ML20247N741
| ML20247N741 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 05/31/1989 |
| From: | Withers B WOLF CREEK NUCLEAR OPERATING CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-011, IEB-88-11, WM-89-0158, WM-89-158, NUDOCS 8906060013 | |
| Download: ML20247N741 (6) | |
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W64.F CREEK NUCLEAR OPERATING CORPORATION
' Bart D. Withers Proendent sind May 31, 1989 chief Executive Officer WM 89-0158 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk
- Mail Station F1-137 Washington, D. C. 20555
Reference:
WM 89-0075, dated March 3, 1989, from E. D. Withers.
WCNOC, to Document Control Desk, NRC
Subject:
Docket No. 50-482: Justification for Continued Operation Regarding NRC Bulletin 88-11, ' Pressurizer Surge Line -
Thermal Stratification
- Gentlemen:
The purpose of this letter is to transmit a copy of Wolf Creek Generating Station's (WCGS) justification for continued operation (JCO) regarding NRC Bulletin 88-11,
' Pressurizer Surge Line Thermal Stratification *.
The attached'JC0 is being submitted in accordance with the commitments made in the Reference letter and NRC Bulletin 88-11 idlich required the JC0 to be submitted within one year of receipt of the bulletin.
Wolf Creek Nuclear Operating Corporation (WCNOC) is participating in a program for partial resolution of this issue through the Westinghouse owners Group (WOG).
The attached JC0 utilizes the information, experience, and monitoring data obtained through the WOG program as well as plant specific information obtained for titis issue.
The JC0 will remain valid and in effect until the final response to Bulletin 88-11 is submitted. The current schedule shows completion of the final response to the bulletin in
' January 1991.
If you have any questions concerning this matter, please contact me or
- Mr. O. L. Maynard of my staff.
Very truly yours, I
Bart D. Withers President and Chief Executive Officer EDW/jad Atc u.hment cci B. L. Bartlett (NRC), w/a E. J Holler (NRC), w/a R. D. Martin (NRC), w/a D. V. Pickett (NRC), w/a gfl P.O. Box 411 / Burhngton, KS 6683D / Phone: (316) 364-8831
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'Attcchm:nt to VM 89-0158 i
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JUSTIFICATION FOR CONTINUED OPERATION REGARDING PRESSURIZER SURGE LINE STRATIFICATION Wolf Creek Generating Station BACKGROUND It was first reported in INPO SER 25-87 that temperature measurements at a German PWR indicated pressurizer surge line thermal transients different than derign.
Recent measurements at several domestic FWR's have indicated that the temperature difference between the pressurizer and the hot leg results in stratified flow in the surge line, with the top of the flow stream being hot (pressurizer temperature) and the bottom being colder (h 5
'8 t*8Perature).
The top-to-bottom temperature difference can reach 250 F to 300 F in certain modes of operation, particularly Modes 3, 4, or 5 during heatup and cooldown.
Surge line stratification causes two effects:
Global bending of the pipe differently than that predicted in the original design.
Fatigue life of the surge line piping could be reduced due to the global and local stresses from stratification and striping.
More recently, the NRC has issued Bulletin 88-11 " Pressurizer Surge Line Thermal Stratification,' December 20, 1988, identifying the following actions to be taken by licensees.
a) Conduct visual inspection - walkdown b) Update stress and fatigue analysis to account for stratification and striping c) Obtain monitoring data, as necessary The bulletin encourages licensees to perform actions b) and c) above through collective efforts with other plants.
In October 1988, Wolf Creek Nuclear Operating Corporation (WCNOC) and other members of the Westinghouse Owners Group (WOG) cuthtrized a program to perform a generic evaluation of curge ina stratification in Westinghouse FWR's that will address portict.s Jf dalletin 88-11.
The WOG pro 2 ram is derigne3 ta bene. fit from the experience gained in the performance of severtl plant rpecific analyses on Westinghouse PWR rurge lines.
The s e-.
dethiled analyses included definition of revised thermal transients (including stratification) and evaluations of pipe stress, fatigue usage factor, thermel striping, fatigua crack growth, leak before-
- break, and sapport loads.
The evermli cnalytical approeth used in all of 3
these analyses has been consistent and has been reviewed, in detail, by the NRC staff.
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. 'Attcchmint to WM 89-0158 Page 2 of 6 As of March 1989, plant specific analyses have been performed on 'five domestic We:tinghouse PWR's.
In addition, twelve. Westinghouse plants have completed or are currently performing an interim evaluation of surge line stratification which includee finite element structural analysis of their specific configuration under stratified loading conditions.
WOG Program Status As part of the current WDG Program, surge line physical and operating. data has been collected and summarized for all domestic Westinghouse PWR's (.55 units).
Information relating to piping layout, supports and restraints, components, size, material, operating history,
'etc.,
has been obtained.
This data has been evaluated in conjunction with available monitoring data and plant specific analyses performed by Westinghouse.
The results of.this evaluation were presented to the NRC in a meeting on April 11, 1989.
The evaluation is being formalized into a Westinghouse topical report (WCAP--
12277, Proprietary and WCAP-12278 Non-proprietary version) scheduled for submittal to the NRC on June 15, 1989.
- This topical report forms the basis for the following justification for continued operation.
JUSTIFICATION FOR CONTINUED OPERATION A.
Stratification Severity 0
Thermal stratification ( AT > 100 F) has been measured on all surge lines for which monitoring has been performed and which have been reviewed by the WOG to date (eight surge lines including Wolf Creek Generating Station).
The amount of stratification measured and its variation with time (cycling) varies.
This variation has been conservatively enveloped and applicability of these enveloping transients has been demonstrated for plant specific analyses.
Various surge line design parameters were tabulated for each plant.
From this, four parameters judged to be relatively significant were identified.
Pipe inside diameter e.
l-B. Piping slope (average)
C. Entcance angle of hat leg nozzic D. Presence cf mid-line vertical riser
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'AttachmInt to WM 89-0158 Psgs 3 of 6 These parameters were used in a grouping evaluation which resulted in the definition of 10 monitoring groups corresponding to various combinations of these parameters at Westinghouse FWR's.
Approximately 40% of the plants fall into one group for which a large amount of monitoring data has already been received and for which the enveloping thermal transients, discussed above, are applicable.
The remaining 60% of Westinghouse PWR's are divided among nine other additional groups.
Although monitoring data has not yet been received representative of all these nine groups, in general, the combination of significant parameters of these groups is-expected to decrease the severity of stratification below that of the enveloping transients.
This conclusion is also supported by a comparison of available monitoring data.
B.
Structural Effects Significant parameters which can influence the structural effects of stratification are:
a.
Location and design of rigid supports and pipe whip restraints b.
Pipe layout geometry and size c.
Type and location of piping components Although the material and fabrication techniques for Westinghouse surge lines are reasonably consistent and of high quality, the design parameters listed above vary among Westinghouse PWRs.
This variation in design is primarily a result of plant specific pipe routing requirements.
A preliminary evaluation, comparing the ranges of these parameters to those of plants for which plant-specific analysis and interim evaluations are available (approximately 20% of Westinghouse PWRs),
has been performed.
This comparison indicates with a high degree of confidence that, from a combined transient severity and structural effects standpoint, the worst configuration has most likely been evaluated.
This conclusion is supported by plant-specific analyses covering five plants and interim evaluations of six additional plants (interim evaluation is in progrees on six more plants as of March 1989).
These analyses and evaluations have included various piping layouts, pipe sizes, support and restraint designs and piping components.
Although the full range of variation in these parameters has not been evaluated, experience gained from these evaluations indicates that further evaluations will not result in a more limiting configuration than those already evalucted.
C.
Operating Procedures The WOG currently has available the surveys of plant-specific operating procedures performed in support cf existing analyses. Experience indict.tes that heatup and cooldown procedures have a significant effect on stratification in the surge line.
All conclusions reached by the WOG to date have assumed a steam bubble mode heatup and cooldoen procedure which may result in a temperature difference between the pressurizer and reactor l
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Attechm:nt to WM 89-0158
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'4 of 6 coolant system (RCS) hot leg of more than 300 F.
In many cases, individual plant operating procedures and_ technical specifications provide limits on this value.
It is also known that some procedures utilize nitrogen, during at least part of the heatup/cooldown cycle, as a means of providing a press'we absorbing space in the pressurizer. Based on information currently s"L11able to the WOG, a high confidence exists that the steam bubble mode heatup, assumed to date, is conservative with respect to Westinghouse PWRs.
D.
Pine Stress and Remaininn Life The design codes for surge line piping have requirements for checking pipe stress limits and the effects of fatigue loadings.
These stress limits provide a means of controlling stress from primary loads such as pressure, dead weight, and design mechanical loading, as well as stress from secondary loads such as thermal and anchor motion effects.
Stratification in the surge line is a. secondary load which will only affect the qualification of secondary stresses.
The qualification of primary stresses is not affected by this loading.
Secondary stresses are controlled to prevent excessive displacements and gross plasticity and to prevent excessive fatigue loadings in the pipe.
The basic characteristic of a secondary stress is that it is self limiting;
- thus, a failure from a single application of a secondary stress is not expected.
For the stratification
- issue, the potential effects of excessive displacements have been investigated at WCGS through a detailed visual observation of the surge line during the December, 1988, refueling outage.
The results of the walkdown required for Bulletin 88-11 action 1.a were found to be satisfactory.
The effects of secondary stresses on the remaining life of the surge line have been evaluated on a generic basis through the WOG program.
The following summarizes the results of this evaluation.
All plant specific analyses performed as of March, 1989 have demonstrated compliance with applicable ASME Codes and a surge line fatigue life in excess of a 40 year plant life.
Review of plant specific fatigue calculations indicates that the surge line fatigue life is primarily dependent on the number of heatup and cooldown cycles, rather than years of operation.
Considering the worst case years of operation (28.5 years) in combination with the worst case number of heatup-cooldown cycles (75, at a different plant) at any Westinghouse PWR. and assuming n 40 year life for all surge
- lines, it it estimated that no more then approxiraately 50% of the fatigue life has been used at any Westinghouse plant to date.
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1 Attschm2nt!to WM'89-0158
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design life. considering 200 heatup-cooldown cycles (used.in plant specific analyses),. this would indicate.approximately 100 remaining cycles.
.This number of remaining cycles far exceeds the-postulated worst case number for the two year time frame needed to resolve the stratification issue.
E.-
Leak Before Break All the plant specific analyses performed to date that.have included the-loadings due to stratification and striping have validated the " leak-before-break" concept and have substantiated a 40-year plant life.
Fatigue crack growth calculations, performed as part of these plant specific analyses, have demonstrated-that any undiscovered crack as large as 10% of the wall thickness would not grow to 'cause leakage within a 40 year plant life.
Nevertheless,' any postulated through wall crack propagation would' most likely result in- " leak-before-break" and thus permit a safe and orderly shutdown.
F.
Inspection History The NDE inspection history at Wolf Creek Generating Station (WCGS), as well as all. other domestic Westinghouse designed PWRs, has not revealed any service induced degradation in the surge line piping that has been attributed to thermal stratification.
Summary of Conclusions From WOG Program Based on information assembled on surge lines'for all domestic Westinghouse PWR's, and evaluation of that information in conjunction with plant-specific and other interim evaluation results, the WOG concludes that:
A high degree of confidence exists that further avaluation will confirm that the worst combination has already been evaluated for stratification
- severity, structural effects and operating procedures.
All plant specific analyses, to date, have demonstrated a 40 year life of the surge line.
Assuming that further evaluation leads to the same conclusion for the remaining Westinghouse PWR's, the worst case remaining life is approximately 100 heatup-cooldown cycles.
- Through wall crack propagation is highly unlikely, however,
" leak-before-break" would permit a safe and orderly shutdown if a through wall leak should develop.
NDE inspection. history demonstrates to the present day that the l-integrity of Westinghouse PWR pressurizer surge lines is acceptable, i
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.While additional monitoring, analyses.- and surveys of operating procedures are expected to further.
substantiate 'the above
. conclusions, the presently available information on surge line stratification indicates that Westinghouse PWR's may. be safely operated while additional data is obtained.
Overa11' Conclusion Based on the above discussions, WCNOC believes it is acceptable for WCGS to continue power operation for. at ' least. ten additional heatup-cooldown g-cycles..
WCNOC has committed to address the requirements of Bulletin 88-11 by January, 1991.
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