ML20247L636
| ML20247L636 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 05/26/1989 |
| From: | Michael Ray TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-011, IEB-88-11, NUDOCS 8906020225 | |
| Download: ML20247L636 (7) | |
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TENNESSEE VALLEY AUTHORITY
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CH ATTANOOGA TENNESSEE 37401 SN 157B Lookout Place MAY 261989 1
U.S. Nuclear Regulatory Commission ATTN:
Document Control Desk-Washington, D.C.
20555 Gentlemen:
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.In the Matter of
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Docket'Nos. 50-327 Tennessee Valley' Authority
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50-328 SEQUOYAH NUCLEAR PLANT (SQN) UNITS 1 AND 2 - NRC BULLETIN 88-11,- PRESSURIZER
' SURGE LINE THERMAL STRATIFICATION The subject bulletin required the following actions to be taken:
1.
Visual' inspection of the pressurizer surge line.
2.
Demonstration that the pressurizer surge line meets applicable design codes.
- 3. ' Collection of data, as necessary.
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~4.
Updating stress and fatigue analysis to account for stratification and l
striping.
In October 1988, TVA and other members of the Westinghouse Owners Group (WOG) authorized a program to perform a generic evaluation of surge line stratification in Westinghouse Electric Corporation (W) plants that will address portions of Bulletin 88-11.
The WOG' program is designed to benefit from the experience gained in the performance of several plant-specific analyses on W surge lines.
These detailed analyses included definition of revised thermal transients (including stratification) and evaluations of pipe stress, fatigue usage factor, thermal striping,. fatigue cra.:k growth, leak before break, and support loads.
The overall analytical approach used 11 all of these analyses has been consistent and has been reviewed, in detail, by NRC staff.
As of March 1989, plant-specific analyses have been performed on five domestic W plants.
In addition, 12 W plants have completed or are currently performing an interim evaluation of surge line stratification that includes finite element structural analysis of their specific configuration under stratification loading conditions.
WOG Program Status As part of the current WOG program, surge line physical and operating data has j
been collected and summarized for all domestic W plants (55 units).
Information relating to piping layout, supports and restraints, components,
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size, material, operating history, etc, has been obtained.
This data has been evaluated in conjunction with available monitoring data and plant-specific
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h-U.S. N'uclear Regulatory Commission MAY 261989 analyses performed by W.
The results of this evaluation were presented to NRC
'in a meeting on April 11, 1989.
The evaluation is being formalized into a W topical report (WCAP-12277, proprietary, and WCAP-12278, nonproprietary version) scheduled for submittal to 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 (AT > 100 degree Fahrenheit [F]) has been measured on all surge lines for which monitoring has been performed and that have been reviewed by the WOG to date (eight surge lines).
The measured amount of stratification and its variation with time (cycling) differs from plant to plant.
This variation has been I
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:
(1) pipe inside diameter, (2) piping slope (average), (3) entrance angle of hot leg nozzle, and (4) presence of mid-line vertical riser.
These parameters were used in a grouping evaluation that resulted in the definition of 10 monitoring groups corresponding to variceus combinations of these parameters at W plants. Approximately 40 percent 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 percent of W plants are divided among the other nine additional groups. Although monitoring data that is representative of all these groups has not yet been received, 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 that can influence the structural effects of stratification are:
(1) location and design of rigid supports and pipe whip restraints, (2) pipe layout geometry and size, and (3) type and location of piping components.
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..U.S.NuclearRegulatoryCommission MAY 261989 E
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Although the material and fabrication techniques for H' surge lines are reasonably consistent and of high quality, the design parameters listed above vary among H plants.
This variation in design is primarily a result of plant-specific routing requirements.
A preliminary evaluation, comparing the ranges of these parameters with
'those of plants for which plant-specific analyses and. interim evaluations are available (approximately 20 percent of H plants), 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 plants and interim evaluations of six additional plants (interim evaluation is in progress on six more plants as of March 1989). These analyses and evaluations have included various 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.
C.
' Operating' Procedures The HOG currently has available the surveys of operating procedures c
l 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 HOG to date have assumed a steam bubble mode heatup and cooldown procedure that may L
result in a temperature difference between the pressurizer and reactor coolant system (RCS) hot leg of more than 300 degrees 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, it, conservative with respect to H plants.
D.
Pipe Strest and Remaining Life The current industry 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.
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, f U.S.SuclearRegulatoryCommission MAY 261989 Stratification in the surge line is a secondary load that will only affect l
the qualification of secondary stresses. The qualification of primary stresses is not affected by this loading, with the exceotion of nozzle evaluations. Secondary stresses are controlled to prevent excessive displacements and gross plasticity and to prevent excessive fatigue loadings in the pipe.
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 the current American Society of Mechanical Engineers (ASME) code (Section III) and surge line fatigue life in excess of a 40-year plant life.
Review of plant-specific fatigue calculations indicates that the surge 1tne fatigue life is primarily dependent on the number of heatup and cooldown cycles, rather than years of operation.
Considering the worst-ca~se years of operation (28.5 years) in combination with the worst-case number of heatup/cooldown cycles (75, at a different plant) at any W plant, and assuming a 40-year life for all surge lines, it is estimated tfiat no more than approximately 50 percent of the fatigue life has been used at any W 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 2-year timeframe needed to resolve the stratification issue.
E.
Leak Before Break All the plant-specific analyses performed to date that have included the loadings because of 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 percent 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 nondestructive examination history at SQN Units 1 and 2, as well as all other domestic W designed plants, has not revealed any service-induced degradation in the surge line piping that has been attributed to either thermal stratification or striping.
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,.- lU.S. Nuclear Regulatory Commission MAY 261989
' Summary of Conclusions from NOG Program Based on information assembled on surge lines for all domestic H plants and evaluation of.that information in conjunction with plant-specific and other interim evaluation results, the HOG concludes that:
.. A hig'h degree of' confidence _ exists-that further evaluation will confirm i
that the worst combination.has already been evaluated for stratification b
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 H plants, 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.
Nondestructive examination history demonstrates the present day integrity
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of H 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 H plants.
may be safely operated while additional data is obtained.
l Sequoyah-Specific Conclusions The potential existence of excessive surge line displacement has been investigated at SQN Unit 2 in accordance with the requirements of Bulletin 88-11. A visual inspection was completed during the Unit 2 Cycle 3 refueling outage.
There was ne evidence of gross discernable distress or structural damage resulting from pipe displacement, unanalyzed impact with pipe whip restraints, overload of pipe supports, or overload of anchor bolts.
j An inspection of the Unit 1 pressurizer surge line, including piping, pipe L
supports, pipe whip restraints, and anchor bolts, will be completed during the first available cold shutdown that exceeds 7 days.
The inspection is currently scheduled for the Unit 1 Cycle 4 refueling outage.
The' code of record for SQN is American National Standard Institute B31.1-1967.
TVA will demonstrate that SQN's pressurizer surge line meets the SQN code of record supplemented by a fatigue evaluation performed in I
accordance with the latest ASME Section III requirements or will participate i
in a data monitoring effort by January 3, 1991. Upon completion of the l
proposed WOG effort, TVA will evaluate the analyses performed by H and l
demonstrate applicability to SQN by January 3, 1991.
To support conclusions i
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t MAY 261989 U.S. Nuclear Regulatory Commi bion regarding plant operability.and design fatigue life as required by Bulletin 88-11, TVA will also update the surge line stress analysis performed in accordance with SQN's code of record to indicate that a supplemental analysis
. and fatigue evaluation have been performed in accordance with the current ASME
.Section III code.
Based upon the above conclusions,.TVA believes it is j
acceptable for SQN Units 1 and 2 to continue power operation for at least 10 additional heatup/cooldown cycles.
j Summary statements of commitments contained in this submittal are provided in the enclosure. Please direct questions concerning this issue to K. S. Whitaker at'(615) 843-7748.
Very truly yours, TENNESSEE VALLEY AUTHORITY
\\l\\ Q Manager, Nuclea'r Lic nsing 1
and Regulatory Affairs i
Enclosure cc (Enclosure):
Ms. S. C. Black, Assistant Director l
for Projects Tt'A Projects Division U.S. Nuclear Regulatory Commission One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852 Mr. B. A. Wilson, Assistant Director for Inspection Programs TVA Projects Division U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia' 30323 Sequoyah Resident Inspector l
Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy, Tennessee 37379 i
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ENCLOSURE COMMITMENTS 1.
A visual inspection of the Unit 1 pressurizer surge line, including piping, pipe supports, pipe whip restraints, and anchor bolts, will be completed during the first available cold shutdown that exceeds 7 days.
This inspection is currently scheduled for the Unit 1 Cycle 4 refueling outage.
2.
TVA will demonstrate that SQN's pressurizer surge line meets the SQN code of record supplemented by a fatigue analysis performed in accordance with the latest ASME Section III requirements or will participate in a collective data monitoring effort by January 3, 1991.
3.
TVA will update SQN's stress and fatigue analysis to account for stratification and striping by January 3, 1991.
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