ML20247R789
| ML20247R789 | |
| Person / Time | |
|---|---|
| Site: | Beaver Valley |
| Issue date: | 05/30/1989 |
| From: | Sieber J DUQUESNE LIGHT CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-011, IEB-88-11, NUDOCS 8906070320 | |
| Download: ML20247R789 (8) | |
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Beve Whey Poweh $tahon SNgunpon. 7% 15077@M I
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JOHN D SIEGER
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May 30, 1989 i
U. S. Nuclear Regulatory-Commission Attn:
Document. Control Desk Washington, DC 20555
Reference:
Beaver-Valley Power Station, Unit No. 1 j
Docket No. 50 ~34, License No. CPR-66 NRC Bulletin 88-11 Justification for Continued Operation
. Gentlemen:
i By letter dated March 1,
- 1989, Duquesne Light Company (DLC) submitted _ an alternative schedule for evaluation of pressurizer surge line thermal stratification as required by NRC Bulletin 88-11 l
" Pressurizer Surge Line Thermal Stratification".
This schedule was subsequently discussed during a
meeting between the NRC Staff and i
Westinghouse Owners Group (WOG) on April 11, 1989.
The NRC staff and the WOG agreed upon actions which would meet the intent of Item 1.b of the Bulletin.
Specifically, these actions are: (1) the WOG will submit i
a Westinghouse topical report (WCAP-12277) to the NRC by June 15, 1989, j
and (2)
WOG utilities will submit a
Justification for Continued Operation - (JCO) to the NRC by May 31, 1989.
The JCO will reference l
WCAP-12277 and together will. form the bacis of a bounding evaluation which meets the intent of Item 1.b of the Bulletin.
The NRC letter of I
May 10, 1989 reviewed this approach and found the schedule acceptable.
The JCO for Beaver Valley Unit 1
(BV-1) is attached.
The JCO 3
concludes that it is acceptable for BV-1 to continue power operation for at least ten (10) additional heatup-cooldown cycles.
This number of remaining cycles far exceeds the postulated worst case number of i
cycles for the two (2) year time frame (from receipt of the Bulletin) needed to resolve the stratification issue.
This schedule n.
consistent with the schedule given in Item 1.d of the Bulletin for completion of detailed plant-specific analyses.
DLC is continuing our l
participation in the WOG program to address the requirements of j
Bulletin 88-11 by January, 1991.
l l
If there are any questions concerning this matter, please contact my office.
i Very truly yours, fb l
8906070320 890530 D. Sieber i
gDR ADOCK0500ggg4 Vice President l
Nuclear Group j
I cc:
Mr. J. Beall, Sr. Resident Inspector j
Mr. W. T. Russell, NRC Region I Administrator Mr.
P. Tam, Sr. Project Manager
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~ COMMONWEALTH.OF PENNSYLVANIA)
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COUNTY OF BEAVER'-
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On this day of
&>/-
, 1989,
- /M N,
a Notary Public in and for seid before me
- j.,
Commonwealth-and County, personally appeared J, D. Sieber, who being duly
- sworn, deposed, and' said that (1) he is Vice President of Duquesne
- Light, (2) he is duly authorized to execute and file the foregoing ~ Submittal on behalf of said Company, and (3) the statements set forth.in the Submittal are true and correct to the best of his
. knowledge, information'and. belief.
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Ne*anat' Seal
$heia M. Fartore, Notary Public shipping;crtBoro BeaverCounty My Comtraen Expres 0d.23,1989 mmber, Pennsylvane /esooaten of Notanes K_____------
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Duqucann' Light Company a
Nuclear Group Beaver Valley Power Station Unit 1 Attachment-JUSTIFICATION FOR CONTINUED OPERATION REGARDING PRESSURIZER SURGE LINE STRATIFICATION BACKGROUND It was first reported in INPO SER 25-87 that temperature measurements at a
German PWR indicated thermal transients different than design.
Recent measurements at.several domestic PWR'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.
(hot leg
. temperature).
The top-of-bottom temperature 0
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:
o-Global bending of the pipe is different than that predicted in the original design.
o.
. 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, Duquesne Light Company (DLC) and other members of the Westinghouse Owners Group (WOG) authorized a
program to perform a
generic evaluation of surge line stratification in Westinghouse PWR's that will address portions of Bulletin 88-11.
1 The WOG program is designed to benefit from the experience gained 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 l
- growth, leak before-break, and 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.
At,techm:nt j
Page 2
)
As of March
- 1989, plant specific analyses have been performed on five domestic Westinghouse PWR's.
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 As part of the current WOG 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 Thermal stratification (Delta T
> 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).
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
1 1
At,tachm@nt Page 3 These parameters were used in a grouping evaluation *hich resulted in the definition of 10 monitoring groups corresponding to various combinations of these parameters at Westinghouse PWR's.
Approximately 40%
of the plants including Beaver Valley Unit 1, 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 the other nine additional groups.
Although monitoring data has not yet been received representative of all these groups, in general, the combination of I
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 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 routing requirements.
A preliminary evaluation, comparing the ranges of these parameters to those of plants which plant-specific analysis and interim evaluations are available (approximately 20%
of Westinghouse PWR's), has been performed.
This comparison indicates 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 cJants and interim evaluations of six additional plants (interim
+" aluation 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 has not been evaluated, experience gained from these evaluations indicates that further evaluations will not result in a more limiting configuration than those already evaluated.
, At.tachment Page 4 i
C.
Operatina 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 available 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 Remainina 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, 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 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 will be investigated through a
detailed visual inspection of the surge line during the walkdown required per Bulletin 88-11, Item 1.a.
This inspection will be conducted during the next available cold
- shutdown, exceeding 7
days duration.
A planned cold
- shutdown, exceeding 7 days duration, is scheduled for September 1989.
DLC will submit a letter to the staff, within 30 days after completion of this inspection, per reporting requirements 1
and 3
of Bulletin 88-11.
Should the results of this inspection indicate any gross discernible distress or structural damage, this letter will also address the need for corrective actions, repair, plant specific analysis and/or monitoring, etc., as appropriutc.
. Attcchm:nt Page 5 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 yr. 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 yr) 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.
For a
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.
Icak 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 Beaver Valley Unit 1, as well as all other domestic Westinghouse designed PWR's, has not revealed any service induced degradation in the surge line piping that has been attributed to thermal stratification.
., Attachment Page 6 Summary of Conclusions From WOG Procrag 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:
o 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.
o 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.
o Through wall crack propagation is highly unlikely, however,
" leak-before-break" would permit a safe and orderly shutdown if a through the wall leak should develop.
o NDE inspection history demonstrates the present day integrity of Westinghouse PWR pressurizer surge lines.
o 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.
Overall Conclusion Based on the above discussions, DLC believes it is acceptable for Beaver Valley Unit 1
to continue power operation for at least ten additional heatup-cooldown cycles.
DLC has committed to address the requirements of Bulletin 88-11 by January, 1991.
I
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