ML20198N824
| ML20198N824 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 01/15/1998 |
| From: | Sorensen J NORTHERN STATES POWER CO. |
| To: | NRC |
| Shared Package | |
| ML20070L182 | List: |
| References | |
| NUDOCS 9801210227 | |
| Download: ML20198N824 (12) | |
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UNITED STATFS NUCLEAR REGULATORY COMMISSION l NORTHERN STATES POWER COMPANY .
l PRAIRIE ISLAND NUCLEAR GENERATING PLANT ' DOCKET NO. 50 282 50-306 3
REVISED REQUEST FOP. AMENDMENT TO l OPERATING LICENSES DPR 42 & DPR 60 i l
LICENSE AMENDMENT REQUEST DATED JANUARY 15,1998 ;
MOVEABLE INCORE DETECTOR THIMBLE REDUCTION !
I Northern States Power Company, a Minnesota corporation, requests authorization for !
chtnges to Appendix A of the Prairie Island Operating License as shown in the !
! attechments labeled Exhibits A, B, C, D, E AND F. Exhibit A contains a description of ;
the proposeo changes, the reasons for requesttg the changes, the supporting safety j evaluations and significant hazards determinat!ons. Exhibit B contains current Prairie Island Technical Specification pages marked up to show the proposed changes. .
4 Exhibit C contains the revised Technical Specification pages. Exhibits D and E contain l proprietary and non-proprietary versions of the Westinghouse Thimble Deletion Study :
for Prairie Island Unit 1 Cycle 19. Exhibit F is a Westinghouse Electric Corporation effidavit for withholding of proprietary information. !
This letter contains no restricted or other efense information. [
p NORTHERN STATES POWER COMPANY I By, e -
plPBorenson' Plant Manager.
ta g 0 a
' ear Generating Plant ;
On this of f/tdIN hfdfore me a mtary public in and for said County, personally a dred Joel/f5bronsen, Plant la eger, Prairie Island Nuclear !
Generating Plant, being first d6ly sworn acknowledged that he is authorized to .}
execute this document on behalf of Northern States Power Company, that he knows the :
contents thereof, and that to the s,t pf his knowie'dge, ,tormation, and belief the !
st ts made in it are true in not in rposed for delay, i
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Exhibit A j Prairie Isl.nnd Nuclear Generating Plant t License Amendment Request Dated January 15,1998 Evaluation of Proposed Changes to the l Technical Specifications Appendix A of Operating License DPR-42 and DPR 60 Pursuant to 10 CFR Part 50, Sections 50.59 and 50.90, the holders of Operating Licenses DPR-42 and DPR-60 hereby propose the following changes to Appendix A, Technical Specifications:
Backaround and Reasons for Channes The Moveable incore Detector System consists of 36 incore flux thimbles to permit measurement of the axial and radial neutron flux distribution within the reactor core.
Moveable fission chamber detectors are avdlable to scan the length of the 36 selected core locations. Specification 3.11.A currently requires at least 75% of the detector thimbles to be operable when performing a flux map to ensure compliance with the peaking factor requirements of Technical Specification Section 3.10.B. Ir. accordance with the surveillance requirements in Specification 3.10.B.2 the peaking factors mJst be determined to be within limits at least once per 31 effective fall power days. If the moveable detoctor system is degraded to the extent that less than 75% of the detector thimbles are available, Technical Specification 3.11.A requires that the measuremer'.'
error allowance due to incomplete mapping shall be substantiated by the licensee.
The Prairie Island Unit 1 moveable incore detectors have experienced sticking problems which have prevented them from fully inserting into all of the thimble tubes.
Due to these recurrent sticking problems, the Unit 1 thimbles were cleaned following Cycle 15 increasing the number of accessible thimbles from 28 to 32, in an attempt to regain the remaining thimbles,6 thimbles were replaced and the slip clutches on the detector drives were either rebuilt or replaced at the end of Cycle 16. The net effect on the system for the beginning of Cycle 17 was that 34 of the 36 thimbles were accessible. In Cycle 17 three thimbles were lost early in the cycle and 31 of 36 remained accessible through the end of the cycle. Following Cycle 17,35 of 36 thimbles were eddy current tested successfulh.
By June of 1996 (early in Unit 1 Cycle 18) the number of accessible detectors had fallen to 29 of 36 (81%). On July 15,1996 NSP submitted a License Amendment request which proposed one-time only changes for Unit 1 Cycle 18 that would allow the use of the moveable incore detector system for measurement of the core peaking factors with less than 75% and greater than or equal to 50% of the detector thlmbles available. This License Amendment flequest was supported by a thimble deletion study performed by Westinghouse for Prairie Island Unit 1 Cycle 18. A License
_ .- . - . ~ - ~ _ . - - =- - -_ - - - - - - .
6 Exhibit A ;
Page 2 cf 11 4
Amendment, allowing the one time only reduction in the required number of ir. core ,
detector thimbles, was epproved by the NRC on October 10,1996. !
During the remainder of Unit 1 Cycle 18,3 additional detector thimbles became insccessible, which resulted in 26 of 36 (72%) of the detector thimbles available at the
- i end of Cycle 18.
During the Fall 1997 Unit 1 Refueling Outage, the incore thimbles were cleaned to attempt to recover access to thimble 6 that had become obstructed. Two incore detectors had been cut off and abandoned inside the thlmbles during previous cycles because they had become stuck in the thimble. At the beginning of the outage attempts were nisde to pull both detectors, one of the detectors was removed from its thimble but the other remained stuck. Prior to refueling, the 35 accessible thlmbles underwent a solvent cleaning that had been successful at other nuclear plants. The '
process included flushing each thimble with an approved solvent and then with high pressure domineralized water. The thimbles were then blown dry with station air ,
folbwed by vacuum drying to remove residual moisture. Each thimble was then tested with the insertion of a test cable to verify no obstructions. During the solvent flush of one of the thimbles, the cleaning tube became stuck in the same mann6r as the detectors. The tube could not be removed and was abandoned. The remaining 34 thimbles were all accessible to their full length as demonsi '*lth the test cable.
Following the refueling outage, a number of the thimbles thm had been obstructed prior to the cleaning proved inaccessible for the first flux' map of the cycle. The subsequent flux maps to date have resulted in continued access to only one of the thimbles that were obstructed prior to the cleaning, in summary, the cleaning and post-cleaning testing performed during the Fall 1997 refueling outage resulted in the recovery of one previously blocked thimble.
The following table illustrates the actual number of accessible thimbles to date for Unit i Cycle 19.
Burnup Accessible FAH Margin FQ Margin Flux Mao { MWD /MTU) % Power Thimbles to Limit to Limit 1 7 30 29 15.4% 45.4% "
2 35 70 27 12.4 % 35.1 %
3* 146 100 27 5.0% 4.1%
4 223 100- 27 4.4% 9.9%
5 230 100 2? 4.6% 10.1%
- Equilibrium map .
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6 Exhibit A Pag) 3 cf 11 Attachment 1 to this Exhibit shows the thimble locations inaccessible during the performance of flux map 3 for Unit 1 Cycle 19. As can be seen from the data above, only 75% (27 of 36) of the incore thimbles were accessible. The available detectors were well distributed throughout the core and the thimble coverage was adequate.
Until corrective actions can be taken, in view of the recent decline in the number of detector thimbles available, Northern States Power is concerned about meeting the 75% criterion called for in Technical Specification 3.11.A for future required moveable incore detection system monitoring / calibration uses. Northern States Power believes that this problem has the potential to worsen throughout Unit 1 Cycle 19. Failure to have at least 27 thimbles accessible (75%) could eventually result in a forced Unit 1 3
shutdown due to the inabil;ty to determine the peaking factors per the requirements of Technical Specifications 3.10.B.1,3.10.B.2 and 3 " 9.3.
In order to be prepared for the possibility that the number of available moveable detector thimbles could fali celow 75%, Northern States Power commissioned Westinghouse Electric Corporation to evaluate, per Technical Specification 3.11.A, the measurement error allowance due to flux mapping with less than 75% of the detector thimbles available. As part of their evaluation, Westinghouse assessed the incremental peaking factor measurement uncertainties and excore calibration impact associated with a roduction to a minimum of 18 (i.e. 50%) of the 36 moveable detector thimbles in Prairie Island Unit i for Cycle 19. The results of the Westinghouse evaluation is provided as Exhibit D.
Based on the results of the Westinghouse evaluation discussed above, this License Amendment Request proposes changes to the power distribution limit requirements in Technical Specifications 3.10.B.1,3.10.B.2,3.10.B.3.b.2 and 3.10.B.3.d.1 to increase the uncertainty factors applied to the peaking factors when a flux map is performed with less than 75% of the thimbles. Cnanges are also proposed to Technical Specification 3.11.B to clarify the requirements for the number of detector thimbles required pu core quadrant when the number of available detector thimbles is less than 75%. The proposed changes are one time only changes, applicable only to the remainder of Prairie Island Unit 1 Cycle 19. They wed allow continued operation of Unit i for the remainder of Cycle 19, at which time further corrective actions will be taken to return inaccessible detector thimbles to service. The possibility of replacement of all of the inaccessible detector thimble tubes at the end of Unit 1 Cycle 19 is being evaluated.
. Exhibit A Page 4 cf 11 Proposed Channes The proposed changes to the Prairie Island Technical Specifications are described below, and the specific wording changes to Technical Specifications are shown in Exhibit B.
A. Eroposed Chanaes to Technical Specification 3.10.B Technical Specifications 3.10.8.1,3.10 B.2,3.10.B.3.b.2 ano 3.10.B.3.d.1 are n?odified, as shown !n Exhibit 8, to note that the footnotes which address the peaking factor measurement uncertainties are appilcable to Unit 1 Cycle 19 rather than Cycle 18. The footnotes, which will be applicable only for Prairie Island Unit 1 Cycle 19, instruct that the peaking factor measurement uncertainty be increased linearly, as a function of the number of available detector thimbles, when the number of available detector thimbles is greater than or equal to 50% and less than 75% of the tott. For F"o the 5% measurement uncertainty, currently specified in the Technical Specifications, increases linearly to a maximum of 8% for the condition with 50% of the detector thimbles available. For F"as the 4%
measurement uncertainty, currently specified in the Technical Specifications, increases linearly to a maximum of 6% for the condition with 50% of the detector thimbles available. These additional uncertainties are in accordance with the results of the Westinghouse evaluation for Unit 1 Cycle 19 discussed above.
B. Proposed Chanaes to Technical Specification 3.11.8 Technical Specification 3.11.B is modified, as shown in Exhibit B, to note that the footnote is applicable to Unit 1 Cycle 19 rather than Cycle 18. The footnote states that with greater th 'n or equal to 50% and less than 75% detector thimbles available, the 2 thhnole per quadrant requirement must be met for each of the four horizontal-vertical quadrants and the four diagonally-bounded quadrants (eight individual quadrants in total). This is consistent with the assumptions in the Westinghouse evaluation for Unit 1 Cycle 19 discussed above and it establishes the bounds of the applicability of the evaluation. Per the Westinghouse evaluation, detector thimbles located on the boundaries between quadrants can be counted toward each of the adjaccnt quadrants at the same time.
Bases for Specification 3.11.B The bases for Specification 3.11.B are revised in accordance with the changes made in the specification as stated above. The changes to the bases are shown in Exhibit B.
l e Exhibit A Page 5 cf 11 Safety Evaluation and Justification of Channes The tioveable incore detector system is used for confirmatory informatic . nd is not requiled for the day to day safe operation of the reactor (daily core pow, " :;ormanco is monitored by the excore detectors). The measurea power distribution .u affected by the 'true" power distribution that exists in the core, the predicted power distribution in the core, and the instrument thimble pattern. The thimbles are distributed nearly uniformly over the core with approximately the same number of thimbles in each quadrant. The number and location of these thimoles allows measurement of F"as to within 4% and F"o to within 5% with at least 75% of the detector thimble locations available.
The 75% detector thimble availahility requirement in Technical Specification 3.11.A was based on the requirements from Revisio ia of the Westinghouse Standard Technical Specifications. The requirement fur maintaining 75% of the detector thimbles available provides for a reasonable amount of failures of the incore detectors while encouraging licensees to strive for maintaining the system as near to 100% available as possible. Technical Specification 3.11.A allows continued use of the moveable incore detector system with less than 75% of the thimbles available if the measurement error allowance due to incomplete flux mapping is substantiated. As shown by the attached Westinghouse evaluation, the reduction of available detector thimbles to 50%,
as proposed by this amendment reque,3t, does not significantly degrade the ability of the detector system to measure core power distributions. However, use of the moveable incore detector system with less than 75% of the detector thimbles available requires the core peaking factor measurement uncertainties specified in Technical Specification Section 3.10.B be increased to compensate for the reduction in the number of available detector thimble locations.
As noted above, Northern States Power commissioned Westinghouse to assess the incremental peaking factor measurement uncertainties and excore calibration impact associated with a reduction to a minimum of 18 of the 36 moveable detector thimbles for Prairie Island Unit 1 Cycle 19. That evaluation is provided as Exhibit D to this submittal. That evaluation, which is based on a Westi.1ghouse generic thimble deletion analysis, indicates that additional uncertainties of 2% for F"as and 3% for F"o are appropriate when the numbcr of instrumented assemblies is reduced from 36 to 18.
The additional uncertainties should be applied linearly from below 75% to grerser than or equal to 50% detector thimble locations.
In addition to the uncertainty changes, the Westinghouse evaluation also specifies that vihanever greater than or equal to 50% and less than 75% of the detector thimbles are avaGable, there should be a minimum of two thimbles available per quadrant, where quadrant includes both horizontal-vertical quadrants and diagonally bounded quadrants (eight individual quadrants in total). This requirement improves the ability to distinguish between random and systematic thimble deletion events and establishes the bounds of
. Exhibit A Page 6 of 11 applicability of the pe aking factor uncertainties. The peaking factor measurement uncertainty analysis in the Westinghouse evaluation makes the assumption that thimbios were randomly deletsd from the core. If the thimbles are somehow systemmically deleted from use, then the calculated peaking factor measurement uncertainties will not apply. Technical Specification 3.11.0 currently requires a minimum of 2 detector thimbles per quadrant. However, the current requirement in Technical Specification 3.11.0 is not sufficient to distinguish between random c.nd systematic deletion events with high confidence. To help insure that thimble deletion is random, a footnote is being added to Technical Specification 3.11.8 to provide the more restrictivo quadrant requirements necessary for Unit 1 Cycle 19 with greater than or equal to 50% and less than 75% detector thimbles available. This footnote will establish the bounds on the number of detector thimbles per quadrant to be consistent with the Westinghouse evaluation and will significantly enhance the ability to distinguish between random and systemeatic events.
The Westinghouse evaluation concludes that with the inclusion of the additional peaking factor uncertainties, operation of the moveable incore detector system with a minimum of 50% of the thimbles available is acceptable with the above provisions on the number of detector thimbles per quadrant.
Burnup on Unit 1 Cycle 19 is currently about 1,000 MWD /MTU of a 18,717 MWD /MTU cycle. All power distribution surveillance parameters (F"as and F"o) currently have sufficient margin to their limits after the current Technical Specification required uncertainties are applied. Froni the flux map taken on December 21,1997, it is calculated that thera is approximately 5.0% margin to F"as (!.e.e Neasured F"as plus its measurement uncertainty in comparison to its Technical Specification limit) and N
approximately 4.1% margin to F o. The predicted peaks (F"as and F"o) provide adequate margin for implementation of the additional measurement uncertainties resulting from the Westinghouse evaluation to meet the Technical Specification limits i
through the remainder of the cycle.
i A summary of the results of the Prairie Island Unit 1 Cycle 19 low power physics testing is provided as Attachment 2 to this exhibit. The Unit 1 Cycle 19 low power physics test results showed good agreement between predictions and measurements.
As noted above, a license amendment allowing a one-time reduction in the required number of incore detector thimbles, was approved for Prairie Island Unit 1 Cycle 18 on October 10,1996. Similar requests for relaxation of the 75% requirement have also been reviewed and found acceptable by the NRC in the past. Similar license amendments approved by the NRC include; Beaver Valley Unit 1 Amendment 61 app oved on January 19,1983, McGuire Unit 1 Amendment 101 approved on December 14,1989 and McGuire Unit 1 Amendment 117 approved on February 27, 1991-
4 Exhibit A Page 7 cf 11 i
Conclusion ;
This proposed change to the Prairie Island Technical Specifications would allow an increase in plant operating flexibility for Unit 1 Cycle 19, while maintaining sufficient data collection capability to ensure that the operation of the core is within licensed limits. This change would be utilized only if further failures of the Unit i detector i thimbles occur during the remainder of Cycle 19. Based upon the preceding justification ,Northern States Power concludes that the proposed amendments are ,
necessary to avoid an unnecessary potantial shutdown of Prairie Island Unit 1 which
. has real benefits in terms of availability, component lifetime (avoiding an unnecessary thermal cycle on the reactor and associated systems), and safety (the potential for ;
plant upset and challenges to safety systems is increased during a plant shutdown). .
Based upon the preceding safety analysis and the results of the Westinghouse evaluation (Exhibit D), Northern States Power believes there is reasonable assurance that the health and safety of the public will not be adversely affected by the proposed Technical Specification changes.
Determination of Slanificant Hazards Considerations The proposed changes to the Operating License have been evaluated to determine :
whether they constitute a significant hazards consideration as required by 10 CFR 50, .
Section 50.91 using the standards provided in Section 50.92. This analysis is provided below ;
- 1. The orooosed amendment will not involve a slanificant increase in the probability or conseauences of an accident previousiv evaluated.
The proposed changes do not involve an increase in the probability of an accident previously evaluated. The moveable incore detector system is used only to provide confirmatory information on the neutron flux distribution and is not required for the daily safe operation of the core. The system is not a process variable that is an initial condition in the accident analyses. The only accident thL the moveable incore detector system could be involved in is the breaching of the detector thimbles which would be enveloped by the small break loss of coolant accident (LOCA) analysis. As the proposed changes do not involve any changes to the system's equipment and no equipment is operated in a new or more harmful manner, there is no increase in the probability of such an accident.
The proposed amendments would not involve an increase in the consequences of an accident previously evaluated. The moveable incore detector system provides a monitoring function that is not used for accident mitigation (the system is not used in the primary success path for mitigation of a design basis accident). The ability of the reactor protection system or engineered safety e r -m ,
J l
l ,
'
- Exhibit A l P ge s af 11 l
l features system instrumentation to mitigate the consequences of an accident will [
not be impaired by the proposed changes. The small break LOCA analysis (and ;
thus its consequences) continues to bound potential breaching of the system's l detector thimbles. -
i With greater than or equal to 50% and less than 75% of the detector thimbles I
available, core peaking factor measurement 'Jncertainties will be increased, which could impact the core peaking factors and as a result could affect the +
l consequences of certain accidents. However, any changes '.1 the core peaking !
factors resulting from increased measurement uncertainties will be compensated ;
for by conservative measurement uncertainty adjustments in the Technical i Specifications to ensure that pertinent core design parameters are maintair,ed. ;
Sufficient additional penalty is added to the power distribution measurements ;
such that this change will not impact the consequences of any accident !
previously evaluated. l Therefore, based on the conclusions of the above analysis, the proposed 1 changes will not involve a significant increase in the probability or consequences !
of an accident previously evaluated.
- 2. The proposed amendment will not create the oossibility of a new or different kind of accident from any accident previggjv analyzed 1 The proposed amendments would not create the possibility of a new or different '
kind of accident previously evaluated as they only affect the minimum complement of equipment necessary for operability of the moveaole incore detector system. There is no change in plant configuration, equipment or equipment design No equipment is operated in a new manner. Thus the changes will not create any new or different accident causal mechanisms. The accident analysis in the Updated Safety Analysis, Report remains bounding, ,
Therefore, based on the conclusions of the above analysis, the proposed f 4 changes will not create the possibility of a new or different kind of accident. i
- 3. The orooosed amendment will not involve i sianificant reduction in the marain of i safety. <
The proposed changes will net involve a significant reduction in a margin of safety. The reduction in the minimum complement of equipment necessary for the operability of the moveable incore detector system could only impact the monitoring / calibration functions of the system. Reduction of the number of ,
available moveable incore detector thimbles to the 50% level does not :
significantly degrade the ability of the system to measure core power <
distributions. With greater than or equal to 50% and less than 75% of the r detector thimbles available, core peaking factor measurement uncedtilnties will- ;
i
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l e Itzhibit A Page 9 of 11 !
i be increased, but will be compensated for by conservative measurement l uncertainty adjustments in th Technical Specifications to ensure that pertinent core design parameters are maintained. Sufficient additional penalty is edded to the power distribution measurements such that this change does not impact the safety margins which currently exist. Also, the reduction of available detector thimbles has negligible impact on the quadrant power tilt and core average axial power shape measurements. Sufficient detector thimbles will be available to ensure that no quadrant will be unmonitored.
Based on these factors, the proposed changes in this license amendment will not result in a significant reduction in the plant's margin of safety, as the core will continue to be adequately monitored.
Conclusion Based on the evaluation above, and pursuant to 10 CFR 50, Section 50.91 Northern States Power Company has determined that operation of the Prairie Island Nuclear Generating Plant in accordance with the proposed license amendment request does ,
not involve any significant hazards considerations as defined by NRC regulations in 10 CFR 50, Section 50.92.
Environmental Assepsment Northern States Power has evaluated the proposed changes and determined that:
- 1. The changes do net involve a significant hazards consideration,
- 2. The changes do not invcive a significant change in the types or significant increase in the amounts of any effluents that may be released offsite, or
- 3. The changes do not involve a r,lgnificant increase in individual or cumulative occupational radiation exposure.
Accordingly, the proposed changes meet the eligibility criterion for categorical exclusion set forth in 10 CFR Part 51 Section 51.22(c)(9). Therefore, pursuant to 10
' CFR Part 51 Section 51.22(b), an environmental assessment of the prnposed changes is not required.
Attachments
- 1. Instrument Thimble Locat:on (Map #119-03)
- 2. Unit One Cycle 19 Low Power Physics Testing Recults rev~~- - - ~ - ~,,~w-, ,.v.n ,a.. - , - . - - . - ~ ,
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4
- Exhibit A P:ge 10 cf 11 Attachment 1 Prairie Island instrument Thimble Locations (Map #119 03) -
1 2 3 4 5 6 7 8 9 10 11 12 13 .
l A 35 B 9 ,
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C 29 5 33 20 D 23 gj
%g;;g E 14 7 lljpg ylpp;:{
F 10 32 26 p
H 27 30 22 *: -
g z. a J 34 6 25 K 31 1S L 16 i M 3 is M Inaccessible hhN$ Location
- Exhibit A !
Page 11 si' 11 ,
Attachment 2 .
UNIT ONE CYCLE 19 LOW POWER PEYSICS TESTING RESULTS Initial Criticality at 02:30 on 12-13-1997.
Measured Prodi ted Review Parameter value value Criteria Error critical Boron concentration, 1800 178. i 50 ppm + 18 ppm ARO (ppm) :
Critical Boron concentration, 1683 1672 i 50 ppm + 11 ppm !
A bank in (ppm)
ITC ARO - (pcm/*r) -0.75 -0.81 > -4.94 40.06
< +5.0 ,
Control Rod By Rod Swap control Bank As Reference Bank (pcm) 1023 1083 i 10% -5.5%
Control Bank B (pcm) 579 541 i 15% +7.0%
Control Bank C (pcm) 952 944 i 156 +0.8%
control Banax D (pcm) 694 685 i 15% +1.3%
shutdown sank A (pcm) 749.5 776 i 15% -3.4%
Shutdown Bank B (pcm) 749.5 776 1 15% -3.4%
Total of All Banks 4747 4a05 i 10% -1.2%
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