ML19261A438
| ML19261A438 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 01/31/1979 |
| From: | Baxter A, Hoppes D, Mceachern D GENERAL ATOMICS (FORMERLY GA TECHNOLOGIES, INC./GENER |
| To: | |
| Shared Package | |
| ML19261A436 | List: |
| References | |
| GA-A13954, NUDOCS 7901180176 | |
| Download: ML19261A438 (68) | |
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CONTENTS
- 1. EX-CORE DETECTOR DECALIBRATION IN THE FORT ST. VRAIN HTGR . . . . . 1 1.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2. Location and Function of Power Rnage Detectors . . . . . . . 4
- 2. ANALYSIS OF DETECTOR DECALIBRATION . . . . . . . . . . . . . . . . 7 2.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Ef fect of Detector Decalibration on Plant Control . . . . . . 7 2.3. Effect of Detector Decalibration on PPS Action. . . . . . . .15 2.3.1. Measurement and Calculation of Detector Decalibration Factors . . . . . . . . . . . . . . . .20 2.3.2. Detector Decalibration Due to Sequential Rod Withdrawal . . . . . . . . . . . . . . . . . . .22 2.3.3. Detector Decalibration Due to Single-Rod Withdrawal Accidents . . . . . . . . . . . . . . . .23 2.3.4. Determination of PPS Trip Setpoints . . . . . . . . .25 2.3.5. Impact of Detector Calibration on Trip Setpoints . . . . . . . . . . . . . . . . . . . . . .41 2.3.6. Conservatisms in Analysis . . . . . . . . . . . . . .42
- 3. PROPOSED HARDWARE CHANGES . . . . . . . . . . . . . . . . . . . . .43 3.1. Floating Setpoint Circuitry Hardware . . . . . . . . . . . .43 3.2. Floating Setpoint Performance . . . . . . . . . . . . . . . .43 3.3. Failure Mode Analysis . . . . . . . . . . . . . . . . . . . .50 3.4. Heat Balance Correction Circuitry . . . . . . . . . . . . . .56
- 4. RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . .58 4.1. Programmed Trip and RWP Setpoints . . . . . . . . . . . . .58 4.2. Calibration Requirements . . . . . . . . . . . . . . . . . .58 APPENDIX. SAFETY MARGIN AND UNCERTAINTIES . . . . . . . . . . . . . . .61 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68 111
FIGURES
- 1. FSV core layout and locations of ex-core detectors . . . . . . . 2
- 2. Flux detector control loop . . . . . . . . . . . . . . . . . . 8
- 3. F3V ramp load decrease to 75% load at 5%/ min with accurate power measurement . . . . . . . . . . . . . . . . . . . . . . . . 10
- 4. FSV ramp load decrease to 75% load at 5%/ min with nuclear instrument decalib ration . . . . . . . . . . . . . . . . . . . . 11
- 5. FSV ramp load decrease from 100% load at 5%/ min with manual shimming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
- 6. FSV load increase from 25% at 5%/ min with manual shimming, decalibration, and rod prohibit at 120% power, rods half in . . . 14
- 7. FSV single-loop trip with accurate power measurement. . . . . . . 16
- 8. FSV single-loop trip with nuclear instrument decalib ration. . . . 17
- 9. FSV turbine trip f rom full load with accurate power taeasurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
- 10. FSV turbine trip from full load with nuclear instrument decalibration . . . . . . . . . . . . . . . . . . . . . . . . . . 19
- 11. Minimum required starting power for RWA to attain 140% true power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
- 12. Trip setpoint ve rsus indicated power envelope for the initial cycle . . . . . . . . . . . . . . . . . . . . . . . . . . 37
- 13. PPS floating trip circuit (typical of six places) . . . . . . . . 44
- 14. IIeat balance calibration . . . . . . . . . . . . . . . . . . . . 45
- 15. FSV floating reactor trip on high reactor power (example of operation showing startup, load-changing, load reduction to 20% power, and rod withdrawal accident) . . . . . . . . . . . . . 46
- 16. PPS floating trip point circuitry for Fort St. Vrain Unit 1 . . . 48
- 17. Nuclear channel test setup, block diagram . . . . . . . . . . . . 51
- 18. PPS channel configuration . . . . . . . . . . . . . . . . . . . . 52
- 19. Recommended program for trip and RWP setpoints. . . . . . . . . . 59 iv
TABLES
- 1. FSV initial core: detector response to sequential rod bank withdrawal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
- 2. Definition of starting point designations and data for 14 assumed starting points, f rom which single-rod withdrawals are analyzed . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
- 3. Detector decalibration due to single-rod withdrawals . . . . . . . 26
- 4. Analysis of single-rod withdrawals . . . . . . . . . . . . . . . . 29
- 5. Detector decalibration following single-rod withdrawals from 14 assumed starting points . . . . . . . . . . . . . . . . . . . . 31
- 6. RWA starting points analyzed in addition to Table 4 cases . . . . 39
- 7. Failure modes and effects analysis . . . . . . . . . . . . . . . . 53 v
SUMMARY
Measurement of the neutron flux level in the Fort St. Vrain (FSV) HTGR core is accomplished by neutron detectors located outside the core in the prestressed concrete reactor vessel (PCRV) immediately adjacent to the core cavity, as shown in Fig. 1. Signals from ex-core detectors are used to monitor core conditions during startup, rise-to power, and normal power operations. These signals are also used in the automatic control system to regulate power level and power changes, to initiate plant protective system (PPS) action at limiting safety system settings on power levels, and to pro-vide input to the megawatt-hour meter and the power level indicator on the operator control panel.
Analyses of the as-assembled FSV initial core, which have now largely been corroborated by operating data, indicate that decalibration of the power-range detectors can occur due to motion of the control rod banks.
Control rod motion can alter the radial core power distribution so that the neutron flux levels seen by the power-range detectors are not directly pro-portional to the true core thermal power level. The detector responses are dependent upon rod bank position. For example, the withdrawal of a rod bank near the core centerline causes the detectors to underpredict the true power changes, while outer bank withdrawal results in overprediction of true power changes. The reason for this behavior is that a majority of the neut.ons reaching the detectors from the core originate in the outer core regions. The relative power level in these outer regions can be changed by rod motion, in a manner which does not accurately reflect the resultant average core power ch:rnge. The deviations of readings of the six detectors, due to the seq'iential withdrawal of one, two, or three control rod banke, are shown in Table 1.
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2
TA'LE 1 FSV INITI AL CORE: DETECTOR RESPONSE TO SEQUENTI AL ROD BANK WITHDILWAL IChannel Indicated Power )(a) l Source
/ ~ l Av True Pcwer gg og Bank (s)
Withdrawn III IV V VI VII VIII Six Data 3A 1.45 1.38 1.19 0.90 0.87 0.96 1.13 Calculation 1.3 1.3 1.1 0.9 0.9 1.0 1.08 Measurement 4D 0.98 0.97 0.96 1.65 1.80 1.77 1.36 Calculation 0.98 0.98 0.97 1.54 1.65 1.64 1.29 Measurement 3B 0.88 0.81 0.86 0.99 1.07 0.93 0.92 Calculation 0.94 0.89 0.93 0.97 1.02 0.99 0.96 Measurement 4E 1.13 1.19 1.00 1.01 1.17 1.30 1.13 Calculation 1.18 1.12 1.02 1.13 1.22 1.35 1.17 Measurement 4A 1.78 1.60 1.57 0.93 0.91 0.96 1.39 Calculation 4C 0.93 0.93 0.92 1.58 1.43 1.36 1.19 Calculation 3D 0.92 0.89 1.02 0.91 0.93 0.85 0.92 Calculation Reg rod 0.97 0.97 0.98 0.98 0.97 0.97 0.91 Calculation (115-190fn.)
3A+4D 1.27 1.27 1.07 1.39 1.48 1.64 1.35 Measurenent 41n3B 0.92 0.87 0.90 1.49 1.68 1.62 1.25 Measurement 3B+4E 1.11 1.00 0.95 1.10 1.24 1.34 1.12 Measurement 4E-4A 2.10 1.79 1.60 1.05 1.11 1.30 1.49 Meas /caleth) 4A+4C 1.65 1.49 1.44 1.47 1.30 1.31 1.44 Calculation 4C+3D 0.86 0.83 0.94 1.44 1.33 1.16 1.09 Calculation 1D&RR 0.89 0.86 1.00 0.89 0.90 0.82 ~0.89 Calculation 3A+4D&3B 1. "] 1.13 1.00 1.35 1.51 1.62 1.30 Measurement 4D&3B+4E 1.09 0.98 0.92 1.69 2.05 2.19 1.49 Measurement 3B+4E+4A 1.97 1.59 1.49 1.02 1.13 1.28 1.41 Meas /cale(b) 4E+4A+4C 1.95 1,67 1.47 1.66 1.59 1.76 1.68 Meas /cale(b) 4A*4CF3D 1.52 1.32 1.47 1.34 1.21 1.11 1.31 Calculation 4C+1D+RR 0.92 1.41 1.29 1.12 1.06 Calculation 0.83 [ 0.80 (a)It is assumed that all detectors are calibrated to read heat-balance power when the bank (s) are fully inserted. The figures represent the decalibration that would occur when the bank (s) are fully withdrawn if there were no intermediate recalibration of the detectors.
(b) Meas /cale indicates that measured decalibration factors were used where available, and calcul etonal results were used where no measurements vero available.
3
The results of the detector decalibration analysis are summarized in this report along with recommended circuit designs to supplement the current single-level trip setpoint with a " floating" trip setpoint. Thfu recom-mended change will ensure compliance with technical specifications and NRC regulations and will not impair the control and operation of the plant.
Trip settings, as a function of power indicated by the detectors, are speci-fled here for the initial cycle only. However, the same methods will be used to determine trip settings for subsequent fuel cycles.
1.2. LOCATION AND FUNCTION OF POWER RANCE DETECTORS The locations of the ex-core detectors are shown in Fig. 1. The power range detectors, 12 in all, are located symmetrically at 60-deg intervals around the core in steel-lined wells in the PCRV. The power range detectors serve two functions:
- 1. The six PPS detectors are fission chambers, one located in each of the six wells shown in Fig. 1 (identified as NE-1133 through NE-1138) at about the core axial midplane. Their range is from 1.5%
to 150% of full power. The six detector signals are combined into three channels for PPS use, each channel combining the signals from two 180-deg-opposed detectors. Thus, NE-1133 and NE-1136 feed channel A, FE-1134 and NE-1137 feed channel B, and NE-1135 and NE-1138 feed chanr.el C. These chan.,els provide a trip signal at 140% of full power on a two-out-of-three channel logic; each channel '.s tripped on a single high detector reading. Sigrals from these six detectors are also fed to six separate indicators on the operator control panel.
- 2. Plant control detectors are a separate set of six fission chambers with a range from 1.5% to 150% of full power located in the same wells as the linear range detectors at about the same axial posi-tion. Signals from these detectors (identified as NE-1133-2 through NE-1138-2) are averaged to form a single input to the flux controller NC-1199 (Ref. 1) which, in turn, regulates the position 4
of the central control pod pair and runback
- rod group to control the power level in the core. The averaged flux level is also sent to the flux recorder (NR-1199), the flux integrator (NM-1199), and the power / flow module (XMS-11262). The flux integrator, in turn, feeds the megawatt-hour meter (NQ-1199) (Ref. 1).
In addition to the PPS trip setpoints, the rod withdrawal prohibits (RWPs) will be affected by detector decalibration. One of the RWPs is acti-vated on high reactor power (120%) with signals from the linear range detec-tors in the same manner as the PPS trip signal at 140% of full power. The other RWPs are activated if the power level is not within the range per-mitted by three positions of the interlock sequence switch (ISS). The pur-pose of the RWPs is to ensure that the correct sequence of protective actions is engaged during the rise to full power. The ISS incorporates three positions with RWP functions:
1 Startup. An RWP will be encountered at 5% power to ensure startup with adequate neutron flux indication and proper rate of increase in the flux. This RWP is activated by a high signal from two nonopposite linear range detectors and can be cleared by switching to Low Power mode.
- 2. Low Power. An RWP will be encountered in this mode if two non-opposite linear power detectors indicate power levels above 30%
of full power. This prohibit can be cleared by switching to the Power Operation mode, which activates the PPS trips required for PPS action. Once in the Power Operation mode, power must be in-creased to the point at which all six linear power detectors indicate greater than 30%; otherwise, a subsequent reduction be-low 30% power will reactivate this RWP.
- Runbuck rods are rods from two rod groups (six rod pairs) preselected for automatic insertion when the plant control system demands a power reduction of more than 10 T' .
5
- 3. Power Operation. Once all detectors indicate above 307. of full power, the RWP on low power will only be activated if the power level drops below 107.. This permits normal operation a nd load changes with power overshoot, while still ensuring correct protec-tive function action following a shutdown or reduction to a power level below the range where electrical power is produced.
6
- 2. ANALYSIS OF DETECTOR DECALIBRATION 2.1. INTRODUCTION The problems associated with the ex-core detector decalibration can be divided into two main areas: safety and control. From a safety standpoint, all PPS actions must occur at or below the setpoints given in the technical specifications (Ref. 2). From the control standpoint, decalibration of the detectors must not interfere with the ability of the plant to make planned load changes.
2.2. EFFECT OF DETECTOR DECALIBRATION ON PLANT CONTROL The plant control loop involving the ex-core detectors is illustrated in Fig. 2. As shown in this figure, primary control of reactor power is on reheat steam temperature (RHST) and turbine generator first-stage pressure.
Analysis of this control loop indicates that the error sensor on averaged measured power can tolerate averaged power sensor offsets of from 31.5% to
-19.5%, with essentially no lag or overshoot during load changes.
Larger offsets can be handled but require a somewhat longer time (of the order of minutes) for the reactor power to come to equilibrium following a load change.
To check the response of the centrol system with the measured and cal-culated detector decalibration, the following plant transients were analyzed using the TAP code (Ref. 3): ramp load changes, loop trip, turbine trip, 8-hr xenon buildup, and repositioning of the regulating rod accompanying movement of a shim rod bank. The examples of plant behavior discussed in this section assume the originally proposed PPS and control configuration when an RWP is set at 120% of full power and a trip is set at 140%.
7
MEASURED HOT REHEAT STEAM TEMPE RATU RE (RHST) r 1 r 1 ri 1r3r1r RHST AVERAGE MEASURED SETPOINT RHST (1000 F)
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Fig. 2. Flux detector control loop 8
Figure 3 shows a ramp load decrease from 100% to 75% load at a rate of 5%/ min in which the reactor power measurement was assumed to be accurate (zero error). Xenon effects were negligible in such a short-term transient and were ignored. '8cweve r , in subsequent runs evaluating nuclear instrument errors, xenon effects were included since counteracting rod motion was re-quired which could add to flux profile changes and, therefore, to the errors in the reactor power measurement.
The nuclear instrument error as a function of regulating rod position was added to the model, and the load decrease transient of Fig. 3 was re-peated assuming that the nuclear instruments had been calibrated at rated load just prior to beginnlag the load change. As can be seen by comp:. ring the resulting transient (Fig. 4) to the reference transient (Fig. 3), intro-duction of nuclear instrument error does not significantly affect the plant transient response. The reheat steam temperature control sytem provides co rection for this error in the following manner:
1 The measurement error initially causes a slight misadjustment of reactor power which results in a slight misadjustment of steam generator inlet helium temperature. This, in turn, results in a small reheat steam temperature error.
- 2. The reheat steam temperature controller generates the reactor power setpoint signal required to drive the reheat steam tempera-ture error to zero. Thus, the integral term of the controller (essentially a capacitor) accumulates an offset equivalent to the measurement error. This setpoint signal permits adjustment of the reactor power to the proper level despite the measurement error.
In this case, the nuclear instrument error causes a reheat steam tem-perature error of less than 4*F, which represents an insignificant change in the reheat steam temperature transient. However, this slight change in the temperature error provides sufficient information for the reheat steam tem-perature controller to correct for the nuclear instrument error as described above.
9
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Important reactor parameters for a similar load change between 100% and 25% of full power are shown in Figs. 5 and 6. In this exampic, it was assumed that the flux detectors were calibrated prior to the load reduction and that the control signal, which is the average of six detectors, was decalibrated linearly by 30% during the load change. It was also assumed that the load change was accomplished under automatic control by regulating rod-pair motion with operator adjustment of the shim bank as necessary to maintain the regulating rod-pair position within its normal operating range.
The results in Figs. 5 and 6 show no adverse (i.e., unsafe) effects due to detector decalibration. A 30% decalibration for the average signal was conservatively chosen as larger than any value anticipated during this type of load change.
If the control rod position changed (e.g., due to xenon effects) or the detectors were recalibrated at low power, then during a subsequent return to full power a decalibration of the detectors would occur. If this happened to be a downward decalibration, the plant control system would accomplish the change in a normal fashion except for the detector meter readings. An upward decalibration would be handled in a similar fashion, except that an RWP would occur at 90% of true power for a 30% decalibration (%120% of the indicated power); the power level would remain there until the detectors were recalibrated, at which time the load change would be completed. No RWP would occur for decalibration less than 20%.
Following a change in the reactor power level, the xenon concentration undergoes a prolonged transient (peaking af ter 8.5 hr), during which signif-icant reactivity changes occur. The regulating rod is repositioned by the flux controller to maintain the desired flux level, but such rod movement causes further decalibration of the nuclear sensors. The xenon transient that follows the ramp load decrease of Fig. 4 was simulated with the xenon concentration increasing at an average rate of about 20 times the expected value in order to model the 8.5-hr period within a single TAP code (Ref. 3) run. Neither the xenon transient nor the nuclear instrument errors result-ing from regulating rod motion cause significant plant parameter upset.
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During upset transients which give a difference larger than 10% between setpoint and indicated power, a group of power runback rods are inserted simultaneously with the regulating rod to accomplish a rapid power reduction.
The nuclear instrument errors caused by the group rod movement, conserva-tively estimated as up to 29%, could add to that caused by the regulating rod movement. To assess these effects, the loop trip and turbine trip transients were considered. Figures 7 and 8 show loop trip transients Lefore and after the addition of the combined decalibration effects. Figures 9 and 10 show turbine trip transients before and after the addition of the combined decal-ibration effcets. In both cases, initial calibratJ<1 s assumed and the worst-case decalibration for the rod group was use e., bank 4A, which is directly in front of the nuclear instrument, causes the most decalibration.
The similarity of the before and after figures demonstrates that the control system adequately corrects for the nuclear instrument errors.
From the above discussion, it is concluded that the introduction of nuclear instrument errors of the magnitude expected in the FSV plant has no significant effect on control system functions. This means that these errors will cause neither control instabilities nor any significant increase in expected temperature excursions provided the controllers are reasonably well tuned initially. In addition, the load change capability of the plant is not impaired by nuclear instrument errors.
2.3. EFFECT OF DETECTOR DECALIBRATION ON PPS ACTION This section describes the analysis done to identify the extent of detector decalibration that could occur due to any sequential rod withdrawal or any single-rod withdrawal accident. A technique for defining a channel floating trip setpoint envelope as a function of indicated power is devel-oped. The technique can be summarized as follows:
- 1. Find the decalibration factors (DF) defining the initial condition for rod withdrawal accidents. This is done by evaluating the decalibration factors for the six detectors for various combina-tions of detector calibration and subsequent motion of rod groups 15
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\g\ 0
\ 1 0 .
0 g 0 0 i 5 0 5 1 1 F 84x o 6 xy2 mS5m Qga2 5$Sg g
in sequence (Section 2.3.3.1). The contribution of the normal motion of rod groups following calibration is the larger part of the total decalibration factor DF.
- 2. Find the maximum power level attainable for each rod pattern (Sec-tion 2.3.3.2).
- 3. Find the minimum rod worth necessary to take the power from the initial power level to a true power of 140% rated (Section 2.3.3.2).
- 4. For each initial condition (i.e., each starting rod configuration),
find the decalibration factor for each detector caused by the withdrawal of a single rod (Section 2.3.3.2). This is the con-tribution to the DF of the rod withdrawal accident.
S. Using combinations of DF from the initial condition and from the rod withdrawal accident, determine the total DF for each detector (Section 2.3.3.3).
- 6. Using total DF for each initial condition and RWA combination, determine the trip setting that will ensure reactor trip at 140%
true reactor power (Section 2.3.4).
2.3.1. Measurement and Calculation of Detector Decalibration Factors Detector decalibration factor is defined as the ratio of a detector's indicated power level to the concurrent heat-balance power level (or true power). Detector decalibration factors, such as those shown in Table 1, hrve been determined by two methods:
- 1. Measurements have been taken as a part of a special startup test (Ref. 4), with well over 100 data points gathered to date for each linear detector. The measurements will continue throughout the B-series rise-to-power tests.
20
- 2. Analytical calculations have been made for over 100 different control rod configurations and core conditions.
As indicated in Table 1, the calculations have agreed with measured values to within 5% to 10%.
Measurements are made about three times a day during periods of power operation. A primary-side heat balance is arformed and ratios are taken to the values from the six linear detectors. For each detector, the ratios are plotted versus rod bank position, and from these plots, the detector decali-bration can be determined for the sequential withdrawal of any rod bank.
For rod groups whose decalibration factors have not yet been measured, the calculation of detector decalibration factors involves two steps. First, a diffusion calculation is performed using the GAUGE code for the subject rod configuration and core conditions. Column-wise power factors are tabu-lated; these factors are normalized to an average core power generation rate.
The power factors are multiplied by a set of influence coefficients, and the sum of these products is proportional to the signal that the detector will register for the given conditions. The ratio of detector signal to true power at any rod configuration is the decalibration factor for that rod configuration with respect to the rod configuration when the detectors were calibrated. (The calibration makes the ratio of maannel indicated power to true power equal to unity.)
Influence coefficients, a , are a set of numbers relating column-wise power factors to the response of each detector:
247 R = a *P i Z. ij j j=1 where R =
response of detector 1, P. =
radial power peaking factor for fuel column j J
= column power density / core average power density.
21 ,
These influence coefficients a are derived from a point kernel transport calculation.
The calculated decalibration factors given in the tables in this report are listed simply as a function of control rod position. The justification for this is as follows. Diffusion calculations which included the effect of core burnup and the buildup of neutron poisons have shown that the decalibra-tion factors are insensitive to changes other than control rod position.
The detector signals are calculated to change by less than 2% during the initial core burnup when the calculations are performed with a single-rod configuration. Clearly, the effect of core depiction on the detector sig-nals is insignificant during the nominal 24-hr period between detector recalibrations. Likewise, short-term changes in the xenon inventory have a very small effect on detector accuracy apart from the possible countering motion required of the control rods to maintain power. Even during extreme transients, the effect of xenon itself on the power distribution can change the detector accuracy by only 2%. In contrast, the countering rod motion can change the accuracy of individual detectors by over 30%.
2.3.2. Detector Decalibration Due to Sequential Rod Withdrawal Table 1 shows the measured and calculated decalibration factors due to the sequential withdrawal of one, two, or three rod banks. These results are most easily understood when seen together with Fig. 1. When a rod is withdrawn near a detector (e.g., rod 26 of bank 4A near detector 3), the neutron flux near the detector increases more than the average core power; hence, the detector overestimates the power change. In contrast, the regu-lating rod, for example, is distant from all detectors. Its withdrawal causes the neutron flux near the detectors to increase less than the average core power; hence, its withdrawal causes an underprediction of power change on all six detectors. The results from Table 1 are considered in Section 2.3.4 for fixing acceptable PPS trip setpoints.
22
2.3.3. Detector Decalibration Due to Single-Rod Withdrawal Accidents In this section, results of analyses of detector decalibrations that occur due to over 100 selected rod withdrawal accidents (RWAs) are presented.
First, the decalibration that can exist at the beginning of an RWA is dis-cussed. Then the decalibration arising from the withdrawal of selected single-rod pairs from a number of initial rod configurations is tabulated.
Finally, the initial conditions and RWAs are combined to determine appro-priate trip setpoints as a function of power level indicated by the detector.
In deriving the trip setpoints, it is assumed, even though it is a very unlikely event, that the most sensitive channel has failed in a nontripped mode. Therefore, tripping of all three channels instead of the usual two out of three is assumed necessary.
2.3.3.1 Possible Starting Conditions for RWAs. Table 2 shows the 14 start-ing points that have been used in this analysis. Two variables define the starting point-
- 1. The rod configuration.
- 2. The decalibration in each detector that initially exists due to rod motion since the last calibration of the detectors.
For the rod configuration variable, seven rod configurations are con-sidered: seven banks inserted (through groun 3A), six banks inserted, etc.,
until only hank 3D is inserted. This spans all rod configurations normally expected from startup to full power. In addition, test measurements con-ducted as part of the rise-to-power program have shown that the decalibra-tion factors change nearly linearly with rod bank position during the with-drawal of a rod bank. Thus, the choice of a partially withdrawn bank as the starting point configuration will not produce worst-case results. For the initial decalibration variable, two cases are chosen for each of the seven starting point rod configurations. The seven " primed" cases in Table 2 assume that the RWAs are initiated with all detectors calibrated to true 23
TABLE 2 DEFINITION OF STARTING POINT DESIGNATIONS AND DATA FOR 14 ASSUMED STARTING POINTS, FROM WilICll SINGLE-ROD WITIIDRAWALS ARE ANALYZED
/Channel Indicated Power,*
)
Starting Rod
- l Maneuvers That Cause Point Banks k epm / Starting Point Designation Inserted (") III IV V VI VII VIII Decalibrations A Through 3A 0.79 0.79 0.93 0.72 0.68 0.61 Calibrate detectors, then insert 4D, 3A A' Through 3A 1.00 1.00 1.00 1.00 1.00 1.00 Calibrate detectors with 3A in B Through 4D 0.92 1.02 1.09 0.59 0.49 0.46 Calibrate detectors, then insert 4E, 3B, 4D 3' 1hrough 4D 1.00 1.00 1.00 1.00 1.00 1.00 Calibrate detectors with 4D in C Through 3B 0.51 0.63 0.67 0.98 0.88 0.78 Calibrate detectors, then insert 4A, 4E, 3B C' Through 3B 1.00 1.00 1.00 1.00 1.00 1.00 Calibrate detectors with 3B in D Through 4E 0.51 0.60 0.68 0.60 0.63 0.57 Calibrate detectors, then insert 4C, 4A, 4E D' Through 4E 1.00 1.00 1.00 1.00 1.00 1.00 Calibrate detectors with 4E in E Through 4A 0.61 0.67 0.69 0.68 0.77 0.76 Calibrate detectors, then insert 4C, 4A E' Through 4A 1.00 1.00 1.00 1.00 1.00 1.00 Calibrate detectors with 4A in F Through 4C 1.08 1.08 1.09 0.63 0.70 0.74 CalibraN d3tectors, then insett 4C F' Through 4C 1.00 1.00 1.00 1.00 1.00 1.00 Calibrate detectors with 4C in G Through 3D 0.93 0.93 0.92 1.58 1.43 1.36 Calibrate detectors, then withdraw 4C G' Through 3D 1.00 1.00 1.00 1.00 1.00 1.00 Calibrate detectors with 3D in (a)This column indicates rod configuration; e.g., Through 4A denotes that all rods in the normal sequence are withdrawn through group 4E. This means that groups 4A, 4C, and 3D are fully inserted and the rod pair in region 1 is withdrawn 115 in.
24
power. The seven other cases assume that the detectors are initially decal-ihrated in the worst manner, i.e., that the detectors underpredict power to the greatest extent possible. The maneuvers required to give these starting point decalibrations are shown in the right-hand column of Table 2.
- 2. 3. 3. 2. RWAs Investigated, Decalibration Due to RWAs. In addition to the possible initial decalibration discussed above, the detectors may be further decalibrated due to the change in power distribution as an RWA occurs. This decalibration due to the RWA itself is shown in Table 3 for 27 different single-rod withdrawals. The selection of the rods analyzed for each start-ing point was dictated by two guidelines:
- 1. Select rods located in the various radial rings of the core:
regulating rod, third ring, and fourth ring. (All second-ring rods are withdrawn early in the rod withdrawal sequence.)
- 2. Select rods whose withdrawal should most delay the PPS trip.
Also shown in Table 3 are the rod worths, Ap, for the RWAs. These values, together with data from Ref. 5 as shown in Fig. 11, were used to determine the initial power level required for the RWA to attain 140% true power. The required power levels are listed in Tables 3 and 4.
2.3.3.3. Total Decalibration for RWA: Initial Plus Withdrawal. A combina-tion of the 14 possible starting point conditions and the 27 different RWAs yields 52 RWA cases, as shown on Table 5. The table shows the decalibration factors that would exist af ter each of the RWAs.
2.3.4. Determination of PPS Trip Setpoints The results from Table 5 show that in certain situations the PPS trip setpoints must be lowered from 140% to ensure that all three PPS channels reach their setpoints before true power exceeds 140%. The limiting case is 25
TABLE 3 DETECTOR DECALIBRATION DUE TO SINGLE-ROD WITllDRAh \LS Minimum Initial True Power Level From Which
[ Channel Indic. Power \
- * ' se True Power to Rod Banks Initiall Rod (b) Ap Rod Inserted a) Withdrawn (BOC)(C) III IV V VI VII VIII BOC EOC None Reg. 0.0034 0.97 0.97 0.98 0.98 0.97 0.97 118% 70%
3D Reg. 0.0038 0.94 0.95 0.98 0.97 0.96 0.94 105 65 11 0.0093 1.43 0.66 0.83 0.69 1.26 0.90 52 <10 V 15 0.0089 0.79 1.46 0.69 0.95 0.63 1.22 54 <10 Through 4C Reg. 0.0050 0.93 0.94 0.96 0.96 0.95 0.93 92 37 19 0.0117 0.58 0.69 1.70 1.34 0.87 0.58 25 <10 u 34 0.0052 0.72 1.13 0.95 2.49 0.76 0.84 90 35 Through 4A Reg. 0.0060 0.93 0.94 0.95 0.96 0.94 0.93 82 20 11 0.0125 1.69 0.61 0.70 0.59 1.31 0.77 20 <10 26 0.0038 2.39 0.85 0.79 0.82 0.92 1.05 105 65 U 34 0.0058 0.72 1.05 1.09 2.54 0.68 0.82 85 25 Through 4E Reg. 0.0067 0.92 0.93 0.94 0.95 0.93 0.93 75 <10 11 0.0094 1.60 0.68 0.64 0.67 1.23 0.95 52 <10 26 0.0039 2.81 0.84 0.65 0.79 0.93 1.18 102 60 V 30 0.0043 0.91 1.50 0.63 0.90 0.79 1.64 95 50 Through 3B Reg. 0.0090 0.90 0.90 0.91 0.92 0.92 0.91 55 <10 y 13 0.0092 1.37 0.85 0.70 0.72 0.84 1.47 52 <10
TABLE 3 (continued)
Minit :m Initial True Power Level From Which IChannel Indic. Power \ se True Power to Rod Banks -
i Initia11 Rod (b) \
Inserted a) Withdrawn 60 R (BOC) d)
c III IV V VI VII VIII BOC EOC Through 3T, 26 0.0056 3.35 0.79 0.72 0.70 0.99 1.25 85% 25%
h 30 0.0084 0.77 1.75 0.63 0.81 0.64 2.08 60 <10 Through 4D Reg. 0.0095 0.90 0.88 0.89 0.91 0.91 0.90 50 <10 13 0.0091 1.43 0.80 0.66 0.70 0.83 1.63 52 <10 w 29 0.0009 0.99 1.03 0.94 0.97 0.95 2.03 132 120 u
V 30 0.0055 0.82 1.60 0.74 0.93 0.75 1.58 87 30 Through 3A Reg. 0.0115 0.89 0.87 0.88 0.90 0.91 0.89 30 <10 13 0.0071 1.32 0.92 0.72 0.67 1.09 1.64 ?? <10 29 0.0011 0.99 1.08 0.93 0.84 0.94 2.26 128 117 V 30 0.0059 0.83 2.07 0.71 0.82 0.94 1.84 82 22
( This column indicates rod configuration; e.g., Through 4A denotes that all rods in the normal sequence are withdrawn through group 4E. This means that groups 4A, 4C, and 3D are fully inserted and the rod pair in region 1 is withdrawn 115 in.
(b) Reg. rod is withdrawn from 115 in. to full-out; all others from full-in to full-out.
(C BOC = beginning of cycle.
(d)It is assume 6 that all detectors are calibrated to read heat-balance power level befort the rod withdrawal. The figures represent the decalibrations caused by the rod withdrawal with no inter-mediate recalibration of the detectors.
0012 -
R00 WILL ALWAYS WITHOR AW 0 011 -
LESS THAN FULL STROKE BEFORE TRUE POWER REACHES 140%
0.010 -
m 0.009 -
8 BEGINNING OF CYCLE
{
0.008 -
5 5
m 0.007 -
AT SOME TIME IN CYCLE FULL S R00 WITHO RAWAL WILL CAUSE o 0.006 - TRUE POWER TO REACH 140%
E ra w
$ 0.005 -
END OF CYCLE 5 0.004 -
5
=
0.003 0.002 -
R00 WITH0RAWAL WILL NOT CAUSE TRUE POWER TO 0.001 - REACH 140%
I ' ' '
0 O 20 40 60 80 130 120 140 POWER (%)
Fig. 11. Minimum required starting power for RWA to attain 140% true power
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%s Pawer Man Fraction of Min Initial True Is This Min Attainable at Seeded tn Attain Safe FPS Trip Starting Power from Which This Initial Power This Starting Tia True P ue r . Setting Basis:
Point Rod i. Can Attain 140t At t a i n.ib l e .i t P. i nt Frew M in Starting Trip cf 3rd PPS Designation Bank Rod u;
- e Power This Startinc Power Channel at 140?.
True ICI Temp Coeff.) True Nwer (Table 2) Inserted *# Withdrawn Rod EOC Temp Coeff.) Feint ? InJic. Av (SOC F' 4C Reg. 0.0050 40 Yes 100 100 0.840 1 34 19 0.0117 <10 Yes 100 100 0.359 13J 34 0.0052 35 Yes 100 100 0.808 1 34 W G 3D Reg. 0.0039 65 Yes 100 100 1.0 3140 O 11 0.0093 <10 Yes 100 >100 0.452 >140 15 0.0039 <10 Yes 100 3100 0.472 >140 C' 3D Reg. 0.0038 65 Yes 100 100 1.0 1 34 11 0.0093 <10 Yes 100 100 0.452 133 15 0.0089 <10 Yes 190 100 0.472 137
- None Reg. 0.0034 70 Yes 1bJ 100 1.0 135 (a)This colu:nn indicates rod configurat tun; e.g. , M denotes that all rods in the normal sequence are withdraw through group 4F. This reans that droups 4A, 4C, and 3D are fully inserted and the rod pair in region 1 is withdrawn 115 in.
(b)EOC = end of cycle.
W = beginning of cycle.
TABLE 5 DETECTOR DECALIBRATION FOLLOWING SINGLE-ROD WITHDRAWALS FROM 14 ASSUMED STARTING POINTS Starting Channel Indic. Power Point Rod Rod Designa- Banks With-( True Power /
tion Inserted drawn III IV V VI VII VIII A Through 3A Reg. 0.70 0.69 0.82 0.65 0.62 0.54 13 1.04 0.73 0.67 0.48 0.74 1.00 29 0.78 0.85 0.86 0.60 0.64 1.3E 30 0.66 1.64 0.66 0.59 0.64 1.12 A' Through 3A Reg. 0.89 0.87 0.88 0.90 0.91 0.89 13 1.32 0.92 0.72 0.67 1.09 1.64 29 0.99 1.08 0.93 0.84 0.94 2.26 30 0.83 2.07 0.71 0.82 0.94 1.84 B Through 4D Reg. 0.83 0.90 0.97 0.54 0.45 0.41 13 1.29 0.82' O.72 0.41 0.41 0.75 29 0.89 1.05 1.02 0.57 0.47 0.93 30 0.74 1,63 0.81 0.55 0.37 0.73 B' Through 4D Reg. 0.90 0.88 0.89 0.91 0.91 0.90 13 1.43 0.80 0.66 0.70 0.83 1.63 29 0.99 1.03 0.94 0.97 0.95 2.03 30 0.82 1.60 0.74 0.93 0.75 1.58 C Through 3B Reg. 0.46 0.57 0.61 0.90 0.81 0.71 13 0.70 0.54 0.47 0.71 0.74 1.15 26 1.71 0.50 0.48 0.69 0.87 0.98 30 0.39 1.10 0.42 0.79 0.56 1.62 C' Turough 3B Reg. 0.90 0.90 0.91 0.92 0.92 0.91 13 1.37 0.85 0.70 0.72 0.84 1.47 26 3.35 0.79 0.72 0.70 0.99 1.25 30 0.77 1.75 0.63 0.81 0.64 2.08 D Through 4E Reg. 0.47 0.56 0.64 0.57 0.59 0.53 11 0.82 0.41 0.44 0.40 0.77 0.54 26 1.43 0.50 0.44 0.47 0.59 0.67 30 0.46 0.90 0.43 0.54 0.50 0.93 D' Through 4E Reg. 0.92 0.93 0.94 0.95 0.93 0.93 11 1.60 0.68 0.64 0.67 1.23 0.95 26 2.81 0.84 0.65 0.79 0.93 1.18 30 0.91 1.50 0.63 0.90 0.79 1.64 E Th ro ugh 4 A Reg. 0.57 0.63 0.66 0.65 0.72 0.71 11 1.03 0..' O.48 0.40. 1.01 0.59 26 1.46 0.57 0.55 0.56 0.71 0.80 34 0.44 0.70 0.75 1.73 0.52 0.62 31
TABLE 5 (continued) nne n c. Power Starting : -
Point Rod Rod g True Power j Designa- Banks With-tion Inserted drawn III IV V VI VII VIII E' Through 4A Reg. 0.93 0.94 0.95 0.96 0.94 0.93 11 1.69 0.61 0.70 0.59 1.31 0.77 26 2.39 0.85 0.79 0.82 9.92 1.05 34 0.72 1.05 1.09 2.54 0.68 0.82 F Through 4C Reg. 1.00 1.02 1.05 1 0.60 0.67 0.69 19 0.63 0.75 1.85 0.84 0.61 0.43 34 0.78 1.22 1.04 1.57 0.53 0.62 F' Through 4C Reg. 0.93 0.94 0.96 0.96 0.95 0.93 19 0.58 0.69 1.70 1.34 0.87 0.58 34 0.72 1.13 0.95 2.49 0.76 0.84 G Through 3D Reg. 0.87 0.88 0.90 1.53 1.37 1.28 11 1.33 0.61 0.76 1.09 1.80 1.22 15 0.73 1.36 0.63 1.50 0.90 1.66 C' Through 3D Reg. 0.94 0.95 0.98 0.97 0.96 0.94 11 1.43 0.66 0.83 0.69 1.26 0.90 15 0.79 1.46 0.69 0.95 0.63 1.22 32
the withdrawal of rod 11 from the starting point designated D in Table 2.
The decalibration factors from Table 5 are listed below:
- Detector :
III IV V VI VII VIII 0.82 0.41 0.44 0.40 0.77 0.54 When the opposite detectors are grouped and their signals auctioneered, the three PPS logic channels would register signals with the following decalibrations:
A (III or VI) B (IV or VII) C (V or VIII) 0.82 0.77 0.54 Thus, for all three channels to trip before true power exceeds 140%, the setpoints must be 1 (140%
- 0.54) = 75.6%.
The remaining cases from Table 5 are treated likewise. The results are summarized in Table 4, which incorporates several additional considera-tions. Column 5 of Table 4 shows the minimum initial true power from which the RWA can attain 140% true power; the values were determined using Fig. 11 and are also listed in Table 3. Figure 11 gives an overestimate (conserva-tive) of the reactivity required to reach 140% power as a function of power for beginning of cycle (BOC) and end of cycle (EOC) (first cycle). This information was obtained from Ref. 5, in which the results of a series of point kinetics calculations were reported. From these calculations at both BOC and EOC, the time required to reach 140% power was converted to the reactivity required to reach 140% power from a knowledge of the reactivity input as a function of time used in the point kinetics calculations for each case. The reactivity input to the point kinetics calculations as a function of time is in the shape of the typical S curve to represent the changing reactivity insertion rate as a function of control rod withdrawal distance (which i; proportional to time). To make the results used for the detector decalibration analysis conservative, the data represented by the two straight 3'aes it. Fig. 11 were obtained by assuming that the reactivity 33
insertion rate is linear with time. The time to reach 140% power was in all cases less than 110 sec, which is about half the time required to withdraw a rod pair; hence, the actual reactivity required to reach 140% power will be less than that given by the two curves of Fig. 11.
Column 7 of Table 4 lists values of maximum power attainable at each starting point. These values were determined from calculations such as those described in Ref. 6. Reactivity calculations were done for various power levels and various times in life, using the measured and calculated values for the following:
1 Control rod worth cumulative to the starting point rod position.
- 2. Control rod worth of rods inserted or withdrawn since last cali-bration point.
- 3. Temperature defect as a function of temperature.
- 4. Worth of Pa-233 decayed to U-233.
The calculations conservatively assumed that no xenon was present at the starting point and that all Pa-233 had decayed. With these assumptions, the most reactive core conditions were used; thus, the assumed starting point power level was as high as possible.
Values in .iumns 5 and 7 of Table 4 are then cross-compared to deter-mine which of the RWA cases can attain 140% true power, and that is indi-cated in column 6. Ten of the cases are thus eliminated from consideration tecause it is not possible to attain the 140% true power limit.
For the remaining RWA cases, it is possible, at some time during the reactor cycle, to reach 140% power. In these cases, high worth rods are to be withdrawn from high initial power levels. In many instances, true power 34
will reach 140% of its rated value before the rod is fully withdrawn. The total decalibration factor for these cases has been calculated as
[^P 140 F ~ ~
D DO (ApRWA),
where F a ca N ation factor when t m power reades 140% power, D
F = initial deca 1 & ation factor ham the starting point of 99 Table 2, f = full-stroke RWA decalibration factor from Table 3, Ap = Ap required to reach 140% power (Fig. 11),
140 A r w A m a es 3 or 4.
RWA For cases in which the rod being withdrawn will cause a trip before reaching the fully withdrawn position, the most restrictive setpoint is found when otarting at the maximum power attainable (given in column 7 of Taole 4).
For example, using starting point C' from the highest power attainable, 100%, an RWA on rod 30 yields a trip point of 127%. When starting from 10%
power, the rod in reg'on 30 can be fully withdrawn before reaching 140%
power (Fig. 11). The trip setpoint required from a 10% power starting point is calculated by finding the starting point decalibration f actor in Table 2, 1.0 for case C', Then the decalibration due to the rod withdrawal is taken from Table 5, 0.81 for the third channel trip. The trip setpoint is 140%
- 0.81 = 113%. This is not nearly the most restrictive accident which may be initiated from 10% power. Other cases show this same trend; the accidents started from the highest power are the most restrictive for those cases where the rod need only be partially withdrawn to reach 140% power.
Column 9 of Table 4 shows the safe PPS trip setpoints, i.e., the set-points which ensure that all three logic channels are tripped before true power exceeds 140%. The trip circuit is designed to initiate a reactor t rip when any two of the ( bannels trip. However, in Section 3.1 it is shown to be possible for single channels to trip in a mode which does not 35
trip the channel and is not detectable except during required surveillance tests. In determining the last column in Table 4, it was assumed that all three channels had to trip before the reactor was scrammed. It is generally observed that as the RWA starting power is lower, the required trip setpoint is lower. The reason for this is as follows. The maneuvers that cause the largest initial decalibration at the RWA starting point are the insertion of banks 4C, 4A, 4E, or 4D without calibration. These maneuvers will neces-sarily reduce true power and will reduce indicated power to an even greater extent. (The starting point indicated power values, as shown in Tabic 4, are equal to starting point true power times the average of the detector decalibration factors.) For example, in the Table 4 results for starting points D and E, the required trip setpoints are low, as are the starting point indicated power levels.
Figure 12 shows an envelope of trip setpoints versus starting point indicated power. The setpoints shown in Fig. 12 are safely lower than set-points required to ensure that scram occurs before true core power reaches 140% for any of the following events:
- 1. Sequential rod withdrawal (Table 1).
- 2. Any RWA from Table 4.
- 3. Any RWA following insertion of runback rods.
The insertion of runback rods, banks 2A and 3C, will affect all detectors, as does the insertion of the regulating rod: the ratios of indicated power to true power are raised. The insertion of these runback rods poses no de-calibrations more adverse than cases considered in Table 4.
The analysis done to determine the setpoints shown in Fig. 12 has con-sidered cases in which no more than three rod banks are withdrawn without recalibration of the detectors. Because of the requirement for daily cali-brations, and based on operating history to date, this assumption seems reasonable. Nevertheless, 53 other cases have been considered in 36
130 - REACTOR TRIP MAY OCCUR AT TRUE POWER EXCEEDING 140%
120 ENVELOPE USING 110 _ RECOMMENDED CAllBRATIONS ,
100 -
\'
90 g _
2 80 -
6
$ 70 -
2
[ REACTOR TRIP ALWAYS OCCURS w 60 -
AT TRUE POWER < 140%
2 50 -
h a.
40 -
30 -
20 10 -
I I I ' I I ' I 0
O 10 20 30 40 50 60 70 80 90 100 INDICATED POWER (%)
76 Pg < 12 S= 0.5795 P, + 69.05 12 s P, s 100 127 P, > 100 Fig. 12. Trip setpoint versus indicated power envelope for the initial cycle 37
which four, five, six, or seven rod banks are moved in sequence without recalibration. The 15 additional starting points for these cases are described in Table 6; the RWA decalibration data are those of Table 3. Of these 53 new cases, only one is outside the envelopc saown in Fig. 12; the data for this case are given below.
Detectors are calibrated with bank 4C fully withdrawn. Rods are inserted through bank 3A without recalibration. This insertion of 0.07 Ap will cause the core to go subcritical at any time in the cycle. Xenon and Pa-233 then decay fully. Without calibration, the reactor is returned to the .aximum attainable power with bank 3A inserted (< 30% true power, <12%
indicated power). The regulating rod is then rapidly withdrawn, raising true power to 140%.
The most important aspect of this unlikely case is that it requires a return to power without recalibration of the detectors. However, if it is required that the detectors be calibrated before the return to power, then this RWA is much less severe than several cases considered in Table 4.
Therefore, a requirement that the detectors be calibrated before the ISS is moved to a higher position is proposed. A requirement that detectors be calibrated before the ISS position is changed will accomplish two things:
- 1. It will ensure that the ISS position is changed at the proper true power level (see Section 1.2).
- 2. It will ensure, for all RWAs, that true power will not exceed 140% if setpoints are adjusted in compliance with the envelope shown in Fig. 12.
The following specific calibration requirements are recommended:
- 1. With the ISS in the Startup mode, a calibration is to be done when heat-balance power is between 2% and 4% of rated power. The methods to determine heat-balance power level are given in Ref. 7.
38
TABLE 6 RWA STARTING POINTS ANALYZED IN ADDITION TO TABLE 4 CASES Rod Banks Inserted Rod Banks Inserted Since Detectors at Starting Points Last Calibrated Through 3A Reg. 3D,4C,4A,4E,3B,4D,3A 3D,4C,4A,4E,3B,4D,3A 4C,4A,4E,3B,4D,3A 4A,4E,3B,4D,3A 4E,3B,4D,3A Through 4D Reg, 3D,4C,4A,4E,3B,4D 3D,4C,4A,4E,3B,4D 4C,4A,4E,3B,4D 4A,4E,3B,4D Through 3B Reg, 3D,4C,4A,4E,3B 3D,4C,4A,4E,3B 4C,4A,4E,3B Through 4E Reg, 3D,4C,4A,4E 3D,4C,4A,4E Through 4A Reg, 3D,4C,4A 39
- 2. When increasing power with the ISS in the Low Power mode, a cali-bration is to be done when heat-balance power is between 24% and 28% of rated power.
- 3. When decreasing power with the ISS in the Power mode, a calibra-tion is to be done when heat-balance power drops below 36% of rated power.
When the ISS is in the Startup position, before the calibration is done at 2% to 4% power, two additional safeguards are in effect to terminate a rod withdrawal accident:
- 1. An RWP occurs if the flux level rises above 5%.
- 2. A PPS reactor trip occurs if the rate of flux change is >5 dpm.
These ctions are deactivated in the Low Power and Power modes.
In summary, compliance with these simple calibration requirements plus the adjustment of trip setpoint to a value no greater than shown in Fig. 12 will ensure that:
- 1. True power will not exceed 140%.
- 2. The ISS position will be adjusted at the proper true power level.
The envelope contained in Fig. 12 depends on the core fuel-loading dis-tribution and the specific control rod sequence used during the initial reactor cycle. It will be necessary to develop envelopes similar to that of Fig. 12 for subsequent cycles. The techniques described in this section can be used to develop envelopes for subsequent cycles.
40
2.3.5. Impact of Detector Calibration on Trip Setpoints Calibrations of detectors are recommended between 2% and 4%, between 24% and 28% power when increasing power level, and at 36% power when de-creasing power level. These calibrations ensure that the ISS switch posi-tion is changed at the required power level. The possibility of several starting points shown in Tables 2 through 5 is eliminated by these calibra-tions. For example, the starting points precluded by the calibration at 36%
power are the cases where a calibration is made at high power and then several rod banks are inserted to reach the starting point of an RWA.
Cases A, B, C, and D are precluded.
When calibrations are made at 36% power and at about 5% power as rods are inserted to reduce power, both the amount of rod motion possible between calibrations and the amount of initial detector decalibration at the begin-ning of an RWA are much less than occur for the most limiting cases in Tables 2 through 5. Consequently, the required setpoints are much less restrictive than for the restrictive cases in Fig. 12, where no calibrations between 5% and 100% power were considered.
Below about 20% power, the limiting case is calibration of detectors
.: 30% power, a decrease in power below 20% by insertion of rod bank 4E, and then full withdrawal of rod 11 to raise true power to 140% power. The required setpoint for this case is 98.5%. Between 20% and 30% power, the limiting case consists of calibrating detectors at 30% power with bank 4A inserted, followed by accidental withdrawal of rod 11. The required trip set ting for this case is 107.8% power. These special required setpoints are shown in Fig. 12 by dashed lines in the indicated power range O to 30%.
41
2.3.6. Conservatisms in Analysis The following major conservatisms have been incorporated in the analy-sis of Section 2.3:
- 1. No credit is taken for the RWPs or reactor trip on high steam temperatures.
- 2. It is assumed riot the PPS reactor trip is delayed until the third PPS logic channel reaches its setpoint. This delay requires that there be an undetected PPS channel failure, with the logic channel failing untripped.
42
- 3. PROPOSED HARDWARE CHANGES 3.1. FLOATING SETPOINT CIRCUITRY HARDWARE The circuitry that is proposed to adjust the PPS trip setpoint as a function of indicated power is described in this section. The RWP setpoint will be handled in the same manner, but the RWP setpoint will always be set a fixed amount below the trip setpoint. In norma' operation, as the detected power increases or decreases, the trip setting rises and falls (floats) accordingly. To prevent the trip point from floating upward during an unacceptable rate of climb in power (such as a rod withdrawal accident),
a special rate detection circuit will be used. If the rate of power in-crease at any time is greater than a high positive rate setpoint, this cir-cuit will prohibit the floating trip point from following the rise in detected power, thus holding it at a constant value until either the trip or RWP occurs or until the rate of increase becomes acceptable.
The hardware to implement the floating trip point is presented in Fig.
- 13. This circuit is duplicated six times, once for each PPS power range detector. Figure 14 represents a heat balance correction fix which will be used to correct non-PPS readouts, including the megawatt-hour meter, the power and flow measurements, and the flux recorder. It will also add a meter to indicate the true power as calculated by the heat balance equations in the data logger. In the case of computer failures, an annunciator will sound and the average uncorrected power output of the flux controller will serve as the input to the above devices until the computer is back on line.
3.2. FLOATING SETPOINT PERFORMANCE Figure 15 illustrates the floating trip point reaction to various load changes, as well as to a sudden rise in power such as an RWA. The trip point will float at a constant offset above the indicated power, between 43
NUCLEAR POWER
( 0 - 150% ) p - - - - - - - - - - - - - - - - - - - - - - - - - - ~- - -]
HIGH POSITIVE l, dN R ATE (35' MIN - 10% MIN) l
. BISTABLE
'r "
dt POWER RATE TRIP l I
I I
I I HOLD ON HIGH HIGH HIGH I POSITIVE RATE l SETPOINT SETPOINT g
l l r r m SAMPLE AND SUMMER M00 LOW SETPOINT SUMMER SETPOINT HOLD CIRCulT IFIER MODULE m ,
l l i
- L A i 15-60% 0 - 30% l g l OFFSET OFFSET I I i l REACTOR TRIP SETPOINT I I L---------- ROD WITHDR AWAL PROHIBIT SETPOINT !
1r l m PR OG R AMM ABL E HIGH NUCLEAR L_ J
" ADDITIONAL FLOATING BISTABLE TRIP POWER TRIP TRIP P0 INT CIRCulTRY 3 7 IN BOX
% PROGRAMMABLE ROD WITHD R AWAL BISTABLE TRIP PROHIBIT Fig. 13. PPS floating trip circuit (typical of six places)
UNCORRECTED FLUX AVERAGE POWER FLUX MW(T) H R CONTROLLER +---p4 INTEGRATOR > METER (NC1199) (NM1199) (NQ1199)
PREVIOUS 103 103 103 HEAT BALANCE DIGITAL-TO' FLUX RECORDER ANALOG CALCULA. 4 CONVERTER
-> (N R1199) (ALSO T10N (DATA RECO RDS P/F)
LOGGER) (DATA LOGGER) d L 103 CORRECTED INPUT 0NLY ON AVERAGE POWER COMPUTER
$ FAILURE 1 r 1r DIVIDER / POWER / FLOW AMPLIFIER ; MEASUREMENT CIRCUlT (XMS 11262) 103 110 l
I CORRECTED ADDITIONAL HEAT
.--> POWER BALANCE CIRCulTRY METER IN BOX 103 L.
Fig. 14. Heat balance calibration
140 TRIP SETPOIN T 120 -
/
SETPOINT HOLDS DUE TO HIGH RATE OF INCREASE 100 - 0F MEASURED POWER AND I TRIP OCCURS AT 70%
$ POWER e
' 80 -
m e
4 > -
e e y 60 -
6 5 Lr R0D WITH D RAWAl E INDICATED ACCIDENT FROM 40 - REACTOR 20% POWE R y POWER 20 -
0 TIME Fig. 15. FSV floating reactor trip on high reactor power (example of operation showing startup, load-changing, load reduction to 20% power, and rod withdrawal accident)
the limits 70% and 120%. When power level is above 100%, the trip holds at 120%; when power is below 50%, the trip point holds at 70%. When the cate of rise 'n power exceeds a preset rnte, the trip point holds at a con-stant value a nd the reactor is scrammed at a true power of less than 140%
of rated.
In Fig. 13, the additional floating trip point circuitry is shown in module form within the dashed box outline. The original bistable trips will be made programmable also, but only up to 140% and 120% of actual power, i.e., they will retain their absolute trip levels. Thus, the origi-nal function of this part of the PPS system has not been changed; rather, the floating trip circuitry is an addition which can produce an RWP or trip below the original values if required. Should any part of the additional circuitry fail, for example, the trip on high rate of power increase, the original trip action will be unaffected and will still occur. This design ensures that the safety margin of the trip system is not reduced.
The entire circuit shown in Fig. 13 is located in the dual linear power channel drawer for both nuclear channels. A more detailed circuit layout is shown in Fig. 16. The circuit consists of a differentiator, dN/
dt, which receives a signal proportional to indicated nuclear power (0 to 150%), and outputs a signal proportional to the rate of change of the nuclear power. This rate signal is fed to a bistable trip. The setpoint on the bistable is adjustable from 3% to 10% power increase per minute.
When the rate exceeds this setpoint, the bistable output goes high. When the bistable output signal is low, the sample and hold (S/H) circuit con-tinually samples its input, the indicated nuclear power. The minimum rate of sampling is once every millisecond, which allows the S/H output to fol-low its input. When the bistable output signal goes high, the S/H circuit holds its output to the last sampled input value before the S/H line goes high. This held output decays less than 3.33% of the last sampled input per minute. The S/H output is fed to one input of the summer modifier circuit.
47
V3tTS PROPO RTION AL VOL TS PROPO RTION AL TO INDIC ATE D TO INDICATED NUCLE AR POWE R 3 - 17% VIN NUCLEAR POWE R 10-150%) (RATE OF POWER CHANGE) (0-15%)
SE TPOIN T dN 'dt BISTABLE ,, S/H SAMPLE V O LT A.G E f (10005EC)
VO LTS PROPO RTIONAL COMPARATOR TO RATE OF HOLD CHANGE OF VO LTS PR OPO R TION AL VOLTS PROPORTION AL TO INDICATE D TO INDICATE D 0-375 POWER POWE R PLUS POWER AT (RWP DF FSET) TRIP 0FFSET LAST SAMPLE I HIGH LIMIT d (100-14N +
+ LOW LIMIT p (60 -140%)
VOLTS PROPORTIONAL 15-60% POWE R TO IN0lCATE D { TRIP OFFSET)
POWE R PLUS TRIP 0FFSFT MINUS RWP OFFSET BIST A BL E 3, SETPCINT BISTABLE
- VO LTAGE m VO LT AGE -
COMPARATOR COMPAR AT O R f I u ir RWP RE ACTO R TRIP Fig. 16. PPS floating trip point circuitry for Fort St. Vrain Unit 1
The summer modifier circuit has two inputs. One is from the S/H cir-cuit; the other is a bias signal which is adjustable between 15% and 60% of rated full power (100%). The summer modifier circuit sums these two inputs; if the sum exceeds an adjustable high setpoint (100 to 140% rated full power), the output holds at the upper limit. Likewise, if the sum is less than an adjustable low setpoint (60 to 140% rated full power), the output holds at the lower limit. Between the lower and the upper limits, the out-put follows the sum of the two inputs. The output goes to the reactor trip bistable programmable setpoint input and to the RWP summer. The bistable input is indicated nuclear power. Whenever the indicated nuclear power ex-ceeds the setpoint, the bistable output feeds the same point as the output of the 140% bistable trip in the previous design of the dual linear power channel drawer.
The summer modifier circuit output is also summed with a bias, adjust-able between 0% and -30% of rated full power (100%) to create an RWP, which actuates before the reactor trip. The output is fed to the RWP bistable programmable setpoint input. The bistable input is indicated nuclear power.
Whenever the indicated nuclear power exceeds the setpoint, the bistable output goes high. Othe rwis e , the output of the bistable is low. The out-put of this bistable feeds the same point as the output of the 120% bistable trip in the previous design of the dual linear power channel drawer.
A testing capability is provided within each drawer to show operability of the floating trip circuit in both channels of each dual linear channel drawer, with provisions to connect and disconnect necessary test equipment.
The tests consist of applying several different input signals to the drawer in place of the nuclear detector input. The test signals consist of an ad-justabic signal between 0 and 150% rated power and a ramp signal adjustable between 0 and 150% of rated power per minute.
The tests are recorded with a strip-chart recorder or equivalent. The recorder looks at the input signal, the output of both the RWP and reactor trip programmable bistables, the output of the power rate bistable, the reactor trip histable setpoint, and the RWP bistable setpoint.
49
A nuclear channel consists of a detector and one-half of a dual linear nuclear drawer, which contains a power supply, linear amplifier, and the circuitry shown in Fig. 13, as well as additional equipment (not shown) used to generate other RWP actions at 5% and 30% power. Figure 17 shows the arrangement of this equipment. During monthly recalibration, a test signal is inserted at the input of the linear amplifier. This signal can be set and the various trip outputs adjusted to an accuracy of 0.01%, using a good quality digital voltmeter. However, conditions present in the field will sometimes preclude such accuracy, and errors as large as 0.5% can, on occasion, be expected. An error analysis of the whole system, including calibration and calculational uncertainties, is given in the appendix.
The output current of the detector for a given flux field degrades to 902 of the original output in 10 nyt (neutron flux times the time). The FSV flux field at the detector iu less than 10 at 100% power, and the usable lifetime of the detector, based on uranium depletion alone, may be l
stated as 10 i 10 = 10 sec (N3000 years). Thus, the degradation of signal because of uranium depletion obviously is not limited by the life of the detector nor does it make a significant contribution to signal error.
Addition of the above possible inaccuracies gives an overall total of less than 1%.
3.3. FAILURE MODE ANALYSIS Figure 18 shows the PPS hardware configuration for channel A, which is typical also for channels B and C, including the added floating setpoint hardware in the dashed box outlines.
Table 7 presents a detailed failure modes and effects analysis of the added floating setpoint circuitry. This analysis shows that no single fail-ure will prevent the initiation or completion of the reactor trip or RWP functions, although there are cases where one channel can be inoperative until the next surveillance test. Thus, the PPS continues to satisfy the single-failure criterion as well as all other requirements of IEEE Standard 279-1968 (Ref. 8).
50
DIGITAL VOLTMETE3 0.01%
d' DUAL LINEAR DRAWER m____ ____________
I l l BISTABLE TRIP FLOATING I TRIP TRIPS OUTPUT I
I FISSION CHAMBER l LINEAR NEUTRON , W AMPLIFIER DETECTOR l 1 0 l u I l I m OTHER l l TRIPS l l l 1 I I I I I I I l
TEST INPUT l (VOLTS)
I I L _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - -. _ _ _ _ _ _ _ _ _ _ _J Fig. 17. Nuclear channel test setup, block diagram
- - - - - - - - - - - - - _ _ _______.,_____y f- . -*
- l I . RWP l l
+ 20UT -> OR -> RWP BISTABLE TRIP - -> 0F3 + l l r-i I -
+ i 1
aL S.P. I g l RWP CHANNEL A FLOATING m
4 .d _ _ _ _ _
1 ' S'P. l g
NE 1136 e, , ,
2
AMPL' BfSTABLE TRIP [ 0F l NEW l L RWP CHANNEL B _]
t l
I m RWP l BISTABLE TRIP ,
d i S.P.
4 A 4 FLOATING l l u g dt SETPOINT I l r S.P.
NE 1133 L, ,
,a 9
l SCRAM ---->
NT ' AMPL BISTABLE TRIP l
l ' l
[_ OUAL LINEAR ORAWER _ j OR OTHER ->
CHANNEL A (TYPICAL OF 3)
RWP = R00 WITH0RAWAL PROHIBIT +
NT = TRANSMITTER AMPL = AMPLIFIER CH ANNEL B %
SP = SETPOINT fCONTACTORS)ROOS NE = FISSION CHAMBER CH ANNEL C ->
Fig. 18. PPS channel configuration
TABLE 7 FAILURE MODES AND EFFECTS ANALYSIS Name Failure rode Channel Effect System Effects I Differentiator Output open or No detection of high flux RWA detectable by trip of 2 shorts to ground. rates. Upper limit remains rt'aining PPS channels.
effective.
Output shorts Hold at last measured rate. Flux rise causes spurious 1-high. Trip point does not float. channel RWP and trip.
II Voltage Output open or No detection of high flux RWA detectable by trip of 2 comparator shorts to ground. rates. Upper limit remains remaining PPS channels.
effective.
Output shorts Hold at last measured rate. Flux rise causes spurious 1-high. Trip point does not fl-^r. <.hannel RWP and trip.
U III Sample and hold Does not hold. No detection of high flux RWA detectable by trip of 2 rates. Upper limit remains remaining PPS channels.
effective.
Does not sample. Setpoint does not float. Flux rise causes spuriouc 1-channel RWP and trip.
Open or short to Setpoint goes to lower Spurious 1-channel RWP and ground, limit, trip when power is above lower limit.
Output high. Setpoint goes to upper RWA detectable by trip of 2 limit. No detection of retaining PPS channels.
high flux rates.
I
TABLE 7 (continued)
Name Failure Mode Channel Effect System Effects IV Modifier
- a. High limiter Output opens. No upper limit. Hold on No effect on rapid withdrawal.
high rate still effective. Remaining 2 PPS channels de-tect rises above absolute limit.
Output shorts to Setpoint goes to zero. Spurious 1-channel RWP and ground. trip.
Output high. No upper limit; no detec- RWA detectable by trip of 2 tion of high flux rates remaining PPS Channels.
- b. Low limiter Output opens. No lower limit.
Output shorts to Setpoint goes to zero. Spurious 1-channel RWP and ground. trip.
Output high. Setpoint rises to 150%. RWA detectable by trip of 2 remaining PPS channels.
- c. Offset Output opens or No offset. Setpoint still Transients cause spurious 1-shorts to ground. floats. channel RWP and trip.
Output shorts Setpoint rises to upper RWA detectable by trip of 2 high. limit, remaining PPS channels.
V Summer Output opens or Setpoint goes to zero. Spurious 1-channel RWP and shorts to ground. trip.
Output high. Setpoint rises to 150%. RWA detectable by trip of 2 remaining PPS channels.
TABLE 7 (continued)
Name Failure Mode Channel Effect System Effects VI Reactor trip Output opens or Spurious channel trip. No effect on RWP.
setpoint shorts to ground.
Output shorts Setpoint rises to 150%. RWA detectable by trip of 2 high. remaining PPS channels, No effect on RWP.
VII RWP offset Output shorts to Spurious 1-channel RWP. No effect on trip.
summer ground or opens.
Output high. No RWP setpoint - 1 channel. RWP by 2 remaining channels.
U
No single channel failure of any portion of the floating trip circuitry can prevent the PPS from detecting a high flux rate or level and scramming the reactor. All sections of this report are based on the assumption that one channel of the PPS is inoperable in the worst possible mode, i.e., will not trip when the detector receives the proper signal.
The floating trip circuitry addition does not make a change in any failure modes of the original system, i.e., on loss of bus voltage or sensor, or on parts failure. However, one additional failure mechanism is intro-duced, namely, the failure of a channel to detect a high rate of flux in-crease. Since the original equipment did not contain any rate-dependent function (for linear channels), and since the upper trip limit is still active even with rate detection failure, the floating trip circuitry with the rate detection portion failed is still equivalent to the original equipment.
Trip levels, given in Fig. 12, were generated assuming that one of the three channels was inoperative.
3.4. IIEAT BALANCE CORRECTION CIRCUITRY The heat balance correction circuit described below, which uses the data logger to provide a corrected power level reading, is the simplest and most convenient method of correcting for the effects of detector decalibration on power and megawatt-hour meters. This method requires a minimum amount of additional equipment in the control room and uses a program which is already available on the data logger. If the data logger is unavailable at any time, the uncorrected signal from the neutron detectors is used. This arrangement is acceptable for the following reasons:
- 1. The data logger downtime should be brief.
- 2. The primary circuit heat balance can be hand calculated, if neces-sary, and uwid as a correction.
56
- 3. Correction tables will be available to the operator showing the ratio of indicated power to true power levels as a function of rod bank position.
- 4. The detectors will normally be calibrated and there is almost no decalibration under normal full-power operation.
- 5. Errors in the megawatt-hour meter, which is a time integrator, will be small and will tend to cancel over core segment lifetime.
The heat balance correction circuit shown in Fig. 14 consists of a s digital-to-analog (D/A) converter which takes a digital output from the data logger computer (CDC 1700) input / output (I/0) channel and provides an analog output proportional to nuclear power, as calculated by the heat balance program or programs in the computer. The output signal ranges from 0 to 5 V corresponding to O to 150% nuclear power. The D/A converter card is located on the data logger I/O channel chassis. The output signal is fed from the data logger I/O terminal board to the divider / amplifier circuit in the 19303 board. An additional signal is fed from the computer to the 19303 1
board. This signal is normally high and goes low when the computer is off-line, disconnected, or failed for any reason.
The divider / amplifier circuit current modifies as necessary the 0- to 5-V input to provide a 0- to 100-mV output for the flux integrator (NM 1199) and flux recorder (NR 1199), and a 0- to 5-V output for the power-to-flow measurement circuitry (KMS 11262) and the correct power meter (new). The divider / amplifier circuit also can switch its input from the data logger D/A converter output to the flux controller (NC 1199) output upon loss of the computer on-line signal.
Note that this circuitry, with an input from the data logger, is not part of the PPS syetem and is not connected with the floating trip point circuitry described earlier.
57
$j i
\
. L s' t g .'5
' a b
Use of a res-tor, cdp setpoint varying with indicated reactor power is 1 recommended to ensqre that a reactor trip is always initiated before true reactor power reaches the trip point at 140% rated power as given in the technical specification.
Figure 19 gives the programming of trip and RWP setpoints, as a function of indicated power, recommended for the initial cycle. The trip setpoint is
~
conserva t ively\ fixed at values of 7% to 28% of rated power below the values nece:sary to ensurefC rea; tor trip before exceeding 140% true power (compare Figs. 12 and D). This,7% to 28% safety margin exceeds the combined uncer-tainty in the true power at the scram point arising from uncertainty in heat balance power used to calibrate the PPS channels, as well as other calibra-tion and calculational uncertainty. A description of this uncertainty analyris is presented in the appendix.
4.2. CALIBRATION REQUIREMENTS The procedure currently used by Public Service Company of Colorado for s cal i&r at ion , S. R. S.4.1.1.4c-9 (Ref. 7) , is adequate. This procedure calls for calibration of each detec;or to a secondary side heat balance, with backup equations given for a primary side heat balance. The following items s60uld be added to the data sheet used for Ref. 7, and entries should be recorded at each calibration:
Contrzl rod bank partially inserted Posith<n (inches withdrawn)
Reg rod position (inches withdrawn) i \
58 1
\
t s
120 -
110 100 -
90 -
80 E
$ TRIP g 70 RWP f
E 60 -
2 C
50 40 -
30 20 10 I I I I I I I I I l 0
10 20 30 40 50 60 70 80 90 100 INDICATED POWER (%)
Fig. 19. Recommended program for trip and RWP setpoints 59
Events which require that a calibration be done are listd below:
- 1. At 1 1st one calibration is required during every 24-hr period when operating in Low Power or Power modes.
- 2. To prevent or clear RWPs which occur due to inaccurate detector readings, a calibration should be done whenever any channel approaches or reaches an RWP setpoint.
- 3. To ensure that the ISS is switched at the proper power level, the following requirements are made:
- a. With the ISS in the Startup mode, a calibration is required when heat-balance power is between 2% and 4% of rated power.
The methods to determine heat-balance power level are given in Ref. 7.
- b. When increasing power with the ISS in the Low Power mode, a calibration is required when heat-balance power is between about 24% and about 28% of rated power.
- c. When decreasing power with the ISS in the Power mode, a cali-bration is required when heat-balance power drops below about 36% of rated power.
In addition, calibration should be done whenever the operator has reason to believe that one or more detectors are giving anomalous readings.
When individual detectors dif fer by more than 10%, the proper functiening of the channels should be verified.
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APPENDIX SAFETY MARGIN AND UNCERTAINTIES This appendix discusses and compares (1) the safety margin between the required setpoint values and the actual programmed setpoint values, and (2) the uncertainties in the PPS trip circuitry, in the calibration of the detectors, and in the analysis presented in this report.
A.1. MARGIN The margin at any indicated power level is defired as the required trip setpoint value minus the programmed trip setpoint value. The required trip setpoint is the value that will ensure that a reactor trip occurs before the true power level exceeds 140% of its rated value. Re-quired trip setpoints are presented as a function of indicated power in Fig.
- 12. The programmed trip setpoint value can be read from Fig. 19. Figure A-1 shows the safety margin as a function of indicated power. Above 30%, in the Power mode, the margin is simply the difference between Fig. 12 setpoint values and Fig. 19 values. Below 30% power, the Low Power mode, the required trip setpoints are the values shown in Fig. 12 by che dashed lines when detector calibrations are made as described in Section 4.2. When credit is properly taken for these calibrations, several of the more limiting RWAs considered in Table 4 (those with a starting power level below 30%) are not possible. For example, the RWAs that have starting point designation D require that the detectors be calibrated with rod bank 4C withdrawn (at near full power), and that rods then be sequentially inserted through bank 4E, reducing true power to 20% or less. No intermediate recal-Ibration is assumed in the analysis presented in Table 4. If recalibration is assumed at 30% power, the required trip setpoints for the D cases are all above 98%. (See section 2.3.5 of the main report.) All other RWAs which start from 30% or less were considered in a similar manner to derive Fig. A-1.
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50 E 40 -
w 3
2 S
h 30 a
E E
$ 20 -
s a
E E
2 10
' ' I I 0
O 20 40 60 80 1')0 INDICATED POWER (% RATED)
MARGIN = (REQUIRED TRIP SETPOINT) -(PROGRAMMED TRIP SETPOINT)
Fig. A-1. Trip setpoint margin. The fact that the curve is discontinuous at 30% results from calibration requirements at 5% and 30%.
Larger detector decalibrations are possible in the 30% to 100%
Power operating range than in the 5% to 30% Low Power range.
Thus, the required trip setpoint is smaller above 30% than it is in the Low Power range.
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b A.2. UNCERTAINTIES A.2.1. Instrumentation Uncertainty As indicated in Section 3.1, the total uncertainty due to the PPS cir-cuit electronics errors expected in setting a channel to a desired output signal and detector se sor degradation is less than !1%. This is one component in the total uncertainty of the PPS circuit output.
A.2.2. Calorimetric Uncertainty The major uncertainty in the output of the PPS trip circuit is the accuracy of the heat balance used to find the core thermal power to cali-brate the detectors. This uncertainty has been calculated as a function of power level (Ref. 9), and the results are listed below:
Uncertainty, 2a Po e el fUncertainty,MW(t)
\ Power Level, MW(t)/ Uncertainty, 20
(%ofrated power (%) (% of rated power) .
25 7.58 1.89 50 !4.20 2.10 75 3.26 2.44 100 2.33 2.33 A.2.3. Total Uncertainty in PPS Trip Circuit Output The total sum-of-squares uncertainty due to instrumentation and calori-metric uncertainties is shown below:
Uncertainty, 20 Po evel fUnce. ainty, MW(t)
\ Power Level, MW(t)/ Uncertainty, 20
(% ofpower rated /j (%) (% of rated power) 25 7.64 1.91 50 4.32 2.16 75 3.41 2.55 100 2.53 2.53 63
A.2.4. Uncertainty in Establishment of Programmed Trip Setpoints An uncertainty of 5% of rated power is attributable to the analysis used to establish the required trip setpoints of Fig. 12. This uncertainty is conservatively high in view of the assumptions used in the analysis and the measurements of decalibration factor made so far. For each control rod bank which is withdrawn while at power, over 30 data points have been gathered covering the full stroke of the rod bank. These measured data have been used in the analysis presented in this report, and the analysis will he updated as measurements are made for new control rod banks at higher power levels.
A.3. MARGIN AFTER CONSIT RATION OF UNCERTAINTIES The programmed trip setpoints of Fig. 19 are set adequately below the required trip points (Fig. 12) to provide margin for uncertainties in meas-urements, calibrations, and calculations, as described below.
The total uncertainty in the PPS trip circuit output due to instrument or calorimetric errors has a small effect on the safety margin. If a neu-tron detector channel is set during its calibration to read a power level higher than the true power (due to calorimetric and setting inaccuracies),
then the channel output will reach the trip setpoint early, i.e., before true power can reach 140% power. To illustrate the point, an example at 100% power is shown in Table A-1. Because of a heat balance error, a chan-nel power may be set 2.5% high, i.e., set at 102.5% power when the true power is 100%. If an RWA occurs from this power level, the channel will trip when the indicated power is 120%. When the channel trips an accurate channel, power is 2.5% below 120%, or 117%*. Since Fig. 12 indicates that
- When channels are inaccurately calibrated, the channel signal will remain at worst a fixed percentage, equal to the error in the setting, offset from the accurate channel output. (This accurate channel output should not be confused with the true reactor power.) A constant percentage offset occurs if the error is in the slope m of the meter output when V = mI where I is the fission chamber output current and V is the channel output voltage.
If a zero-point error is made in setting, the absolute error is a constant percent of rated power rather than a constant percent of the indicated power.
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TABLE A-1 TRIP SETPOINT MS.RGINS Cali-bration Accurate Accurate Uncer- Required Channel Margin Required Channel Margin True tainty,(a) Trip Trip Reading T-A Trip Trip ReadinR T-A Power, P 2a Setpoint Setpoint OS(P-2s) (% of Setpoint Setroint @S(P+2:) (t of (Z) (%) P-20 S(P-20)(b) TIcJ (A) rated power) P+2a S(P+20)(b) T(C) (A) rated pewst) 15 %10 13.5 70 98.5 77.0 21.5 16.5 70 98.5 63 35.5 30 7.64 27.7 70 107.8 75.3 32.4 32.3 70 107.8 64.7 43.1
$ 50 4.32 47.8 70 98.0 73.0 25.0 52.2 77.2 98.0 69.1 28.9 75 3.41 72.4 92.4 112.5 95.7 16.8 77.6 97.6 112.5 9. 3 18.2 90 %3.0 87.3 107.3 121.2 110.5 10.7 92.7 112.7 121.2 109 11.9 100 2.53 97.5 117.5 127.0 120.5 6.5 102.5 120.0 127.0 ./.o 10.0 7.5 92.5 , 112.5 127.0 120.9 6.1 107.5 120.0 127.0 i11.0 16.0 (a)See Section A.2.3.
(b)From Fig. 19.
C S(Pt2c) = 0.5795(P!20) + 69.05 (Fig. 12).
the required trip point at 100% power is 127% power, the channel trips at 127% - 117% = 10% early.
If the channel is calibrated to read a power lowe - than the true power, the accurate power indication will exceed the trip poilt by the calibration error if an RWA should occur. However, the low indicated setting will cause the floating trip circuitry to generate a trip setpoint lower than pro-grammed for the true power. As shown in Table A-1, a 2.5% calibration error can yield a channel output of 97.5% when the true power is 100%.
From Fig. 19, the trip point is then 117.5%. The required trip for the true power is 127%, the channel trips at an indicated power of 117.5%, while the accurate channel output is 120.5%. Therefore, the margin is 127% - 120.5%
= 6.5%. This is a larger margin than is needed to account for the uncer-tainty in the calculation of required trip setpoint.
To illustrate the sensitivity of the margin to the channel power cali-bration error, an error at 6 standard deviations was used at 100% power.
Here the margin is reduced f~ ..n 6.5% to 6.1% by a f actor of 3 increase in the channel calibration ;see the last entry in Table A-1).
Margins be) ?>% power were determined under the assumption that cali-brations at 5% and 36% were made as specified in Section 4.2 of the report.
Required trip points when these calibrations are used are plotted in Fig. 12.
Table A-1 reports the difference between the required trip setpoint and the actual power at which the channel will trip. This difference is called the margin. Errors made in core power measurements setting the channel, channel drift, and instrument accuracy have been considered in computing the margin. Therefore, the margin must exceed the uncertainty in the calculations and measurements of decalibration factors used to establish the required se cints in Fig. 12. Examination of Fig. A-1 and Table A-1 indicates that the margin at all power leyels exceeds the 5%
decalibration factor uncertainty.
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A.4. CONCLUSION Results of the analysis of uncertainty in trip cetpoint presented in this appendix indicate that there is sufficient margin between the pro-grammed trip setting and the required trip setting to ensure that the reactor is tripped at a power level not exceeding 140% of rated power even when uncertainties in the following are considered:
Instrument inaccuracies and drift.
Uncertainty in power determination.
Inaccuracies in setting channel output.
Uncertainties in calculation establishing trip setpoints.
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REFERENCES
- 1. " Rod Control System Equipment 1-9303. Operations and Maintenance Manual," General Atomic Report E-115-265, revised August 1973.
- 2. " Fort St. Vrain Nuclear Generating Station, Technical Specifications,"
Public Service Company of Colorado, April 1972, pp. 3.3-2.
- 3. Hastings, G. A., and J. Louis, " Control Design Development for an HTGR Power Plant by Digital Computer Simulation," Proceedings, Seventh Power Industry Computer Applications Conference, May 1972, 71C26-PWR, pp. 463-470.
- 4. " Fort St. Vrain Nuclear Generating Station, Request for Test RT-322, Nuclear Detector Decalibration Test During FSV Rise-to-Power B-Series Tests," Public Service Company of Colorado, April 8, 1975.
- 5. Brown, J. R., and R. J. Nirschl, " Fort St. Vrain HTGR Maximum Rod Worth and Rod Uithdrawal Accident Calculations l'or Reactor Thermal Powers from 2% to 100%," USAEC Report Gulf GA-B10872, Gulf General Atomic Company, July 3, 1972.
- 6. " Fo r t St. Vrain Nuclear Generating Station, System Operating Procedure 12-02, Reactivity Calculations," Public Service Company of Colorado, May 28, 1976.
- 7. " Fort St. Vrain Nuclear .anerating Station, Technical Specifications, Surveillance Procedure 5.4.1.1.4.c-D Linear Power Channel Scram and RWP Calibration," Public Service Company of Colorado, December 2, 1975.
- 8. IEEE Standard: " Criteria for Protection Systems for Nuclear Power Gene-rating Stations," IEEE 279-1968.
- 9. Almodovar, S., " Heat Balance Code Fort St. Vrain Data Logger," Gulf General Atomic Company Report Gulf-GA-B10763, May 15, 1972.
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