ML20247Q779
| ML20247Q779 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 06/01/1989 |
| From: | Drawbridge B YANKEE ATOMIC ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| BYR-89-95, IEB-88-011, IEB-88-11, NUDOCS 8906070045 | |
| Download: ML20247Q779 (8) | |
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.YANKEEATOMICELECTRIC COMPANY
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June 1, 1989 BYR 89-95 United States Nuclear Regulatory Commission Document Control Desk Washington, DC 20555
References:
(a) License No. DPR-3 (Docket No. 50-29)
(b) NRC Bulletin No. 88-11, " Pressurizer Surge Line," dated December. 20, 1988 (c) Letter, Yankee Atomic Electric Company to USNRC,
" Alternative Schedule for Evaluation of Pressurizer Surge Line at YNPS." dated March 3, 1989
Subject:
Bulletin No. 88-11, " Pressurizer Surge Line Thermal Stratification"
Dear Sir:
The referenced bulletin requests that licensees perform inspection and analysis to demonstrate that the pressurizer surge line meets the applicable.
- design codes, and other Final Safety Analysis Report (FSAR) and regulatory commitments. This analysis must consider the phenomena of thermal stratification and thermal striping. Yankee Atomic Electric Company (YAEC) has completed the prescribed inspections (Bulletin Action 1.a) and generic bounding analysis (Bulletin Action 1.b) for the Yankee Nuclear Power Station (YNPS),.and hereby submits the results of this work.
In Reference (c), YAEC requested a schedule extension for the completion of Bulletin Action 1.b.
Since that submittal, YAEC has evaluated the options of performing a plant-specific analysis or joining the analysis effort being
. perfomed by Westinghouse for the Westinghouse Owner's Group (WOG). We have concluded that joining the WOG effort will result in a more complete evaluation of the surge line stratification phenomena. Westinghouse has performed a generic bounding analysis for the WOG which includes, as required by the bulletin, consideration of the effects of thermal stratification and thermal striping. The Westinghouse analysis has been reviewed by the staff.
The risults of this generic analysis have been applied to YNPS. Further plant-specific analysis, as required by Bulletin Action 1.d, will be performed and completed in accordance with the bulletin schedule. Until a plant-specific analysis proving compliance with applicable codes for the remaining life of the plant is completed, the bulletin requests that a Justification for Continued Operation (JCO) be prepared to provide assurance that continued operation is acceptable.
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l United States _ Nuclear Regulatory Commission June 1, 1989 Document-Control Desk Page 2
,BYR 89-95f The. Westinghouse bounding analysis,:together with.YAEC's' operating-history,:.provides the. basis for the attached JCO. Based on the discussion therein,LYAEC believes it is acceptable-for the YNPS to continue power
' operation for.at.least-ten (10) additional heatup/cooldown cycles from the=
-date'of.this letter. This'will allow sufficient time to perform the plant-specific analysis ~ required to address this issue for the long-term operation of the plant and'for full compliance with the bulletin.
YAEC is committed to addressing all actions of Bulletin No. 88-11 by January 3, 1991.
We trust the information provided herein is satisfactory.
If there are any' questions, please do not hesitate to contact us.
ry truly
- urs,
/
A B. L. Draw Vice President and Manager of Operations BLD/ pac /0374v
. Attachment.
cc: USNRC Region I USNRC Resident Inspector - YNPS W. T. Russe 11'- Regional Administrator, USNRC Region I COMMONWEALTH OF MASSACHUSETTS)
)ss MIDDLESEX COUNTY
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-Then personally appeared before me, B. L. Drawbridge, who, being duly sworn, did state that he is Vice President and Manager of Operations of Yankee l
Atomic Electric Company, that he is duly authorized to execute and file the foregoing document in the name and on the behalf'of Yankee Atomic Electric I
Company and that the statements therein are true to the best of his knowledge l
- and belief.
Cd
%Ih tDrn? /w r Helen D. Sammarco Notary Public My Commission Expires November 7, 1991 1____
i
YANKEE ATOMIC ELECTRIC COMPANY
' JUSTIFICATION FOR CONTINUED OPERATION REGARDING PRESSURIZER SURGE LINE STRATIFICATION YANKEE NUCLEAR POWER STATION BACKGROUND It was first reported in INPO SER 25-87 that temperature measurements at a German Pressurized Water Reactor (PWR) indicated thermal transients that were different than those assumed in the design. Recent measurements at several domestic PWRs have indicated that the temperature difference between the pressurizer and the hot leg piping results in stratified flow in the pressurizer surge line, with the top of the flow stream being hot (pressurizer temperature) and the bottom being colder (hot leg temperature). At some plants the top-to-bottom temperature difference can reach 250'F to 300'F in certain modes of operation, particularly Modes 3, 4, er 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," dated December 20, 1988, 3dentifying actions to be taken by licensees. These actions are:
(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, Yankee Atomic Electric Company (YAEC) 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 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 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. 0374v
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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 Program Status
.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, L
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 repert (WCAP-12277, proprietary, and WCAP-12278, nonproprietary version) scheduled for submittal to the NRC on June 15, 1989.
This topical report forms the basis for the following-justification for continued operation.
A.
Eiratification Severity Thermal stratification (AT >100*F) has been measured on eight surge lines for which monitoring has been performed and which have been reviewed by the WOG to date. Temperature data measured at Yankee Nuclear Power Station (YNPS) during startup from our last refueling outage indicated a'AT of less than 70'F.
This indicates that the effects of stratification should be minimal.
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.
(1) Pipe Inside Diameter (2) Piping Slope (average)
(3) Entrance Angle of Hot Leg Nozzle (4) Presence of Mid-Line Vertical Riser These parameters were used in a grouping evaluation which resulted in the definition of ten 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. The remaining 60% of Westinghouse PWRs nre divided among the other.nine additional groups.
Although monitoring data has not yet been received representative of all 0374v
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 supForted by a. comparison of'available monitoring data.
B.-
Etructural Effects Significant parameters which 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.
(3) Type and location of piping components.
b Although the materia 1Eand 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 (approximately 20% of Westinghouse PWRs), 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 mest likely been evaluated. this conclusion is supported by plant-specific analyses covering five plants and interim evaluations of six additional plants.
Interim evaluations were 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.
C.
Operating 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 WCG 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. At INPS, this value is limited by Technical Specifications to 225'F.
It is also known that some plant 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. 0374v
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Ploe Stress and' Remaining Life The design codes _for surge line piping have requirements for checking pipe stress limits and the effects of fatigue loadings. These stress i
limits provide a means of controlling stress from primary loads.such as pressure, deadweight, and design mechanica1' 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.
i Secondary stresses are controlled to prevent excessive displacements and i
_ 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 for 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 u
observation of the surge line during the walkdown required per Bulletin 88-11, Action.1.a.
A visual inspection of the YNPS surge line was performed in January 1989, prior to, and during.heatup from our refueling outage. The-inspection.
1 was performed in accordance with ASME Section XI.
The surge line was inspected prior to insulation rmoval. The inspection did not reveal any signs of insulation damage which would be associated with excessive movement or distress of the pipe. Additionally, the piping was inspected for restrictions from supports or wall penetrations. No restrictions were observed.
After insulation removal piping and supports were again inspected. No
- l signs'of piping' deformation were observed.
During heatup,. physical-dimensions from the pipe to the nearest restrictions (walls, penetrations, etc.) were taken at surge line temperatures of 180*F and again at 350*F. Adequate clearance at all restrictions was present at both temperatures.
The effects of secondary stresses on the remaining life of the surge line j
have been evaluated on a generic bcsis through the WOG program. The l
l following summarizes the results of this evaluation.
All plant-specific analyses performed as of March 1989 have demonstrated i
compliance with applicable ASME codes and a surge line fatigue life in j
excess of a 40-year plant life. Review of plant-specific fatigue i
calculations indicates that the surge line fatigue life is primarily dependent on the number of heatup and cooldown cycles, rather than years of operation.
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' 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 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. To date, there have been 54 heatup/cooldown cycles at YNPS.
For a design life considering 200 heatup/cooldown cycles (use] 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.
The YNPS surge line has been analysed considering a top-to-bottom AT of 225'F (i.e., Technical Specification limits). The surge line meets the stress requirements of ANSI B31.1-1977 when subjected to this loading condition.
E.
Leak-Bef m -Atgah All the plant-specific analyses performed to date that have included the loadings due to stratification and striping have validated the
" leak-bef ore-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 postulcted throughwall crack propagation would most likely result in " leak-before-break" and thus permit a safe and orderly shutdown.
F.
Inspection _llistory The Nondestructive Examination (NDE) inspection history of the surge line not:1es at the hot leg and the pressurizer at YNPS, as well as the NDE history of other domestic Westinghouse design PWRs, has not revealed any service-induced degradation in the surge line piping that has been attributed to thermal stratification.
ELumnary of Contl.usions From WOG Program 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:
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 PWRs, the worst-case remaining j
life is approximately 100 heatup/cooldown cycles.
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o Throughwall crack propagation is highly unlikely; however,
" leak-before-break" would permit a safe and orderly shutdown if a throughwall leak should develop.
o NDE inspection history demonstrates the present day integrity of Westinghouse PWR pressurizer surge lines.
While additional monitoring, analyses, and surveys of operating o
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 ebtained.
Overall Conclusion Based on the above discussions, YAEC believes it is acceptable for YNPS to continue power operation for at least ten additional heatup/cooldown cycles.
A heatup is defined as heating the plant from Mode 5 to Modes 3, 2, or 1.
A cooldown is defined as cooling the plant from Modes 1, 2, or 3, to Mode 5.
One heatup/cooldown cycle includes one heatup plus one cooldown as defined above. YAEC has committed to address the requirements of Bulletin 88-11 by I
January 3,1991 as prescr3 bed by the bulletin.
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