RS-08-145, Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency
| ML083080059 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 10/31/2008 |
| From: | Simpson P Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RS-08-145 | |
| Download: ML083080059 (17) | |
Text
Exelon Generation www.exeloncorp.com 4300 Winfield Road Warrenville, I L 60555 Exel6n RS-08-145 10 CFR 50.90 October 31, 2008 U. S. Nuclear Regulatory Commission ATTN : Document Control Desk Washington, DC 20555-0001 LaSalle County Station, Units 1 and 2 Facility Operating License Nos. NPF-11 and NPF-18 NRC Docket Nos. 50-373 and 50-374
Subject:
Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency References :
1.
Letter from Mr. P. R. Simpson (Exelon Generation Company, LLC) to U. S. NRC, "Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency," dated July 25, 2008 2.
Letter from U. S. NRC to Mr. C. G. Pardee (Exelon Generation Company, LLC), "LaSalle County Station, Units 1 and 2 - Request for Additional Information Related to License Amendment Request to Revise Local Power Range Monitor Calibration Frequency (TAC Nos. MD9414 and MD9415)," dated October 1, 2008 In Reference 1, Exelon Generation Company, LLC (EGC) requested an amendment to the facility operating license for LaSalle County Station (LSCS), Units 1 and 2.
Specifically, the proposed changes will revise Technical Specification (TS) 3.3.1.1, "Reactor Protection System (RPS) Instrumentation," Surveillance Requirement (SR) 3.3.1.1.8 and TS 3.3.1.3, "Oscillation Power Range Monitor (OPRM) Instrumentation,"
SR 3.3.1.3.2 to increase the frequency interval between Local Power Range Monitor (LPRM) calibrations from 1000 effective full power hours (EFPH) to 2000 EFPH.
In Reference 2, the NRC requested that EGC provide additional information in support of their review of Reference 1. The NRC request for additional information and the specific EGC responses are provided in the attachment to this letter.
EGC has reviewed the information supporting a finding of no significant hazards consideration that was previously provided to the NRC in Reference 1. The additional information provided in this submittal does not affect the bases for concluding that the Nuclear
October 31, 2008 U. S. Nuclear Regulatory Commission Page 2 proposed license amendment does not involve a significant hazards consideration. No new regulatory commitments are established by this submittal.
If you have any questions concerning this letter, please contact Mr. Timothy A. Byam at (630) 657-2804.
I declare under penalty of perjury that the foregoing is true and correct. Executed on the 31 St day of October 2008.
Respectfully, Patrick R. Simpson Manager - Licensing Exelon Generation Company, LLC
Attachment:
Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency In reviewing the Exelon Generation Company's (Exelon's) submittal dated July 25, 2008.
related to your request for a license amendment to revise Local Power Range Monitor (LPRM) calibration frequency, far LaSalle County Station (LSCS), Units 1 and 2, the NRC staff has determined that the following information is needed in order to complete its review.
1.
Provide the plant specific LPRM uncertainty analysis to demonstrate that the LPRM response uncertainty value used in the Minimum Critical Power Ratio (MCPR) safety limit analysis would remain bounding if the LPRM calibration interval were extended from 1000 Effective Full Power Hours (EFPH) to 2000 EFPH. This analysis should account for 25 percent extension (i.e., 2500 EFPH) allowed by SR 3.0.2.
1.1.
Include a description of the method and assumptions used far this analysis.
Response :
As discussed in Reference 1, the LSCS LPRM calibration currents were collected for the period from 1996 through 2006. All the data collected was for the NA300 model LPRM detectors. Traversing Incore Probe (TIP) calibrations have been performed approximately every month during this time period. The TIP calibration determines the expected LPRM reading from the TIP scan data. This process determines a gain adjustment factor (GAF).
LSCS adjusts the actual LPRM calibration current only if the GAF value is outside of the range from 0.95 to 1.05. Pseudo calibration currents were calculated by using the actual calibration currents and GAF values. In this way calibration currents are available for every LPRM at every calibration interval. These are all treated as measured calibration currents.
The NA300 detectors are doped with U-234 in order to extend the life of the LPRM. This allows the response to remain relatively constant for an extended period of time. Subsequent to this constant period of sensitivity, the sensitivity decays exponentially. The cutoff exposure at which the sensitivity begins to decay is typically around 1.0 snvt. The value currently used for LSCS is 1.81 snvt. Predicted calibration currents used by the core monitoring system for exposures beyond the cutoff exposure are determined by the following equation :
where In is the predicted calibration current In-l is the previous calibration current Page 1 of 15
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency A is the effective exposure decay factor E is the accumulated LPRM exposure (snvt) since the previous calibration Predicted calibration currents are calculated for the actual calibration interval as well as for hypothetical calibration intervals by skipping one or more of the intermediate calibrations. The actual calibration current is independent of the previous calibration so a predicted calibration current for an extended calibration interval is simulated by generating the predicted calibration current using the sum of the intermediate accumulated LPRM exposure values. This is illustrated in Tables 1 and 2 for a selected LPRM. Table 1, columns 1 through 5, are plant-collected data. A value of -99.0 in column 5 indicates that the calibration current was not modified from the previous calibration.
Data in column 6 is determined by retaining the previous calibration current for detectors that did not adjust the calibration current at the time of the core calibration. This is presented primarily for convenience in determining the value in column 7. The calibration currents calculated from the actual calibration current and the core monitoring system GAF values are shown in column 7. These are the calibration currents that would have been used for setting the detector calibration current if they were adjusted. The core monitoring system makes this adjustment internally if the calibration current was not physically altered. Columns 8 through 11 show various methods to calculate an LPRM calibration exposure increment used for the purpose of calculating a predicted calibration current. The corresponding predicted calibration currents, based upon equation 1.1, are shown in Table 2, columns 3 through 6. The relative difference between the predicted and actual calibration currents are correlated with the corresponding cycle exposure calibration interval (shown in Table 3). The cycle exposure calibration interval is determined by taking the difference in the core average exposure between two calibration events. The data for all of the LPRM's are collected together. The resultant data covers a wide array of calibration intervals. In order to get a meaningful statistical approximation of the uncertainty at a particular calibration interval, a range of calibration intervals above and below the desired value are selected and the standard deviation of this population is calculated. This standard deviation is calculated for a variety of desired calibration interval values.
Page 2 of 15
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency Table 1 Illustration of Intermediate Calibration Interval Generation Page 3 of 15 Delta LPRM Exposure Date Core Avererage Exposure (MWd/MTU LPRM Exposure SNVT LPRM GAF Measure Calibration Current rnA)
Previous Calibration Current mA Pseudo Calibration Current mA)
Every Calibration Skip One Calibration Skip Two Calibrations Skip Three Calibrations 8/9/1996 369 0.000 1.000 1103.0 1,103.0 1,103.0 9/12/1996 1276 0.255 0.991 1060.0 1,060.0 1,060.0 0.255 8/25/1998 2824 0.263 0.960 1104.2 1,104.2 1,104.2 0.008 0.263 9/29/1998 3685 0.343 0.957 1153.8 1,153.8 1,153.8 0.080 0.088 0.343 12/29/1998 5733 0.553 1.013
-99.0 1,153.8 1,139.0 0.210 0.290 0.298 0.553 2/4/1999 6618 0.649 1.014
-99.0 1,153.8 1,137.8 0.096 0.306 0.386 0.394 3/11/1999 7477 0.742 1.008
-99.0 1,153.8 1,144.6 0.093 0.189 0.399 0.479 4/15/1999 8305 0.836 1.009
-99.0 1,153.8 1,143.5 0.094 0.187 0.283 0.493 5/20/1999 9159 0.941 0.995
-99.0 1,153.8 1,159.6 0.105 0.199 0.292 0.388 6/24/1999 9908 1.026 1.001
-99.0 1,153.8 1,152.6 0.085 0.190 0.284 0.377 8/4/1999 10769 1.138 1.016
-99.0 1,153.8 1,135.6 0.112 0.197 0.302 0.396 9/17/1999 11716 1.273 0.984
-99.0 1,153.8 1,172.5 0.135 0.247 0.332 0.437 11/30/1999 12553 1.401 1.123 1027.4 1,027.4 1,027.4 0.128 0.263 0.375 0.460 12/27/1999 13220 1.455 1.002
-99.0 1,027.4 1,025.4 0.054 0.182 0.317 0.429 2/2/2000 14128 1.531 1.025
-99.0 1,027.4 1,002.3 0.076 0.130 0.258 0.393 3/8/2000 14988 1.602 1.010
-99.0 1,027.4 1,017.2 0.071 0.147 0.201 0.329 4/12/2000 15849 1.676 1.009
-99.0 1,027.4 1,018.2 0.074 0.145 0.221 0.275 5/10/2000 16540 1.735 1.016
-99.0 1,027.4 1,011.2 0.059 0.133 0.204 0.2801 6/14/2000 17433 1.812 1.036
-99.0 1,027.4 991.7 0.077 0.136 0.210 0.281 7/20/2000 18363 1.891 1.045
-99.0 1,027.4 983.2 0.079 0.156 0.215 0.289 8/24/2000 19269 1.970 1.049
-99.0 1,027.4 979.4 0.079 0.158 0.235 0.294
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency Table 2 Illustration of Predicted Calibration Currents Page 4 of 15 Predicted Calibration Current Date Pseudo Calibration Current mA Every Calibration Skip One Calibration Skip Two Calibrations Skip Three Calibrations 8/9/1996 1,103.0 9/12/1996 1,060.0 1103.0 8/25/1998 1,104.2 1060.0 1103.0 9/29/1998 1,153.8 1104.2 1060.0 1103.0 12/29/1998 1,139.0 1153.8 1104.2 1060.0 1103.0 2/4/1999 1,137.8 1139.0 1153.8 1104.2 1060.0 3/11/1999 1,144.6 1137.8 1139.0 1153.8 1104.2 4/15/1999 1,143.5 1144.6 1137.8 1139.0 1153.8 5/20/1999 1,159.6 1143.5 1144.6 1137.8 1139.0 6/24/1999 1,152.6 1150.5 1123.7 1115.1 1099.1 8/4/1999 1,135.6 1140.8 1138.8 1112.2 1103.7 9/17/1999 1,172.5 1121.6 1126.7 1124.7 1098.4 11/30/1999 1,027.4 1158.8 1108.5 1113.5 1111.5 12/27/1999 1,025.4 1022.3 1153.1 1103.0 1108.0 2/2/2000 1,002.3 1018.2 1015.2 1145.0 1095.3 3/8/2000 1,017.2 995.8 1011.6 1008.6 1137.6 4/12/2000 1,018.2 1010.3 989.1 1004.7 1001.7 5/10/2000 1,011.2 1012.7 1004.9 983.7 999.3 6/14/2000 991.7 1004.1 1005.6 997.8 976.8 7/20/2000 983.2 984.5 996.8 998.3 990.5 8/24/2000 979.4 976.0 977.4 989.6 991.1
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency Table 3 Illustration of the Corresponding Calibration Interval Page 5 of 15 Calibration Interval (Cycle Exposure, MWd/MTU Date Core Avererage Exposure MWd/MTU Every Calibration Skip One Calibration Skip Two Calibrations Skip Three Calibrations 8/9/1996 369 9/12/1996 1276 907.2 8/25/1998 2824 1547.5 2454.7 9/29/1998 3685 861.0 2408.5 3315.7 12/29/1998 5733 2048.0 2909.0 4456.5 5363.7 2/4/1999 6618 885.0 2933.0 3794.0 5341.5 3/11/1999 7477 859.0 1744.0 3792.0 4653.0 4/15/1999 8305 828.0 1687.0 2572.0 4620.0 5/20/1999 9159 854.0 1682.0 2541.0 3426.0 6/24/1999 9908 749.0 1603.0 2431.0 3290.0 8/4/1999 10769 861.0 1610.0 2464.0 3292.0 9/17/1999 11716 947.0 1808.0 2557.0 3411.0 11/30/1999 12553 837.0 1784.0 2645.0 3394.0 12/27/1999 13220 667.0 1504.0 2451.0 3312.0 2/2/2000 14128 908.0 1575.0 2412.0 3359.0 3/8/2000 14988 860.0 1768.0 2435.0 3272.0 4/12/2000 15849 861.0 1721.0 2629.0 3296.0 5/10/2000 16540 691.0 1552.0 2412.0 3320.0 6/14/2000 17433 893.0 1584.0 2445.0 3305.0 7/20/2000 18363 930.0 1823.0 2514.0 3375.0 8/24/2000 19269 906.0 1836.0 2729.0 3420.0
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency 1.2.
Include the derivation of the values of the standard deviations summarized in and provide an example of a calculation. This example should include the applicable LPRM calibration data and the associated data deduction that are used to calculate the exposure uncertainty for a LPRM.
Response :
Standard deviations are calculated from the relative differences between the plant actual LPRM currents and the predicted LPRM currents that are calculated by the equation 1.1. The calculation was based on all available LPRM data for both units. Data from a single LPRM is used here to provide an example calculation.
Columns 6 through 9 of Table 4 show the relative differences between the calibration currents and the predicted currents for different exposure intervals based on the data represented in Table 2. Columns 2 through 5 are the corresponding cycle exposure intervals.
The standard deviation for exposure intervals between 500 and 1500 megawatt-days/metric ton uranium (MWd/MTU), between 1500 and 2500 MWd/MTU, and between 2500 and 3500 MWd/MTU which are representative of an average value of calibration intervals of 1000, 2000, and 3000 MWd/MTU, respectively, are shown in Table 5. These values were generated based upon a single LPRM detector shown in Table 1. The standard deviation presented in the LAR was based upon data from all of the LPRM's from LSCS.
These analyses were performed in terms of MWd/MTU since the data in Table 1 was provided in the units of MWd/MTU. Conversion to units of EFPH is discussed in the response to Question 1.5 below.
1.3.
Explain how the LPRM decay factor was obtained and provide an example calculation to show the result of the calculated decay factor from the exposure data for one LPRM. Also, provide information to demonstrate that the value specified in the attachments is adequate for determining the LPRM uncertainties.
Response :
The uncertainty analysis was done for a fixed decay factor value of -0.092 for both units. By choosing a fixed sensitivity, one expects to maximize the uncertainty in the predicted response. The actual value currently in use for core monitoring is -0.1189.
Different decay factors were used to evaluate the impact on the uncertainty results. The results showed that the uncertainty increase is not very sensitive to different decay factors. In other words, the change in the predicted calibration uncertainties is independent of the choice in a fixed Page 6 of 15
Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency Table 4 Example for Relative Difference Calculation for one LPRM
- M/P = Measured/Predicted ATTACHMENT Page 7 of 15 Calibration Interval (Cycle Exposure, MWd/MTU Relative Difference (WP-1*
Date Every Interval Skip One Calibration Skip Two Calibrations Skip Three Calibrations Every Difference Skip One Calibration Skip Two Calibrations Skip Three Calibrations 9/12/1996 907.2
-0.039 8/25/1998 1547.5 2454.7 0.042 0.001 9/29/1998 861.0 2408.5 3315.7 0.045 0.088 0.046 12/29/1998 2048.0 2909.0 4456.5 5363.7
-0.013 0.032 0.075 0.033 2/4/1999 885.0 2933.0 3794.0 5341.5
-0.001
-0.014 0.031 0.073 3/11/1999 859.0 1744.0 3792.0 4653.0 0.006 0.005
-0.008 0.037 4/15/1999 828.0 1687.0 2572.0 4620.0
-0.001 0.005 0.004
-0.009 5/20/1999 854.0 1682.0 2541.0 3426.0 0.014 0.013 0.019 0.018 6/24/1999 749.0 1603.0 2431.0 3290.0 0.002 0.026 0.034 0.049 8/4/1999 861.0 1610.0 2464:0 3292.0
-0.005
-0.003 0.021 0.029 9/17/1999 947.0 1808.0 2557.0 3411.0 0.045 0.041 0.043 0.067
. 11/30/1999 837.0 1784.0 2645.0 3394.0
-0.113
-0.073
-0.077
-0.076 12/27/1999 667.0 1504.0 2451.0 3312.0 0.003
-0.111
-0.070
-0.075 2/2/2000 908.0 1575.0 2412.0 3359.0
-0.016
-0.013
-0.125
-0.085 3/8/2000 860.0 1768.0 2435.0 3272.0 0.022 0_.00_6
_0_
.009
-_0.106 4/12/2000 861.0 1721.0 2629
.0 3296
.0 0.008 0.029 0.013 0.016 5110/2000 691.0 1552.0 2412.0 3320.0
-0.001 0.006 0.028 0.012 6/14/2000 893.0 1584.0 2445.0 3305.0
-0.012
-0.014
-0.006 0.015 7/20/2000 930.0 1823.0 2514.0 3375.0
-0.001
-0.014
-0.015
-0.007 8/24/2000 906.0 1836.0 2729.01 3420.0 - -0.003
--0.0021
-0.0101
-0.012
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency Table 5 Example of the Standard Deviation for one LPRM decay constant. However, the analysis based on the plant LPRM calibration history data indicates that the decay factor used in the POW ERPLEX-III input deck should be adjusted for 2500 EFPH extended calibration interval to be consistent with the plant historical data. Therefore, the POWERPLEX-III input deck will be adjusted as part of the implementation actions associated with this amendment. The average decay factors calculated from actual plant data are -0.10 for both units.
Table 6 provides the calculated uncertainty changes for different decay factors using only the Unit 1 data (Unit 2 data is consistent with Unit 1).
Table 6 Uncertainties for Different Decay Factors The average decay constant value is determined by performing a least squares fit of the logarithm of the actual calibration currents as a function of detector exposure (SNVT). The data is illustrated in Figure 1 for an individual LPRM detector.
Page 8 of 15 Decay Factor 1000 MWd/MTU 2500 MWd/MTU Uncertainty Increase
-0.080 0.0331 0.0398 0.0068
-0.092 0.0332 0.0406 0.0074
-0.100 0.0333 0.0412 0.0079
-0.1189 0.0335 0.0427 0.0092 Exposure Interval MWd/MTU 500-1500 1500-2500 2500-3500 Standard Deviation 0.030 0.038 0.041
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency Figure 1 Calibration Current for LPRM #344 Pseudo Calibration Current -
Least Squares Fit 1.4.
Explain how the accumulated exposure values were obtained.
Response :
The accumulated exposure values were obtained from the core monitor system (i.e., POWERPLEX-II and POWERPLEX-III). MICROBURN-B, which was implemented in POWERPLEX-II, calculates the flux at the LPRM location and integrates this value over time. MICROBURN-B2, which was implemented in POWERPLEX-III, has the same capability. The resultant values from both core simulators were demonstrated to produce similar results. The MICROBURN-B and MICROBURN-B2 methodologies have been reviewed and approved by the NRC as noted in Reference 1.
1.5.
The analysis summarized in Attachment 5 evaluates the increase in LPRM response uncertainty when accounting for the TS SR 3.0.2 allowed 25 percent extension of the calibration interval (i.e., 2500 EFPH, however the upper bound calibration interval used was of 2500 MWDIMT This value equates to 2336 EFPH. When comparing the results shown in Attachment 4 to those in Attachment 5, it was noted that the difference between them increases as the exposure interval increases. Based in the information provided, the NRC staff does not have confidence that if the analysis was performed for 2500 EFPH instead of 2336 EFPH, the increase of standard deviation not would be significantly higher than stated in Attachment 5.
- Thus, Page 9 of 15
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency the uncertainty value assumed in the MCPR analysis could be exceeded.
Provide an analysis that confirms that the change in LPRM calibration frequency continues to allow the 25 percent extension.
Response :
An additional analysis was performed for 2500 EFPH (i.e., 2675 MWd/MTU) instead of 2500 MWd/MTU. The results shown in Table 7 demonstrate that the uncertainty increase is still bounded by the value that is used in the MCPR safety limit analysis (an increase of 0.9%). However, based on the plant LPRM calibration history data and as part of implementation actions, the decay factor used in POWERPLEX-III input deck will be adjusted to
-0.100 for a nominal 2000 EFPH extended calibration interval.
Table 7 Uncertainties for Different Decay Factors
- 2.
Provide a description of the method used to collect the data used by the analysis referenced in Attachments 4 and 5.
2.1.
Include the nature of the data points used by Attachment 5 for each interval and the reason of why the range selection of +/-500 MWDIMT is conservative and acceptable.
Response :
In Attachment 4 to Reference 1, the selected ranges fit the actual plant data better, the data with exposure intervals up to 1400 MWd/MTU (1000 EFPH x 1.07 x 125% = 1337 MWd/MTU) were used for 1000 EFPH with 25%
extension, and the data with exposures from 1400 MWd/MTU to 2800 MWd/MTU (2000 EFPH x 1.07 x125% = 2675 MWd/MTU) were used for 2000 EFPH with 25% extension. Attachment 5 to Reference 1 documents the analysis that specifically quantifies the uncertainty at the upper end of the 25% extension. The value of 500 was chosen in order to balance actual plant calibration data around the desired calibration interval and provide sufficient data for a meaningful standard deviation. The results showed this approach is conservative compared to the analysis in Attachment 4 for a nominal value of 2000 EFPH.
Page 10 of 15 Decay Constant 1000 MWd/MTU 2675 MWd/MTU Uncertainty Increase
-0.080 0.0331 0.0407 0.0076
-0.092 0.0332 0.0415 0.0083
-0.100 0.0333 0.0422 0.0089
-0.1189 0.0335 0.0439 0.0103
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency 2.2.
Explain the criteria for double counting the collected data used and how the double counting of data contributed to the total uncertainty.
Response :
Intermediate calibration currents were calculated from the actual calibration current from a previous calibration current adjustment and the measured GAF values for calibration of interest. The GAF value is used to determine the adjustment to the LPRM calibration current when no physical change in the calibration current was made. Each calibration point is independent of the previous calibration point. Evaluation of the standard deviation of a collection of points measured the uncertainty of that population ; in this case the parameter of interest is the calibration interval. Use of the same data points for a different standard deviation of a different collection of points is not really double counting but is used to identify a different population. For example, data between 1500 and 2500 MWd/MTU allows us to determine the expected value for 2000 MWd/MTU, while data between 1750 and 2750 MWd/MTU allows us to determine the expected value for 2250 MWd/MTU.
2.3. The third paragraph on page 6 of Attachment 1 indicates that the calculation data between 2000 MWDIMT and 2500 MWDIMT were used to calculate the standard deviations for both 2000 MWDIMT and 2500 MWDIMT calibration intervals. Provide an analysis to quantify the effect of the use of overlapping data range on the calculated standard deviations for the 2000 MWDIMT and 2500 MWDIMT calibration intervals.
Response :
There are the two data sets for representing calibration interval 2000 and 2500 MWd/MTU based on the +/-500 MWd/MTU range. The standard deviations of 2000 and 2500 MWd/MTU intervals are compared separately to 1000 MWd/MTU. The overlapping data range has no effect on the uncertainty calculation. The independence of the data points is described in the response to RAI question 1.1.
- 3.
The first paragraph on page 5 of Attachment 1 states that "LSCS currently uses an improved POWERPLEX -111 core monitoring system and newer design LPRM chambers that exhibit more consistent sensitivity behavior than the older LPRM detectors. " Provide a comparison of the LPRM exposure uncertainties based on appropriate core monitoring systems (CMS) and LPRM calibration data to show that the newer CMS and LPRM detectors provides more accurate power indications than that based on the older CMS and LPRM detectors.
Response :
All LPRM calibration data collected by LSCS for use in the LPRM uncertainty analysis are from NA300 LPRM detectors. The NA200 LPRM detectors are no longer used at LSCS and therefore their data was not applied in any of the analysis. The LPRM detector exposures were obtained from several versions of Page 1 1 of 15
ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency the CMS. The latest POWERPLEX-III version calculates a more accurate neutron flux than POWERPLEX-II and therefore the exposure obtained for the LPRMs are more accurate but not significant enough to impact the results obtained by the analysis.
- 4.
Address the newer design of LPRM chambers in more detail. How was the plant exposure data collected from 1996 through 2006 applied to the new LPRM chambers?
Response :
All LPRM calibration data collected by LSCS for use in the LPRM uncertainty analysis are from GE Reuter-Stokes NA300 LPRM detectors. The LPRM assembly consists of four sensors located at predetermined elevations and a calibration tube for the traversing incore probe detectors. The LPRM detector is a fission chamber with a mineral insulated coaxial signal cable. Improved seal technology improves detector signal reliability and ensures long detector life. The NA300 has an additional seal below the core and outside the fast neutron flux region. The fission chamber consists of a cathode and anode assembly with high-purity argon gas. These detectors are long life due to the cathode being metallurgically bonded with a mixture of U-234 and U-235 oxides. This particular bonding prevents fission product bombardment from causing the uranium coating to flake off of the cathode as observed in NA200 LPRMs. As U-235 is depleted, the U-234 is converted to U-235, so the fission chamber extends its life and output current.
- 5.
The last paragraph on page 4 of Attachment 1 states that the current frequency interval is based, in part, on operating experience with previous core monitoring systems and that LSCS currently uses an improved POWERPLEX -111 core monitoring system. The NRC staff found several records that document LSCS issues regarding POWERPLEX - 11 from 1999 to 2002. Specifically, the issues are: POWERPLEX -11 use of non-conservative steam tables for core monitoring, an error in one input into POWERPLEX - 11 calculation resulting in exceeding the authorized thermal power; feedwater flow input problems; and Unit 2 use of a revised POWERPLEX - 11 deck which did not contain all the gamma TIP data constants necessary to monitor core thermal limits to the correct accuracy. How does the LSCS transition to POWERPLEX -111 assure that errors of this nature are not repeated?
Response :
LSCS currently uses the POWERPLEX-III core monitoring system. The transition to POWERPLEX-III was thoroughly planned and executed. All validation and verifications were performed to ensure the appropriate outputs are obtained.
POWERPLEX-III input decks are generated and verified to ensure that they are correct using approved procedures and processes to ensure issues like those previously identified are not encountered again. Exelon has a robust corrective action program, which identifies issues and ensures that the appropriate actions Page 12 of 15
- 6.
Provide a discussion to confirm that the OPRM and RBM systems will not be adversely affected by the requested extension of calibration frequency with respect to the LPRM input to these systems.
Response :
Rod Block Monitor (BM) System ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency are taken to prevent reoccurrence of issues. LSCS has operated with POWERPLEX-III installed since December 2003 and there have been no repeat occurrences of these issues.
A reduced LPRM (TIP Set) calibration frequency will not adversely affect the RBM function. The RBM uses the LPRM signals that are nulled prior to rod movement.
When a rod is selected, the RBM will go through a null sequence that adjusts the LPRM gains to match the APRM power. Specifically, the LPRM outputs associated with the selected rod are averaged and then compared with the reference APRM signal. If the average LPRM signal is greater than or equal to the APRM reference signal it will be used, if it is less than the APRM reference signal the gain circuitry will increase the gain until it is equal or slightly greater than the reference. This new averaged LPRM signal is compared to the flow reference signal (i.e., Reactor Recirculation total flow signal). A rod block will be generated if the LPRM average power exceeds the flow reference set point. Therefore, since the actual LPRM signal gain is corrected/adjusted to the reference APRM signal, a reduced LPRM calibration frequency will not adversely affect the RBM function.
Oscillation Power Range_ Monitor f OPRM) Svstem The OPRM System trip depends on the relative change in LPRM average power and is not dependent on the specific LPRM gains.
The OPRM set point (Period Based) requires that oscillations be in frequency with that characterized by thermal-hydraulic oscillations (i.e., period of 1.0 to 3.5 seconds) and continue in this frequency for 14 counts. Then if the counts reach the set point, the amplitude is checked, and a trip signal is generated if the amplitude has increased to 1.11 times the prior 5 second LPRM average power.
The next trip algorithm is the amplitude based, and its set point is 1.3 times the prior 5-second LPRM average power. The last algorithm is the growth rate based trip, which has a growth rate factor setpoint that is 1.3 times the prior 5-second average LPRM power.
Since the trips of the OPRM system are dependent on the relative change in LPRM average power and are not dependent on the specific LPRM gains, a reduced LPRM calibration frequency will not adversely affect the OPRM function.
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ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency
- 7.
Demonstrate that there is no reduction in opportunity to detect the failure of the LPRM by the requested extension of calibration frequency. If there is a reduction in opportunity to detect the failure of the LPRM, demonstrate that the increased risk is small and acceptable.
Response :
There are several ways that a LPRM detector failure can be detected.
The POWERPLEX core monitoring system monitors all installed LPRMs for electronic drift. Criteria have been established to detect and identify those detectors whose responses change unexpectedly.
If the indication for a detector is outside of the acceptance band, a drift file (DRF) is created and a message is sent to the POWERPLEX typer in the control room.
POWERPLEX monitoring cases run every two hours and any LPRM detector failure or detector change can impact calculated thermal limit values and preconditioning envelope values.
The plant process computer (PPC) also continuously monitors individual LPRM detectors and has a similar drift algorithm.
If the acceptance criteria for an LPRM is violated, an alarm is sent to the PPC terminal in the main control room and the detector changes color on the PPC LPRM display.
In addition, there are two Technical Specification (TS) Surveillance Requirements (SR) that require Average Power Range Monitor (APRM) readings to be routinely checked. SR 3.3.1.1.1 requires a Channel Check of the APRM readings every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. SR 3.3.1.1.2 requires a weekly verification that the absolute difference between the APRM channel readings and the reactor power is not more than 2%
of rated thermal power (RTP) while operating at greater than or equal to 25% RTP.
Not only are these APRM readings recorded and checked twice daily during the operator rounds, the APRM GAFs are continuously calculated and displayed for the operators. Errors in detectors that are significant will cause the APRM values to change and the GAFs to change. Any GAFs that fall outside of the TS required 2% accuracy limit will change color and prompt an investigation by the operators.
Operators also have indication for all LPRM detectors above the LPRM cabinets in the control room. An indicator bulb illuminates if any of the detectors fail high or low during normal operation. This requires prompt investigation.
Finally, during the LPRM calibration, there is an opportunity to detect LPRM errors.
LPRM GAFs are determined during the calibration process so that the detector gains may be adjusted to match the values corresponding to the incore probe traces. These GAFs are checked to make sure that they are not abnormal relative to those typically observed. Any GAF that falls outside of a range between 0.95 and 1.05 is investigated, and any GAF that falls outside of a range between 0.9 and 1.1 is corrected by changing the LPRM current.
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ATTACHMENT Additional Information Supporting the Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency Since the LPRM response is frequently monitored between calibration intervals, extending the calibration frequency from 1000 EFPH to 2000 EFPH will not have an adverse impact on the ability to identify a detector error.
References :
1.
Letter from Mr. P. R. Simpson (Exelon Generation Company, LLC) to U. S. NRC, "Request for a License Amendment to Revise Local Power Range Monitor Calibration Frequency," dated July 25, 2008 Page 15 of 15