ML13331A328
| ML13331A328 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 06/01/1989 |
| From: | Nandy F Southern California Edison Co |
| To: | NRC/IRM |
| References | |
| IEB-88-011, IEB-88-11, NUDOCS 8906060054 | |
| Download: ML13331A328 (10) | |
Text
Southern California Edison Company P. 0.
BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD. CALIFORNIA 91770 F. R. NANDY TELEPHONE MANAGER OF NUCLEAR LICENSING (818) 302-1896 June 1, 1989 U. S. Nuclear Regulatory Commission Attention:
Document Control Desk Washington, D.C. 20555 Gentlemen:
Subject:
Docket No. 50-206 NRC Bulletin No. 88-11, Pressurizer Surge Line Thermal Stratification San Onofre Nuclear Generating Station Unit 1
References:
- A)
Letter dated March 6, 1989, from F. R. Nandy (SCE) to NRC, NRC Bulletin 88-11, Pressurizer Surge Line Thermal Stratification B) Letter dated May 19, 1989, from F. R. Nandy (SCE) to NRC, NRC Bulletin No. 88-11, Pressurizer Surge Line Thermal Stratification This is regarding-NRC Bulletin No. 88-11 (Pressurizer Surge Line Thermal Stratification), dated December 20, 1988, as applicable to San Onofre Unit 1.
By Reference A, SCE requested an alternate schedule of two years for completing the action required by Item 1 (b) of the bulletin. By Reference 8, SCE informed NRC that a Justification for Continued Operation (JCO) to support this alternate schedule would be submitted at a later date. The enclosed JCO contains a generic bounding analysis which meets the intent of Item 1 (b) of the bulletin. Therefore, the alternate schedule requested by Reference A is no longer needed.
The JCO is based on Westinghouse's evaluation of surge line physical and operating data collected from 55 domestic PWR's, including San Onofre Unit 1.
The portion of these data relating to San Onofre Unit 1 have been reviewed by SCE and are technically accurate.
8906060054 890601 PDR ADOCK 05000206 Q
PNU
Document Control Desk
-2 The JCO concludes that it is acceptable for San Onofre Unit 1 to continue power operation for at least ten additional heatup-cooldown cycles, while detailed resolution of the pressurizer surge line stratification issue, as required by Item 1 (d) of the bulletin, is in progress.
If there are any questions, please call me.
Very truly yours, cc: J. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3
Enclosure Page 1 of 8 JUSTIFICATION FOR CONTINUED OPERATION REGARDING PRESSURIZER SURGE LINE STRATIFICATION SAN ONOFRE NUCLEAR GENERATING STATION UNIT 1 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 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:
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, 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
Enclosure Page 2 of 8 The bulletin encourages licensees to perform actions b) and c) above through collective efforts with other plants.
In October 1988, Southern California Edison Company 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 a portion 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.
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, including San Onofre Unit 1 (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.
Enclosure Page 3 of 8 JUSTIFICATION FOR CONTINUED OPERATION A. Stratification Severity 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).
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 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. The remaining 60% of Westinghouse PWRs (including San Onofre Unit 1) 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 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.
Enclosure Page 4 of 8 B. Structural 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 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 (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 most likely been evaluated.
This conclusion is supported by plant-specific analyses covering five plants and interim evaluations of six additional plants (interim evaluation was 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 WOG to
Enclosure Page 5 of 8 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.
At San Onofre Unit 1, this temperature difference is being administratively controlled to 200*F for the life of the plant.
The steam-bubble mode heatup, which WOG has assumed to date, is conservative with respect to San Onofre Unit 1.
D. Pipe 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 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 have been investigated at San Onofre Unit 1 through a detailed visual observation of the surge line during the walkdown required per Bulletin 88-11 Action 1.a.
No signs of unusual pipe movement were observed.
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.
Enclosure Page 6 of 8 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 40 years.
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 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 150 pressurizer heatup-cooldown cycles (see Chapter 5 of San Onofre Unit 1 updated FSAR), this would indicate at least 75 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.
Enclosure Page 7 of 8 F. Inspection History The NDE inspection history at San Onofre Unit 1, 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 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 at least a 40 year life of the surge line. Assuming that further evaluation will lead to the same conclusion, for San Onofre Unit 1, the remaining life is at least 75 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 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 PWRs may be safely operated while additional data is obtained.
Enclosure Page 8 of 8 Overall Conclusion Based on the above discussion, Southern California Edison Company believes it is acceptable for San Onofre Unit 1 to continue power operation for at least ten additional heatup-cooldown cycles.
By Reference A below, Southern California Edison Company has committed to address the remaining requirements of Bulletin 88-11 within two years of receipt of the bulletin, i.e., by January 1991.
Reference Ai Letter dated March 6, 1989, from F. R. Nandy (SCE) to NRC, NRC Bulletin 88-11, "Pressurizer Surge Line Thermal Stratification" IAA:presstat