ML20005F060
| ML20005F060 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 01/03/1990 |
| From: | Bradham O SOUTH CAROLINA ELECTRIC & GAS CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-011, IEB-88-11, NUDOCS 9001120233 | |
| Download: ML20005F060 (7) | |
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January 3, 1990 t
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Document Control Desk U.!S. Nuclear. Regulatory Commission Washington, D. C.
20555
Subject:
Virgil C. Summer Nuclear Station Docket No. 50/395 o -
Operating License-No. NPF-12 Response to NRC Bulletin 88-11 i
JC0 Response To Item 1.b Centlemen; i
On January 3, 1989 ' South' Carolina Electric & Gas Company (SCE&G) received
.NRC Bulletin 88-11. " Pressurizer Surge Line Thermal Stratification."
Action 1.b of the Bulletin requested that a plant specific or bounding analysis be performed to demonstrate that the pressurizer surge line meets
' design codes and other regulatory requirements..The Virgil C.-Summer Nuclear Station (VCSNS) is participating in the Westinghouse Owners-Group (WOG) effort which is addressing the surge line stratification issue on a generic R. sis. 'The WOG has. issued'. topical report WCAP-12277, " Westinghouse Owners Group Bounding Evaluation for Pressurizer Surge Line Thermal.
. Stratification." SCE&G has confirmed that VCSNS is bounded by WCAP-12277 which provides the technical basis for the attached Justification for Continued 0peration (JCO). The JC0 and WCAP-12277-constitute the VCSNS response to Item 1.b of NRCB 88-11. This submitting of a JC0 does not
-indicate or. imply that VCSNS could not comply with design and licensing 4
-requirements for the duration of the license. SCE&G is. submitting a JC0 as part of the W0G program to address NRCB 88-11. This is a more conservative approach and provides a sound basis for safe continued operation.
I declare that the statements and matters set forth herein are true and
- correct to the best of my knowledge, information and belief.
Should you have any questions, please call at your convenience.
Very truly yours,
- 0. S. Bradham JMC/0SB: led
- Attachment c:
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O' Attachment to Document Control-Desk Letter
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) January 3. 1990 Page l'of 5 JUSTIFICATION FOR CONTINUED OPERATION REGARDING PRESSURIZER SURGE LINE STATIFICATION BACKGROUND It was'first reported in INP0 SER 25-87 that temperature measurements at a German PWR indicated thermal transients different than design.
Recent-measurements at several domestic PWRs 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 (hot leg temperature).
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 is different than that predicted in the a
original design.
Fatigue life of the 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 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, South Carolina Electric & Gas and other members of the Westinghouse Owners Group (WOG) authorized a program to perform a generic evaluation of surge line stratification in Westinghouse PWRs that will address portions of Bulletin 88-11.
The WOG Program is designed to benefit from the experience ge.ined in the performance of several plant specific analyses on Westinghouse PWR surge
. lines. These detailed analyses included definition of revised thermal transients (including stratification) and evaluations of pipe stress, fatigue usage factor, thermal striping, fatigue crack growth, leak before-break, and l
support loads. The overall analytical approach used in all of these analyses has been consistent and has been reviewed, in detail, by the NRC staff.
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' Attachment to Document Control Desk Letter
.j-iJanuary 3, 1990
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Page 2 of 5 As of March 1989, plant specific analyses have been performed on five domestic Westinghouse PWRs.
In addition, twelve-Westinghouse plants have completed or are currently performing an interim evaluation of surge line stratification which includes finite element structural analysis of their specific configuration under stratified loading conditions.
WOG Procram Status-6 As part of the current WOG Program, surge line physical and operating data has been collected and summarized for all domestic Westinghouse PWRs (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 4
evaluation were presented to the NRC in a meeting on April 11, 1989. The-evaluation has been formalized into a Westinghouse topical report (WCAP 12277, Proprietary and WCP-12278 Non-proprietary version) issued June 21, i
1989.
.This topical report forms the basis for the following justification for continued operation.
- JUSTIFICATION FOR CONTINUED OPERATION A..
Stratification 1everitv 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).
4 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.
a..
Pipe inside diameter b.
Piping slope (average) c.
Entrance angle of hot leg nozzle d.
Presence of mid-line vertical riser
-l 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 PWRs. 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.
V. C. Summer Nuclear Station is part of this group. The remaining 60% of Westinghouse PWRs are divided among the other nine additional groups. Although monitoring data has not yet been received
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-Attachment to Document Control Desk Letter Danuary 3, 1990 Page;3 of 5 representative of all these groups, in general, the combination of
-significant parameters of these nine 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 eff ects 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 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(approximately20%ofWestinghousePWRs),hasbeenperformed. This comparison i_dicates a high degree of confidence that, from a combined n
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 Ladditional plants (interim evaluation is in progress 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 have not been evaluated, experience gained from these evaluations indicates that further evaluations will not result in a more limiting configuration than those already evaluated.
C.
Operatino Procedures The WOG currently has available the surveys of operating procedures performed in support of existing plant-specific analyses. Experience indicates 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 cooldown procedure which may result in a temperature difference between the pressurizer and reactor 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 pressure absorbing space in the pressurizer. Based on information currently cvailable to the WOG, a high confidence exists that the steam bubble mode heatup, assumed to date, is conservative with respect to Westirghouse PWRs.
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,a-LAttachment to Document Control Desk Letter 1
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Tanuary 3, 1990
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D.
PiDe Stress and Remainino Life The-design codes for surge line piping have requirements for checking _ pipe
J stress limits and the effects of fatigue loading. These stress limits provide-a'means_of controlling stress from primary loads such as pressure.
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-deadweight, 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 1
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 loading is not expected.
For.the stratification issue, the potential effects of excessive displacements have been investigated through a detailed visual observation of the surge line during the walkdown required per Bulletin 88-11 action 1.a as reported in our March 1, 1989 letter to the NRC. This inspection did not identify any evidence of abnormal thermal movement or identify any damage.
l The effects of secondary stresses on the remaining life'of the surge line R
have been evaluated on a generic basis through the WOG program. The following summarizes the results of this evaluation, b
AllLplant' specific analyses performed as of March, 1989 have demonstrated y
. 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 yrs) in combination with~
the worst case number of heatup/cooldown cycles (75, at a different plant) at any Westinghouse PWR, and assuming a 40 year life for all surge lines, it is estimated that no more than approximately 50% of the fatigue life has been used at any Westinghouse plant to date.
1 For a design life considering 200 heatup/cooldown cycles-(used in plant specific analyses), this would indicate approximately 100 remaining cycles.
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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 loading 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.
r Attachment to Document Control Desk Letter-
' January 3 1990 Page 5 of 5-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 V. C.-Summer Nuclear Station, 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 Proaram Based on information assembled on surge lines for all domestic Westinghouse PWRs, 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 evaluation 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 i
life of the surge line. Assuming that further evaluation leads to the same conclusion for the remaining Westinghouse PWRs the worst t
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 the present day integrity of Westinghouse PWR pressurizer surge lines.
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 PWRs may be safely operated while additional data is obtained.
Overall Conclusion Based on the above discussions, South Carolina Electric & Gas Company believes it is acceptable for V. C. Summer Nuclear Station to continue power operation for at least twenty additional-heatup/cooldown cycles. South Carolina Electric & Gas Company has committed to address the remaining requirements of Bulletin 88-11 by January 3, 1991.
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