an amplitude resolution of 0.1 mV.
In order to provide sufficient resolution, the sampling should be at a rate greater than one sample every 10 milliseconds.
To determine the promptness of the instrumentation under test it is necessary to know when the power change - that is, the reactor trip - actually occurred in the recorded data. Accordingly, one of the instruments measured by the data acquisition system 10 is the trip mechanism. The trip mechanism may be a pushbutton or other trigger for initiating the trip event. The pushbutton may cause the release of the neutron-absorbing rods or the injection of the neutron-absorbing poison to precipitate the reactor shutdown event. Therefore, one of the sets of recorded data will indicate the precise occurrence of the trip initiation. It then takes a finite amount of time for the trip mechanism to deploy and begin affecting the neutron reactions within the core and to propagate as far as the individual detectors.
The computers 20 are suitably programmed to control the various hardware components and perform the data sampling and storage functions.
The programming of the computers 20 to perform these functions will be within the understanding of a person of ordinary skill in the art of computer software programming. The software developed for controlling the operation of the computers 20 provides a user interface to allow for a user to set certain parameters, including the sampling rate.
B. Determination of Effective Prompt Fraction The effective prompt fraction that may be determined from the measurements obtained using the test system described above is related to the actual prompt fraction. Using mathematical models of the ICFD dynamics and the trip event, the relationship between the actual prompt fraction and the effective prompt fraction may be determined.
The ICFD produces an output voltage signal Vo(f), which is related to the convolution of the time dependent flux ~(t) and the detector impulse response function h(t). The latter comprises the actual prompt fraction p and the N number of delayed components with time constants 7" and relative amplitude k", respectively, where n = 1,..., N. This relationship is shown in the following expression:
h(t) = p8(t) + ~ k" exp -t (1 ) n=i Zn Zn N
where 8(t) comprises the Dirac-delta function, and wherein p+~kn = 1.
n=i The detector voltage may be modeled as a function of the detector's flux-to-voltage conversion factor Co, which itself includes the flux-to-current conversion factor C"~, the current-to-voltage conversion factor C"", and the voltage amplifier gain factor Gyrv. The first parameter is the ICFD's sensitivity or current yield, which may change over the life of the ICFD. The latter two parameters are set in the converterlamplifier of the signal instrumentation and may be adjusted to compensate for changes in the detector sensitivity. Using the conversion factor Co, the detector voltage may be modeled using the expression:
t VD (t) = CD ,~h(t -t') x d~(t')dt'+VB (2) where VB is a constant voltage offset, measured as zero power. The actual zero-power offset voltage, VB, may vary from signal to signal due to possible bias in station instrumentation and data acquisition recording channels. The differences between the zero-power offset of different signals are typically in the range of X10 mV or 0.2% of the monitored voltage range. This uncertainty may result in a X0.5% bias in the estimated prompt fraction.
Equations (1 ) and (2) may be combined to arrive at the following expression of detector output voltage Vo as a function of time-dependent flux:
vD (t) = CD P~(t) + ~ kn j~~(t') exp(- t t~)dt~ + VB
n=t zn -w Zn The parameter values k" and T" of the N number of delayed components are not necessarily related to actual physical processes, but rather are the best exponential fit to the measured detector response. One can obtain design or nominal values for the parameters k" and T" by measuring detector response signals and fitting exponential curves to the measured signals. The delayed parameters are related to the prompt fraction by way of the following normalization equation:
N
~7+~~Cn =1 n=t Having obtained a modeling equation, Equation (3), for the detector voltage VD, the reactor trip may be modeled as a linear flux change from an initial static flux ~o at time t = 0, to a later constant flux ~~ after a time interval fit. As a function of time t, the flux may be expressed as:
t<0 Wi(t)=~°-~° ~' xt 0<t<~t (5) ~t Ot < t For the purposes of assessing the delayed current component of a nominal IGFD, the above linear flux change equation is a good approximation of the real flux drop during a reactor trip. The advantage of this simplified function is that it gives an analytical formula for the trip response of the ICFD
voltage signal, which is needed to perform a parameter sensitivity study. If the linear ramp-down expression in Equation (5) is inserted into Equation (3), for times t > fit, and using the pre-trip equation Vo (0) = Goo + Ve, the prompt fraction p may be expressed as a function of the measured detector signal Vo, as follows:
_, exp ~t -1 N
p- 1_ Vn(t)-VB 1_ ~I -~k 1-exp - ~ z" (6) VD (0) - VB d~ ° ~=, n zn 0t ~n where VD (t) VB represents the relative drop in detector voltage after the Vn (0) _ VB
reactor trip, while ~' represents the relative flux drop in the location of the core detector. The first term in Equation (6) defines the "°effective prompt fraction", which can be directly calculated from the measured time series signals using the following expression:
_, Pes(t)= 1-~° ~)-~B 1-~1 (7) D( ) B 0 Accordingly, Equation (6) may be rewritten as:
exp ~t _ 1 p - p~ ltl - ~ kn 1 - eXp - t . Zn n=1 Zn ~t Zn Because the detector voltage signal Volt) is sampled continuously and frequently during the reactor trip test, the time f may be chosen such that ~f <
t < T" so as to minimize the effect of the second half of Equation (8). If this is done, then Equation (8) may be approximated as follows:
exp ~t -1 kn 1- exp - t ~" = ~ kn 1- exp - t ~ (9) n°t Zn ~t n=1 Zn Zn If the time instant t is chosen to be close to ~t relative to the shortest time constant, min{ T"} then the right hand side expression is close to zero.
In this case, the prompt fraction p is approximately equal to the effective prompt fraction Pe~(t).
Another approximation that may be used to simplify the modeling is that the relative flux drop ~' from Equation (7) may be approximated by the relative signal drop of the ion chamber signal. In reality the relative flux drop may depend upon the detector location, since the flux shape after the trip is different from the pre-trip static flux shape. If a X15% variation in the pre-trip static flux at the detector locations and a uniform flux shape after the trip are assumed, then the application of the relative flux drop assumption introduces a variation of X1.5% in the effective prompt fraction.
Accordingly, the effective prompt fraction of an ICFD within the reactor may be estimated using the measured time recorded signal data from the instrumentation, using a time t satisfying the condition flt < t < min{T"} and the following expression:
_ yD (t) - YB vr~ (t) - v8, -pe~(t) 1- ~'D(o)-vB 1- v~~(o)-vB~ (10) where y'c(t)-~B' is the relative signal drop of the ion chamber signal.
vl~ (o) -YB.
In another embodiment, the post-trip signals in Equation (10) are averaged over a time interval (t~, t2) satisfying the condition ~t < t, < t2 <
min{T"}. In one embodiment, t~ is 2.5 seconds and t2 is 3.5 seconds.
In yet another embodiment, the reactor trip may occur following a power change; typically a power ramp-down from full power to a fraction of full power, such as 60%. After the power-ramp down, the reactor is kept at steady state power for hours before the trip is initiated.
In this case, the power reduction may be a linear flux reduction from ~o to ~~ over a time interval DT starting at time tR. The power is then kept constant until time tr when the power trip occurs. During the power trip, the flux decreases from ~~ to ~2 over a time period At. The two ramps may be expressed as follows:
~(t) _ ~, + ~ Q ~1 x (tR + DT - t) if tR < t < tR +AT (11 ) ~(t) _ ~Z + ~Z ~z x (tT + Ot -t) if tT < t < tT +At (12) 0t By way of example, ~T may be 4 to 10 minutes. The power hold between the power ramp-down and the trip event is typically between 30 minutes and 8 hours, although it may be longer or shorter.
Using the same definition for an effective prompt fraction that lead to Equation (7), we may define the effective prompt fraction for this case as follows:
_, peg (t) _ ~D (tT ) YD (t) ~1 ~2 13 vD(tT) ~B ~l The detector voltage from Equation (3) may be combined with the expressions for flux to arrive at an expression for the detector voltage for times t > fT + ~t that includes two exponential signal transients corresponding to the two linear power ramps:
exp ~T _ 1 _ N t-t z vD(t)-CD~2 +~'~'D(~0 ~1)~kn exp __ R n n=, Zn OT
Zn ~t exp - -1 N t-tN Zn +CD (~, +~2 )~kn exp __ (14) n = 1 Z n Ot Zn For times t > tT + dt, the detector signal in Equation (14) shows the superimposing effects of the two independent power reductions. It will be understood that in a further embodiment having multiple power ramps the above equation may be modified to include additional terms.
The pre-trip detector signal is given by the expression:
N exp 0T _ 1 vD(tr)=CD~1 +CD(~° -~1)~kn exp -tT tR Zn (15) n=1 Zn OT
Zn From Equations (14), (15), and (13) we may obtain an expression for the effective prompt fraction for times t > tT + fit:
exp ~t -1 P + ~ kn 1 _ exp _ t zn n=1 Z-n Ot Peg (t) _ exp ~T -1 1 + ~ ° -1 ~ k exp - tT tR zn ~, nm n zn ~T
Zn DT
exp _ 1 N _ ~' ~ 1 - - exp - zn k exp t tT
tR
~ n z z 0T
2 -1 n n n-, ~1 Z'n - (16) dT
\ -1 exp N _ 1~-~0 1~k -tT Zn - eXp tR
n=1 Zn ~T
Zn It will be understood that the effective prompt fraction given in Equation (16) is not identical with the actual prompt fraction p; however, if the correct time intervals are chosen then the difference can be minimized. It will also be understood that Equation (16) includes the case of a trip under steady state conditions with no preceding power ramp-down by setting ~o = ~1. In this case, Equation (16) becomes the same as Equation (8).
C. Correction of Effective Prompt Fraction As will be understood from the above Equations, correction terms may be incorporated into the measurement-based determination of effective prompt fraction. One possible correction arises in a case where the fastest delayed component is close to the measurement interval and its relative contribution is significant. Another possible correction arises in a case where there are one or more power changes that take place prior to the measurement interval, such as a power ramp down. In such a case, the ICFD current signals are still in a non-equilibrium state and are therefore slightly biased. A further possible correction may arise in a case where the trip is preceded by a long full power operation that has built up fission products which produce long-term decay gamma fields, causing residual ICFD current. Other cases may give rise to other corrections which may be applied to the measurement-based effective prompt fraction estimate.
In the first case, the effective prompt fraction may overestimate the detector's true prompt fraction as a result of the bias attributable to the trip response of the fastest delayed component. In one embodiment, this component has a time constant of about 3.9 seconds with 1.6% relative contribution. Three seconds after the start of the flux drop, the trip response of the fastest delayed component is contributing to the prompt response of the ICFD.
To remove the effects of this delayed component, trip response determinations are made with known detector parameters. A lookup table of values based upon modelling provides a mechanism for correcting the measurement-based effective prompt fraction. The effect of the delayed component may then be calculated using the right-hand term of Equation (8) or Equation (9).
In the case of a power ramp, lookup tables may also be used to provide a correction term. The lookup table is generated through modelling.
Appropriate correction terms may be calculated as described above with reference to Equations (14) to (16).
With respect to the third case, the long-term decay gamma field generates a residual ICFD current that provides a constant °'background"
detector signal over the "zero-power voltage floor°°. This effect causes the ICFD to appear less prompt since the relative drop of the signal is smaller than the relative drop of the reference signal.
The magnitude of the decay gamma effect on the individual ICFDs cannot be estimated easily from reactor theory; however, the measurements themselves give a solution over the long term. At a sufficiently long time after the trip, the measured signals comprise the following components:
1. design value of the NOP amplifier's "zero-floor"
2. zero-offset bias of NOP amplifiers (assumed to be X10 mV) 3. zero-offset bias of recording channels (measured before signal connection) 4. residual ICFD signal generated by the long-term decay gamma field Accordingly, the time averages of the ICFD signals are taken a significant time after the trip. In one embodiment, the measurements are taken about a day after the trip. These averages may be used as a zero-power voltage floor to remove the effect of the gamma field residual voltage and unknown instrument bias. By setting a cut-off time for the ICFD response signal, the delayed components, which are regarded as part of the detector response, are separated from the long-term gamma residual component.
These averages are then used in normalization of the ICFD signals.
Reference is now made to Figure 3, which shows, in flowchart form, a method 100 for testing ICFDs in a nuclear reactor.. The method 100 begins in step 102 with the initiation of data collection using the data acquisition system 10 (Fig. 1 ). The data acquisition system 10 begins collecting data regarding the instrumentation, including the ICFDs, the ion chambers, and the trip mechanism. This pre-trip data may be used later to normalize readings.
In step 104, the reactor power trip is triggered. During the power change, the data acquisition system 10 continues to accumulate data from the instrumentation within the reactor. The data is stored locally and if more than one local data store is created, as shown by the six separate channels in Figure 1, then in step 106 the data is compiled together. As described above, the data collection may continue for hours after the trip event in order to collect data with regard to the gamma field residual voltage and zero-power instrument bias for use in correcting the effective prompt fraction estimation through normalization. The normalization of readings takes into account the pre-trip average values and the post-trip average values.
In step 108, the measured data is used to determine the relative signal change over a prompt period. The prompt period is a period of time following initiation of the trip event that is shorter than the time constant of the fastest delayed component. As described above, the prompt period in one embodiment is set to be 3 seconds. Also as described above, in one embodiment, an average signal between 2.5 seconds and 3.5 seconds is used to determine the relative signal drop versus the beginning signal level at power trip initiation.
In step 110, the measured data is used to determine the relative flux drop over the same prompt period. The dynamic response of 100% prompt instrumentation may be employed as a proxy for determining the relative flux drop. In one embodiment, that instrumentation includes ion chambers. The relative change in the output signal from the ion chambers is taken to be indicative of the relative flux drop.
As noted above, the foregoing determinations may be based upon averages over a predetermined sample period, such as one second. The starting signal values used in determining the relative signal changes may be steady-state averaged values prior to the trip event. All signal values may be normalized with respect to pre-trip and/or post-trip measurements to eliminate voltage-offset bias and other effects.
Once relative signal changes for an ICFD have been determined for a prompt period and the relative flux change has been determined, then these quantities may be used in step 112 to determine the effective prompt fraction.
The effective prompt fraction is determined basecl upon a ratio of the relative ICFD signal change to the relative flux change, which is the relative ion chamber signal change in one embodiment. The determination of the effective prompt fraction may be made in accordance with Equation (10~. If the trip event is preceded by a power ramp, then the effective prompt fraction may be determined in accordance with Equation (13).
In step 114, various corrections may be incorporated to refine the effective prompt fraction depending upon the circumstances. For example, time averages of the ICFD signals long after the trip event may be used to established the zero-power offset bias. Once determined, this bias may be eliminated from the determination of the effective prompt fraction. Various other corrections or refinements may be incorporated into the determination of the effective prompt fraction, as described above with reference to the Equations.
Although the foregoing method 100 describes the corrections as being applied after calculation of the effective prompt fraction, it will be understood by those of ordinary skill in the art that in some instances it will be more appropriate to build any correction terms into the determination of relative signal changes or the determination of the effective prompt fraction, as the case may be. For example, calculations of zero-power voltage floor may be incorporated into the calculation of the relative signal drop of the ICFD.
The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Certain adaptations and modifications of the invention will be obvious to those skilled in the art. Therefore, the above discussed embodiments are considered to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
DETECTORS
FIELD OF THE INVENTION
This invention relates to flux detectors for nuclear reactors and, in particular, to testing in-cure flux detectors.
BACKGROUND OF THE INVENTION
Permanently installed in-core flux detectors (ICFDs) are used in many reactor facilities to prevent local over-power in the reactor core. The ICFDs are part of one or more safety shutdown systems in a nuclear reactor. The ICFDs are designed to detect a sudden increase in flux caused by an accident. Accordingly, the ICFDs must quickly respond to flux increases within a specified time interval in order to meet safety requirements. ICFDs may also be called neutron-over-power (NOP) or regional over-power (ROP) detectors.
ICFDs are self-powered flux detectors that produce a measurable current in the,uA range. The ICFDs are sensitive to both neutron and gamma fields. Under steady state conditions, the generated current is proportional to the local thermal neutron flux. In the case of a transient in neutron flux, the detector's current signal may lag behind the flux change since the ICFD°s current generating processes include slow-responding components, causing signal delays. The dynamic transfer function of the ICFD's flux-to-current conversion includes a large prompt response and a series of smaller exponentially delayed components, with either positive or negative amplitudes, making the ICFD either under-prompt or over-prompt.
The dynamic response of an ICFD can change over time with changes (burn-up) of material and as a result of defects. in order to ensure that the ICFDs can respond quickly enough to meet safety requirements, it is necessary to ensure that the prompt response component is above the minimum allowable limit. Therefore, it is desirable to engage in the periodic testing of the prompt fractions of all safety related ICFDs.
A typical CAIVDU nuclear reactor employs a large number of ICFDs, distributed throughout various redundant safety system channels. For safety reasons, these channels must be kept isolated from each other, so that an anomaly on one channel does not affect the other channels, thereby compromising the whole safety infrastructure. The need to test all ICFDs under similar conditions without compromising the isolation requirements presents a significant challenge.
SUMMARY OF THE INVENTION
The present invention provides a system and method of testing in-core flux defectors that determines an effective prompt fraction for an in-core flux detector based upon the dynamic response of the detector to a power change event within the reactor. The response is analyzed over a time interval following the power change event that is shorter than the time constant for the fastest delayed component.
In one aspect, the present invention provides a method for testing an in-core flux detector (ICFD) in a nuclear reactor. The method includes steps of initiating a step change in the nuclear reactor power, receiving response signals from measurement instruments, including the ICFD, digitizing and recording the response signals, and determining an effective prompt fraction for the ICFD based upon the recorded response signals.
In another aspect, the present invention provides system for testing an in-core flux detector (ICFD) in a nuclear reactor, the nuclear reactor having a mechanism for triggering a step change in the nuclear reactor power. The system includes a data acquisition system for receiving and recording response signals from measurement instruments, including the ICFD, and an analysis module for determining an effective prompt traction for the ICFD
based upon the recorded response signals.
In a further aspect, the present invention provides a computer program product having a computer readable medium tangibly embodying computer executable instructions for testing an in-core flux detector (ICFD) in a nuclear reactor, the nuclear reactor including measurement instruments, including the ICFD, the measurement instruments producing response signals as a result of a step change in the nuclear reactor power. The computer executable instructions include computer executable instructions for receiving and recording the response signals from the measurement instruments, including the ICFD; and computer executable instructions for determining an effective prompt fraction for the ICFD based upon the recorded response signals.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference will now be made, by way of example, to the accompanying drawings which show an embodiment of the present invention, and in which:
Figure 1 shows a block diagram of a data acquisition system for testing in-core flux detectors;
Figure 2 shows a block diagram of a data acquisition subsystem within the data acquisition system from Figure 1; and Figure 3 shows, in flowchart form, a method of testing an in-core flux d etector.
Similar numerals are used in different figures to denote similar components.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The present invention provides a method of determining the effective prompt fraction for an ICFD based upon the measured response of the ICFD
' q. ' to a step change in reactor power.
The prompt fraction for an ICFD is a ratio of the ICFD response to a reactor flux change divided by a reference instantaneous response to the same reactor flux change. The actual response signal produced by an ICFD
in situ contains certain delayed components. Some of these delayed components are attributable to the detector and some are attributable to delayed gamma fields within the reactor. Measurements of the signals generated by an ICFD in response to a step change in reactor power will reflect a delayed response dynamic that is attributable to both the detector characteristics and to external characteristics including delayed gamma fields and physical considerations such as the location of the detector. All of these sources of delay impact the effective prompt fraction determined based upon the measured response.
The reactor power change used to test the promptness of ICFDs is intended to model an accident situation in which 'there is a sudden increase in the local neutron flux. Ideally, the test would involve the introduction of a positive step change in reactor power; however, this is rarely possible for safety reasons. Accordingly, in most embodiments, the reactor power change is a step down in reactor power. In some embodiments, the power change includes a reactor power trip. In many reactors, these power outages are regularly planned and scheduled to occur every two years for maintenance and testing.
The use of a decrease in power instead of an increase in power to test the ICFD promptness necessitates an assumption that the dynamics of the detector response to a step change in reactor power - that is, the resulting change in neutron and gamma field flux - are approximately linear and symmetric in both directions. Gertain delayed components of the neutron or gamma field, such as the delayed gamma field resulting from built-up fission products, may have dynamics that depend upon whether the change in reactor power is positive or negative. To rely upon the symmetry of detector signal promptness a further assumption is made that the fission products build up with the same dynamics after a step-up change in the thermal neutron flux as they decay after a reactor trip.
Byway of example, the Platinum-clad Inconel ICFD has a mixed neutron and gamma sensitivity. Forty percent of its current is induced by the ~ reactor's gamma field interacting with the detector's material via the prompt process of ('y, a ); however, one third of the gamma field is delayed with respect to a change in the neutron flux, resulting in a thirteen percent ~-related delayed component in the detector's current. Although the interaction that takes place in the detector, ('y, a ), is prompt, the delay is still attributed to the detector. The source of the delayed gamma field is the fission products that build up after a long full-power operation. When the reactor is tripped, the neutron and gamma fields go down to zero immediately; however, the slow decay of fission products stored in the fuel and other core components still produce a significant gamma field. The cascade of various beta and gamma decays of fission products have certain dynamics, which partly determines the delayed current of the ICFD signal responding to a reactor trip.
The inclusion of delayed components not attributable to the detector in an effective prompt fraction estimate is justifiable in the sense that in a real accident scenario those non-detector delays will still be applicable to the effective prompt fraction for the detector signal. In other words, the promptness of the detector signal - as opposed to the detector itself - is still influenced by the location of the detector within the core, the length of its lead cable, and the power history of the reactor core.
The effective prompt fraction of an ICFD signal comprises the ratio between the immediate change in the iCFD signal following a reactor change and that of a one-hundred percent prompt reference detector signal measured during the same reactor change. In one embodiment, the reactor change is a reactor trip and the one-hundred percent prompt reference detector signal is obtained from one or more ex-core ion chambers. The ion chambers are responsive to the prompt changes in the neutron flux and are -not affected by delayed gamma flux. In one embodiment, the effective prompt fraction is calculated as the post-trip drop of a normalized 1CFD
signal averaged over a time interval after the trip divided by the average signal drop of the normalized reference ion chamber signal.
After initiation of a trip, there is a short delay of approximately 0.4 seconds before the flux signals start to respond. This initial dead-time is partly attributable to the delay in the trip mechanism reaching the relevant area of the core. For example, where the trip mechanism involves dropping shutoff rods to stop the reactions in the core, there is a finite time for the rods to drop into the calandria and to begin to affect the reactions. Another type of trip mechanism is the injection of liquid poison (Gadolinium) to stop the reactions in the core. Again, it takes a finite amount of time for the poison to be injected into the calandria and to affect the reactions in the local region of the detector. The extent of this delay depends partly on the location of the detector within the core.
Once the flux signals begin to respond, then the neutron flux signals drop to a low level over a time interval of approximately 0.3 to 0.8 seconds.
The time constant of the fastest delayed component is about 3.9 seconds, meaning that there is sufficient separation to estimate the prompt fraction in accordance with the present invention.
In one embodiment, the effective prompt fraction of an lCFD is calculated from the normalized drop of the ICFD signal measured three seconds after the start of the flux drop, relative to the normalized drop of the reference ion chamber measured at three seconds. The signals are normalized between their pre-trip values and the nominal zero-power voltage output of their amplifiers {"zero floor"). These nominal floor values vary from station to station. The actual "zero floor" values may differ from design values due to instrument zero-offset bias of NOP amplifiers (station hardware) and recording channels (data acquisition systems).
The determination of an effective prompt fraction in accordance with the present invention may include two steps. The first is to determine an -uncompensated effective prompt fraction from the measurement data obtained during a reactor trip. The second is to apply correction terms to the uncompensated effective prompt fraction to obtain the effective prompt fraction. In some embodiments, these two steps are not performed in distinctive and consecutive fashion.
The correction terms applied to the measurement-based uncompensated effective prompt fraction comprise modelling-based corrections that typically alter the uncompensated effective prompt fraction by a few percent. The relatively modest impact of the correction terms allows for the use of nominal ICFD parameters in place of actual specific ICFD
parameters and for the use of a simplified time function for flux change.
The correction terms may be used to remove the effects of pre-trip power changes, such as power ramps wherein the reactor power may have been steadily ramped down from full power to sixty percent before the full reactor trip was initiated. They may also be used to remove the effects of the fastest delayed components of the detector current already active and contributing to the ICFD trip response at three seconds, the effects of the zero-offset bias of the data acquisition recording channels, and the effects of the post-trip long-term delayed gamma field emitted by the fission products in delayed beta-decays.
A. Data Acquisition System As noted above, a typical nuclear reactor employs a large number of ICFDs, distributed throughout various redundant safety system channels.
The safety channels each monitor a set of ICFDs and other instrumentation, including ion chambers. The instrumentation is distributed throughout the reactor area, both within the core and external to the core. In order to meet safety requirements, the safety channels need to be kept isolated from one another. If the safety channels were to be interconnected, there is a risk that a fault in one channel could affect other channels, thereby compromising the whole safety system.
Reference is now made to Figure 1, which shows a block diagram of a data acquisition system 10 for testing in-core flux detectors. The data acquisition system 10 is distributed across six safety system channels shown as 12a, 12b, ..., 12f. The six channels correspond to two safety systems having three channels each. The data acquisition system 10 comprises a number of subsystems shown as 14a, 14b, ..., 14f, corresponding to the six safety system channels 12a, 12b, ..., 12f. Each data acquisition subsystem 14 is coupled to an instrument panel 16 that receives signals from a number of instruments; in one embodiment, up to twenty-four instruments. The data acquisition subsystem 14 includes an isolation amplifier 18 for receiving instrumentation signals 30 from the instrument panel 16 in parallel. The isolation amplifier 18 prevents any feedback from the data acquisition system 10 to the instrument panel 16 and outputs isolated instrumentation signals 32.
In one embodiment, the isolation amplifier 18 is a 1:1 amplifier with no filtering.
The data acquisition subsystem 14 also includes a signal conditioning unit 22. The signal conditioning unit 22 receives the isolated instrumentation signals 32 and outputs conditioned signals 34. The signal conditioning unit 22 may perform a number of tasks, including analog amplification, DC offset adjustment, and analog low-pass filtering.
The data acquisition subsystem 14 further includes a computer 20 for receiving the conditioned signals 34, digitizing them and storing them locally.
The data recorded by the computers 20 (shown as 20a, 20b, ..., 20f) during a trip event is later used to determine the effective prompt fraction for one or more ICFDs that are included in the reactor instrumentation. In order to perform this analysis the data for a subject ICFD may be compared with data corresponding to ion chambers, other ICFDs, or other instrumentation. To make a useful comparison, the data needs to have a common reference time.
The computers 20 in the data acquisition system 1U typically each have their own clock, since they operate independently of the other data acquisition subsystems 14. Accordingly a difficulty that arises us in synchronizing the computers 20 without compromising the safety system isolation requirements.
_g_ To address this difficulty, one of the computers 20, such as computer 20a, is designated as a "master" computer and the remainder of the computers 20b, 20c, ..., 20f, as '°slaves", although they are not in a master-slave relationship normally seen in computing wherein the master has some level of control over the slaves. The master computer 20a outputs a clock signal 36 that is fed to the other computers 20b, 20c, ..., 20f. The clock signal 36 passes through an isolation barrier 24 between each computer 20 so as to ensure the computers 20 remain isolated from one another. Each of the slave computers 20b, 20c, ..., 20f receives this clock signal 36 and may timestamp any data recordings or otherwise link the data recordings of the conditioned signals 34 to the reference clock signal 36 from the master computer 20a. Accordingly, the locally stored data at each computer 20 may later be collected and combined for analysis using a common time reference.
In one embodiment, the isolation barrier 24 is an optical isolator. The isolation barrier 24 may include other mechanisms for isolating signals, including a galvanic isolator and others. The isolation barrier 24 should be selected to provide an adequate signal-to-noise ratio and temperature sensitivity. The isolation barrier 24 may provide for double isolation.
In one embodiment, the clock signal 36 includes a clock signal and a time verification signal. The time verification signal may be recorded for later analysis and correction of any errors in the clock signal. In one embodiment, the time verification signal is a triangle wave signal.
Existing CANDU nuclear reactors include a communications system for communicating between rooms within the reactor through headsets. In some embodiments, this existing communication system may be employed to provide the interconnections of components for the data acquisition system 10 shown in Figure 1. In particular, the existing communications system together with the isolation barriers 24 may provide a mechanism for coupling the various data acquisition subsystems 14 to distribute the clock signal 36.
Reference is now made to Figure 2, which shows a block diagram of one of the data acquisition subsystems 14. The signal conditioning unit 22 includes a filter 40, an amplifier 42 and a DC-offset adjuster 44 for each isolated instrumentation signal 32. The filter 40 may include an anti-aliasing low pass filter. The computer 20 includes an analog-to-digital (AID) converter 46 for digitizing the conditioned signals 34. The digitised signals created by the AID
converter 46 are provided to a microprocessor 48, which stores the digital data in a memory 54. The microprocessor 48 co-ordinates and manages the data acquisition subsystem 14, including the sending of control signals 56 to the signal conditioning unit 22 to control the filters 40, amplifiers 42, and DC-offset adjusters 44.
The computer 20 further includes a clock 50, which in some embodiments is a part of the microprocessor 48, and which generates the clock signal 36.
The computer 20 may also include a display 52 and other output and input devices (not shown) to enable a user to interact with the computer 20.
It will be understood by those of ordinary skill in the art that the computer may be a personal computer or other workstation. The computer 20 may also be a microcontroller or other programmable processing device for use in managing a data acquisition process.
In one embodiment, the data acquisition subsystem 14 receives, 20 digitizes, and records data with respect to twenty-four instruments. The data collected has sufficient resolution to provide accurate information regarding the promptness of the ICFDs. In one embodiment, the data acquisition subsystem 14 samples the instrument signals - that is, the AID converter 46 samples the conditioned signals 34 - at 500 Hertz with an amplitude resolution of 0.1 mV.
In order to provide sufficient resolution, the sampling should be at a rate greater than one sample every 10 milliseconds.
To determine the promptness of the instrumentation under test it is necessary to know when the power change - that is, the reactor trip - actually occurred in the recorded data. Accordingly, one of the instruments measured by the data acquisition system 10 is the trip mechanism. The trip mechanism may be a pushbutton or other trigger for initiating the trip event. The pushbutton may cause the release of the neutron-absorbing rods or the injection of the neutron-absorbing poison to precipitate the reactor shutdown event. Therefore, one of the sets of recorded data will indicate the precise occurrence of the trip initiation. It then takes a finite amount of time for the trip mechanism to deploy and begin affecting the neutron reactions within the core and to propagate as far as the individual detectors.
The computers 20 are suitably programmed to control the various hardware components and perform the data sampling and storage functions.
The programming of the computers 20 to perform these functions will be within the understanding of a person of ordinary skill in the art of computer software programming. The software developed for controlling the operation of the computers 20 provides a user interface to allow for a user to set certain parameters, including the sampling rate.
B. Determination of Effective Prompt Fraction The effective prompt fraction that may be determined from the measurements obtained using the test system described above is related to the actual prompt fraction. Using mathematical models of the ICFD dynamics and the trip event, the relationship between the actual prompt fraction and the effective prompt fraction may be determined.
The ICFD produces an output voltage signal Vo(f), which is related to the convolution of the time dependent flux ~(t) and the detector impulse response function h(t). The latter comprises the actual prompt fraction p and the N number of delayed components with time constants 7" and relative amplitude k", respectively, where n = 1,..., N. This relationship is shown in the following expression:
h(t) = p8(t) + ~ k" exp -t (1 ) n=i Zn Zn N
where 8(t) comprises the Dirac-delta function, and wherein p+~kn = 1.
n=i The detector voltage may be modeled as a function of the detector's flux-to-voltage conversion factor Co, which itself includes the flux-to-current conversion factor C"~, the current-to-voltage conversion factor C"", and the voltage amplifier gain factor Gyrv. The first parameter is the ICFD's sensitivity or current yield, which may change over the life of the ICFD. The latter two parameters are set in the converterlamplifier of the signal instrumentation and may be adjusted to compensate for changes in the detector sensitivity. Using the conversion factor Co, the detector voltage may be modeled using the expression:
t VD (t) = CD ,~h(t -t') x d~(t')dt'+VB (2) where VB is a constant voltage offset, measured as zero power. The actual zero-power offset voltage, VB, may vary from signal to signal due to possible bias in station instrumentation and data acquisition recording channels. The differences between the zero-power offset of different signals are typically in the range of X10 mV or 0.2% of the monitored voltage range. This uncertainty may result in a X0.5% bias in the estimated prompt fraction.
Equations (1 ) and (2) may be combined to arrive at the following expression of detector output voltage Vo as a function of time-dependent flux:
vD (t) = CD P~(t) + ~ kn j~~(t') exp(- t t~)dt~ + VB
n=t zn -w Zn The parameter values k" and T" of the N number of delayed components are not necessarily related to actual physical processes, but rather are the best exponential fit to the measured detector response. One can obtain design or nominal values for the parameters k" and T" by measuring detector response signals and fitting exponential curves to the measured signals. The delayed parameters are related to the prompt fraction by way of the following normalization equation:
N
~7+~~Cn =1 n=t Having obtained a modeling equation, Equation (3), for the detector voltage VD, the reactor trip may be modeled as a linear flux change from an initial static flux ~o at time t = 0, to a later constant flux ~~ after a time interval fit. As a function of time t, the flux may be expressed as:
t<0 Wi(t)=~°-~° ~' xt 0<t<~t (5) ~t Ot < t For the purposes of assessing the delayed current component of a nominal IGFD, the above linear flux change equation is a good approximation of the real flux drop during a reactor trip. The advantage of this simplified function is that it gives an analytical formula for the trip response of the ICFD
voltage signal, which is needed to perform a parameter sensitivity study. If the linear ramp-down expression in Equation (5) is inserted into Equation (3), for times t > fit, and using the pre-trip equation Vo (0) = Goo + Ve, the prompt fraction p may be expressed as a function of the measured detector signal Vo, as follows:
_, exp ~t -1 N
p- 1_ Vn(t)-VB 1_ ~I -~k 1-exp - ~ z" (6) VD (0) - VB d~ ° ~=, n zn 0t ~n where VD (t) VB represents the relative drop in detector voltage after the Vn (0) _ VB
reactor trip, while ~' represents the relative flux drop in the location of the core detector. The first term in Equation (6) defines the "°effective prompt fraction", which can be directly calculated from the measured time series signals using the following expression:
_, Pes(t)= 1-~° ~)-~B 1-~1 (7) D( ) B 0 Accordingly, Equation (6) may be rewritten as:
exp ~t _ 1 p - p~ ltl - ~ kn 1 - eXp - t . Zn n=1 Zn ~t Zn Because the detector voltage signal Volt) is sampled continuously and frequently during the reactor trip test, the time f may be chosen such that ~f <
t < T" so as to minimize the effect of the second half of Equation (8). If this is done, then Equation (8) may be approximated as follows:
exp ~t -1 kn 1- exp - t ~" = ~ kn 1- exp - t ~ (9) n°t Zn ~t n=1 Zn Zn If the time instant t is chosen to be close to ~t relative to the shortest time constant, min{ T"} then the right hand side expression is close to zero.
In this case, the prompt fraction p is approximately equal to the effective prompt fraction Pe~(t).
Another approximation that may be used to simplify the modeling is that the relative flux drop ~' from Equation (7) may be approximated by the relative signal drop of the ion chamber signal. In reality the relative flux drop may depend upon the detector location, since the flux shape after the trip is different from the pre-trip static flux shape. If a X15% variation in the pre-trip static flux at the detector locations and a uniform flux shape after the trip are assumed, then the application of the relative flux drop assumption introduces a variation of X1.5% in the effective prompt fraction.
Accordingly, the effective prompt fraction of an ICFD within the reactor may be estimated using the measured time recorded signal data from the instrumentation, using a time t satisfying the condition flt < t < min{T"} and the following expression:
_ yD (t) - YB vr~ (t) - v8, -pe~(t) 1- ~'D(o)-vB 1- v~~(o)-vB~ (10) where y'c(t)-~B' is the relative signal drop of the ion chamber signal.
vl~ (o) -YB.
In another embodiment, the post-trip signals in Equation (10) are averaged over a time interval (t~, t2) satisfying the condition ~t < t, < t2 <
min{T"}. In one embodiment, t~ is 2.5 seconds and t2 is 3.5 seconds.
In yet another embodiment, the reactor trip may occur following a power change; typically a power ramp-down from full power to a fraction of full power, such as 60%. After the power-ramp down, the reactor is kept at steady state power for hours before the trip is initiated.
In this case, the power reduction may be a linear flux reduction from ~o to ~~ over a time interval DT starting at time tR. The power is then kept constant until time tr when the power trip occurs. During the power trip, the flux decreases from ~~ to ~2 over a time period At. The two ramps may be expressed as follows:
~(t) _ ~, + ~ Q ~1 x (tR + DT - t) if tR < t < tR +AT (11 ) ~(t) _ ~Z + ~Z ~z x (tT + Ot -t) if tT < t < tT +At (12) 0t By way of example, ~T may be 4 to 10 minutes. The power hold between the power ramp-down and the trip event is typically between 30 minutes and 8 hours, although it may be longer or shorter.
Using the same definition for an effective prompt fraction that lead to Equation (7), we may define the effective prompt fraction for this case as follows:
_, peg (t) _ ~D (tT ) YD (t) ~1 ~2 13 vD(tT) ~B ~l The detector voltage from Equation (3) may be combined with the expressions for flux to arrive at an expression for the detector voltage for times t > fT + ~t that includes two exponential signal transients corresponding to the two linear power ramps:
exp ~T _ 1 _ N t-t z vD(t)-CD~2 +~'~'D(~0 ~1)~kn exp __ R n n=, Zn OT
Zn ~t exp - -1 N t-tN Zn +CD (~, +~2 )~kn exp __ (14) n = 1 Z n Ot Zn For times t > tT + dt, the detector signal in Equation (14) shows the superimposing effects of the two independent power reductions. It will be understood that in a further embodiment having multiple power ramps the above equation may be modified to include additional terms.
The pre-trip detector signal is given by the expression:
N exp 0T _ 1 vD(tr)=CD~1 +CD(~° -~1)~kn exp -tT tR Zn (15) n=1 Zn OT
Zn From Equations (14), (15), and (13) we may obtain an expression for the effective prompt fraction for times t > tT + fit:
exp ~t -1 P + ~ kn 1 _ exp _ t zn n=1 Z-n Ot Peg (t) _ exp ~T -1 1 + ~ ° -1 ~ k exp - tT tR zn ~, nm n zn ~T
Zn DT
exp _ 1 N _ ~' ~ 1 - - exp - zn k exp t tT
tR
~ n z z 0T
2 -1 n n n-, ~1 Z'n - (16) dT
\ -1 exp N _ 1~-~0 1~k -tT Zn - eXp tR
n=1 Zn ~T
Zn It will be understood that the effective prompt fraction given in Equation (16) is not identical with the actual prompt fraction p; however, if the correct time intervals are chosen then the difference can be minimized. It will also be understood that Equation (16) includes the case of a trip under steady state conditions with no preceding power ramp-down by setting ~o = ~1. In this case, Equation (16) becomes the same as Equation (8).
C. Correction of Effective Prompt Fraction As will be understood from the above Equations, correction terms may be incorporated into the measurement-based determination of effective prompt fraction. One possible correction arises in a case where the fastest delayed component is close to the measurement interval and its relative contribution is significant. Another possible correction arises in a case where there are one or more power changes that take place prior to the measurement interval, such as a power ramp down. In such a case, the ICFD current signals are still in a non-equilibrium state and are therefore slightly biased. A further possible correction may arise in a case where the trip is preceded by a long full power operation that has built up fission products which produce long-term decay gamma fields, causing residual ICFD current. Other cases may give rise to other corrections which may be applied to the measurement-based effective prompt fraction estimate.
In the first case, the effective prompt fraction may overestimate the detector's true prompt fraction as a result of the bias attributable to the trip response of the fastest delayed component. In one embodiment, this component has a time constant of about 3.9 seconds with 1.6% relative contribution. Three seconds after the start of the flux drop, the trip response of the fastest delayed component is contributing to the prompt response of the ICFD.
To remove the effects of this delayed component, trip response determinations are made with known detector parameters. A lookup table of values based upon modelling provides a mechanism for correcting the measurement-based effective prompt fraction. The effect of the delayed component may then be calculated using the right-hand term of Equation (8) or Equation (9).
In the case of a power ramp, lookup tables may also be used to provide a correction term. The lookup table is generated through modelling.
Appropriate correction terms may be calculated as described above with reference to Equations (14) to (16).
With respect to the third case, the long-term decay gamma field generates a residual ICFD current that provides a constant °'background"
detector signal over the "zero-power voltage floor°°. This effect causes the ICFD to appear less prompt since the relative drop of the signal is smaller than the relative drop of the reference signal.
The magnitude of the decay gamma effect on the individual ICFDs cannot be estimated easily from reactor theory; however, the measurements themselves give a solution over the long term. At a sufficiently long time after the trip, the measured signals comprise the following components:
1. design value of the NOP amplifier's "zero-floor"
2. zero-offset bias of NOP amplifiers (assumed to be X10 mV) 3. zero-offset bias of recording channels (measured before signal connection) 4. residual ICFD signal generated by the long-term decay gamma field Accordingly, the time averages of the ICFD signals are taken a significant time after the trip. In one embodiment, the measurements are taken about a day after the trip. These averages may be used as a zero-power voltage floor to remove the effect of the gamma field residual voltage and unknown instrument bias. By setting a cut-off time for the ICFD response signal, the delayed components, which are regarded as part of the detector response, are separated from the long-term gamma residual component.
These averages are then used in normalization of the ICFD signals.
Reference is now made to Figure 3, which shows, in flowchart form, a method 100 for testing ICFDs in a nuclear reactor.. The method 100 begins in step 102 with the initiation of data collection using the data acquisition system 10 (Fig. 1 ). The data acquisition system 10 begins collecting data regarding the instrumentation, including the ICFDs, the ion chambers, and the trip mechanism. This pre-trip data may be used later to normalize readings.
In step 104, the reactor power trip is triggered. During the power change, the data acquisition system 10 continues to accumulate data from the instrumentation within the reactor. The data is stored locally and if more than one local data store is created, as shown by the six separate channels in Figure 1, then in step 106 the data is compiled together. As described above, the data collection may continue for hours after the trip event in order to collect data with regard to the gamma field residual voltage and zero-power instrument bias for use in correcting the effective prompt fraction estimation through normalization. The normalization of readings takes into account the pre-trip average values and the post-trip average values.
In step 108, the measured data is used to determine the relative signal change over a prompt period. The prompt period is a period of time following initiation of the trip event that is shorter than the time constant of the fastest delayed component. As described above, the prompt period in one embodiment is set to be 3 seconds. Also as described above, in one embodiment, an average signal between 2.5 seconds and 3.5 seconds is used to determine the relative signal drop versus the beginning signal level at power trip initiation.
In step 110, the measured data is used to determine the relative flux drop over the same prompt period. The dynamic response of 100% prompt instrumentation may be employed as a proxy for determining the relative flux drop. In one embodiment, that instrumentation includes ion chambers. The relative change in the output signal from the ion chambers is taken to be indicative of the relative flux drop.
As noted above, the foregoing determinations may be based upon averages over a predetermined sample period, such as one second. The starting signal values used in determining the relative signal changes may be steady-state averaged values prior to the trip event. All signal values may be normalized with respect to pre-trip and/or post-trip measurements to eliminate voltage-offset bias and other effects.
Once relative signal changes for an ICFD have been determined for a prompt period and the relative flux change has been determined, then these quantities may be used in step 112 to determine the effective prompt fraction.
The effective prompt fraction is determined basecl upon a ratio of the relative ICFD signal change to the relative flux change, which is the relative ion chamber signal change in one embodiment. The determination of the effective prompt fraction may be made in accordance with Equation (10~. If the trip event is preceded by a power ramp, then the effective prompt fraction may be determined in accordance with Equation (13).
In step 114, various corrections may be incorporated to refine the effective prompt fraction depending upon the circumstances. For example, time averages of the ICFD signals long after the trip event may be used to established the zero-power offset bias. Once determined, this bias may be eliminated from the determination of the effective prompt fraction. Various other corrections or refinements may be incorporated into the determination of the effective prompt fraction, as described above with reference to the Equations.
Although the foregoing method 100 describes the corrections as being applied after calculation of the effective prompt fraction, it will be understood by those of ordinary skill in the art that in some instances it will be more appropriate to build any correction terms into the determination of relative signal changes or the determination of the effective prompt fraction, as the case may be. For example, calculations of zero-power voltage floor may be incorporated into the calculation of the relative signal drop of the ICFD.
The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Certain adaptations and modifications of the invention will be obvious to those skilled in the art. Therefore, the above discussed embodiments are considered to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (39)
1. A method for testing an in-core flux detector (ICFD) in a nuclear reactor, the method comprising the steps of:
(a) initiating a step change in the nuclear reactor power;
(b) receiving response signals from measurement instruments, including the ICFD;
(c) digitizing and recording said response signals; and (d) determining an effective prompt fraction for the ICFD based upon said recorded response signals.
(a) initiating a step change in the nuclear reactor power;
(b) receiving response signals from measurement instruments, including the ICFD;
(c) digitizing and recording said response signals; and (d) determining an effective prompt fraction for the ICFD based upon said recorded response signals.
2. The method claimed in claim 1, wherein said step of determining includes a step of determining a ratio of a dynamic response of the ICFD and a flux change within the nuclear reactor.
3. The method claimed in claim 2, wherein said response signals include an ICFD response signal for the ICFD, and wherein said dynamic response comprises a relative change in said ICFD response signal over a predetermined interval following said step of initiating.
4. The method claimed in claim 3, wherein said ICFD response signal includes delayed components, said components each having a time constant, and wherein said predetermined interval comprises a time period following initiation of said step change that is shorter than the shortest of said time constants.
5. The method claimed in claim 4, wherein said time period is less than about 3.9 seconds.
6. The method claimed in claim 3, wherein said response signals include an ion chamber response signal, and wherein said step of determining a ratio includes determining said flux change based upon the relative change in said ion chamber response signal over said predetermined interval.
7. The method claimed in claim 3, further including a step of normalizing said ICFD response signal over said predetermined interval with respect to long-term pre-step change and post-step change readings of said ICFD response signal.
8. The method claimed in claim 3, wherein said step of determining a ratio includes calculating an average ICFD response signal over a time interval following said predetermined interval and using said average ICFD response signal to determine said relative change.
9. The method claimed in claim 2, wherein said step of determining includes determining a measurement-based effective prompt fraction and further includes a step of incorporating modelling-based correction terms to alter said measurement-based effective prompt fraction.
10. The method claimed in claim 9, wherein said correction terms include a term for removing the effect of built-up fission products.
11. The method claimed in claim 1, wherein said step change includes a reactor power trip.
12. The method claimed in claim 1, wherein a data acquisition system includes multiple subsystems that each receive a subset of said response signals from said measurement instruments, and wherein said step of receiving and recording includes providing a clock signal from one of said subsystems to the other of said subsystems.
13. The method claimed in claim 12, wherein said step of providing a clock signal includes passing said clock signal through an isolation barrier.
14. The method claimed in claim 12, wherein said step of providing a clock signal further includes a step of providing a time verification signal.
15. A system for testing an in-core flux detector (ICFD) in a nuclear reactor, the nuclear reactor having a mechanism for triggering a step change in nuclear reactor power, the system comprising:
(a) a data acquisition system for receiving and recording response signals from measurement instruments, including the ICFD; and (b) an analysis module for determining an effective prompt fraction for the ICFD based upon said recorded response signals.
(a) a data acquisition system for receiving and recording response signals from measurement instruments, including the ICFD; and (b) an analysis module for determining an effective prompt fraction for the ICFD based upon said recorded response signals.
16. The system claimed in claim 15, wherein said effective prompt fraction includes a ratio of a dynamic response of the ICFD and a flux change within the nuclear reactor.
17. The system claimed in claim 16, wherein said response signals include an ICFD response signal for the ICFD, and wherein said dynamic response comprises a relative change in said ICFD response signal over a predetermined interval following said step change.
18. The system claimed in claim 17, wherein said ICFD response signal includes delayed components, said components each having a time constant, and wherein said predetermined interval comprises a time period following initiation of said step change that is shorter than the shortest of said time constants.
19. The system claimed in claim 18, wherein said time period is less than about 3.9 seconds.
20. The system claimed in claim 17, wherein said response signals include an ion chamber response signal, and wherein said flux change comprises the relative change in said ion chamber response signal over said predetermined interval.
21. The system claimed in claim 17, wherein said analysis module includes a normalization module for normalizing said ICFD response signal over said predetermined interval with respect to long-term pre-step change and post-step change readings of said ICFD response signal.
22. The system claimed in claim 17, wherein analysis module includes an averaging module for averaging said ICFD response signal over a time interval following said predetermined interval and wherein said analysis module uses said average ICFD response signal to determine said relative change.
23. The system claimed in claim 16, wherein said analysis module determines a measurement-based effective prompt fraction, and wherein said analysis module incorporates modelling-based correction terms to alter said measurement-based effective prompt fraction.
24. The system claimed in claim 23wherein said correction terms include a term for removing the effect of built-up long-term fission products.
25. The system claimed in claim 15, wherein said data acquisition system includes an isolation amplifier for receiving said response signals and providing isolated signals, a conditioning module for conditioning said isolated signals and outputting conditioned signals, and a computer having an input for receiving said conditioned signals, said computer including a digitizer for digitizing said conditioned signals and memory for storing said digitized signals.
26. The system claimed in claim 15, wherein said data acquisition system includes more than one subsystem, each subsystem corresponding to a safety system channel, the nuclear reactor including more than one safety system, one of said subsystems outputting a clock signal, wherein the other subsystems receive said clock signal.
27. The system claimed in claim 26, wherein said data acquisition system further includes at least one isolation barrier in the path of said clock signal between said subsystems to isolate said subsystems.
28. The system claimed in claim 26, wherein said isolation barrier comprises an optical isolator.
29. The system claimed in claim 26, wherein said one of said subsystems outputs a time verification signal and wherein said other subsystems receive said time verification signal.
30. A computer program product having a computer readable medium tangibly embodying computer executable instructions for testing an in-core flux detector (ICFD) in a nuclear reactor, the nuclear reactor including measurement instruments, including the ICFD, the measurement instruments producing response signals as a result of a step change in the nuclear reactor power, the computer executable instructions comprising:
(a) computer executable instructions for receiving and recording the response signals from the measurement instruments, including the ICFD; and (b) computer executable instructions for determining an effective prompt fraction for the ICFD based upon said recorded response signals.
(a) computer executable instructions for receiving and recording the response signals from the measurement instruments, including the ICFD; and (b) computer executable instructions for determining an effective prompt fraction for the ICFD based upon said recorded response signals.
31. The computer program product claimed in claim 30, wherein said computer executable instructions for determining include computer executable instructions for determining a ratio of a dynamic response of the ICFD and a flux change within the nuclear reactor.
32. The computer program product claimed in claim 31, wherein said response signals include an ICFD response signal for the ICFD, and wherein said dynamic response comprises a relative change in said ICFD response signal over a predetermined interval following said step of initiating.
33. The computer program product claimed in claim 32, wherein said ICFD
response signal includes delayed components, said components each having a time constant, and wherein said predetermined interval comprises a time period following initiation of said step change that is shorter than the shortest of said time constants.
response signal includes delayed components, said components each having a time constant, and wherein said predetermined interval comprises a time period following initiation of said step change that is shorter than the shortest of said time constants.
34. The computer program product claimed in claim 33, wherein said time period is less than about 3.9 seconds.
35. The computer program product claimed in claim 32, wherein said response signals include an ion chamber response signal, and wherein said computer executable instructions for determining a ratio include computer executable instructions for determining said flux change based upon the relative change in said ion chamber response signal over said predetermined interval.
36. The computer program product claimed in claim 32, further including computer executable instructions for normalizing said ICFD response signal over said predetermined interval with respect to long-term post-step change readings of said ICFD response signal.
37. The computer program product claimed in claim 32, wherein said computer executable instructions for determining a ratio include computer executable instructions for calculating an average ICFD
response signal over a time interval following said predetermined interval and using said average ICFD response signal to determine said relative change.
response signal over a time interval following said predetermined interval and using said average ICFD response signal to determine said relative change.
38. The computer program product claimed in claim 31, wherein said computer executable instructions for determining include computer executable instructions for determining a measurement-based effective prompt fraction and further include computer executable instructions for incorporating modelling-based correction terms to alter said measurement-based effective prompt fraction.
39. The computer program product claimed in claim 38, wherein said correction terms include a term for removing the effect of built-up long-term delay fission products.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002456284A CA2456284A1 (en) | 2004-01-26 | 2004-01-26 | Method and system for testing in-core flux detectors |
| KR1020050006852A KR101118553B1 (en) | 2004-01-26 | 2005-01-25 | Method and system for testing in-core flux detectors |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002456284A CA2456284A1 (en) | 2004-01-26 | 2004-01-26 | Method and system for testing in-core flux detectors |
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| CA2456284A1 true CA2456284A1 (en) | 2005-07-26 |
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| CA002456284A Abandoned CA2456284A1 (en) | 2004-01-26 | 2004-01-26 | Method and system for testing in-core flux detectors |
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| CA (1) | CA2456284A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR830002067B1 (en) * | 1979-10-19 | 1983-10-08 | 제이. 비. 훼르거슨 | Reactor Flux Measurement |
| US4582672A (en) * | 1982-08-11 | 1986-04-15 | Westinghouse Electric Corp. | Method and apparatus for preventing inadvertent criticality in a nuclear fueled electric powering generating unit |
| JP3785847B2 (en) | 1999-03-09 | 2006-06-14 | 株式会社日立製作所 | Reactor power measuring device |
| JP3800396B2 (en) | 2000-12-13 | 2006-07-26 | 株式会社日立製作所 | Reactor power measuring device |
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2004
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| KR101118553B1 (en) | 2012-02-24 |
| KR20050077031A (en) | 2005-07-29 |
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