ML20247Q932

Provides Evaluation of Operation for NRC Bulletin 88-011, Pressurizer Surge Line Thermal Stratification. Util Participating in Program for Partial Resolution of Issue Through Westinghouse Owners Group

ML20247Q932
Person / Time
Site:	Byron, Braidwood, Zion, 05000000
Issue date:	05/31/1989
From:	Richter M COMMONWEALTH EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
IEB-88-011, IEB-88-11, NUDOCS 8906070084
Download: ML20247Q932 (7)

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~yj' - d 72 West Adams Street, Chicago, Illinois 4.J

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Chicago, Ilhnois 60690 0767 3

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I May 31,'1989  !!

.i 1 U.S. Nuclear. Regulatory. Commission Attn 7 Document Control Desk Washington, D.C. 20555 o

Subjects Byron: Station. Units 1 and 2 .)

Braidwood Station Units 1.and 2 Zion Station Units 1 and 2 . . ,

Evaluation of Operation for NRC Bulletin 88-11 l Pressurizer. Surge Liny Tlyermal Stratification .j j NRC Docket Nos. 50-454/455, 50L456/457, and 50-295/304 1

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11 References (a) HRC Bulletin 88-11, dated December 20, 1988. l 1

.i (b) M. Richter letter to U.S. NRC, dated March 7, 1989. W

Dear Sir:

Peference (a) requests all addressees to establish and implement a .)

program to' confirm. pressurizer surge line integrity in view of the occurrence  :

j of thermal stratification, and requires them to inform the staff of the actions taken to resolve this issue.

i1 As. indicated in Reference (b), Commonwealth Edison Company (Edison) is participating in'a program for partial resolution of this issue through the  ;

, -Westinghouse Owners Group (WOG). Reference (b) requested an alternate  ;]

schedule for Action 1.b of Reference (a), and committed to submit a j f ~ justification for continued operation (JCO). A subsequent meeting between the j

'NRC'and WOG on April 11, 1989, further clarified the work performed by the WOG  !

on this issue and the expectations of the NRC for Action 1.b. As a result of

,the: meeting between the NRC and WOG, it is Edison's understanding that a JCO

.in~ conjunction with the Westinghouse Topical Report (WCAP-12277/12278),

scheduled for submittal to the NRC on June 15, 1989, is intended to fulfill the requirements of Action 1.b. *{herefore, the alternate schedule requested

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r by-Reference (b) is no longer required. 1 Presently, for Action 1.b, the WOG cannot demonstrate full design life, using a single bounding approach, therefore, a JCO (see attached) is being provided for Byron, Braidwood, and Zion Stations in accordance with Reference (a).

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I Edison has performed the visual inspection of the pressurizer surge line requested by Action 1.a for Dyron Station Unit 2, Braidwood Station Units 1 and 2, and Zion Station Units 1 and 2. The inspections revealed that no discernable distresa or structural damage in the pressurizer surge line was evident.

Based on the visual inspections and the work of the WOG to date (precented in the JCO and WCAP-12277/12278), Edison believes it is acceptable for each Byron, Braidwood, and Zion Unit to continue power operation for at least ten (10) additional heatup-cooldown cycles. Edison has committed to address the requirements of Bulletin 89-11 by the end of December 1990.

Please address any questions that you or your staff may have concerning this response to this office.

Respectfully,

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M.H. Richter Nuclear Licensing Administrator rf/9001k cc: A.D. Davis Resident Inspector - BY/BW/Z Subscribea and Qworn Lo [r^ ~ ^:::=::=::::=::::=

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. JUSTIFICATION FOR CONTINUED OPERATION' REGARDING1 PRESSURIZER SURGE LINE STRATIFICATION y FOR

', BYRON, BRAIDWOOD, AND ZION STATIONS

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BACKGROUND

.It was first reported in INPO SER 25-87 that temperature measurements st.a German Pressurized Water Reactor.-(PWR) indicated thermal' transients-different than design. Recent measurements at several domestic PWRs haveL

' 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 2500F-to 3000F in certain modes of operation, particularly Modes 3, 4, or 5 during heatup and cooldown.

3 Surge line stratification causes two effects

- . Global bending of'the pipe is different than that predicted in.the 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," on December 20, 1988,. identifying actions to be taken by licensees.

a)f Conduct visual inspection -walkdown b) _ Update stress and fatigue analysis to account'for stratification and striping c)- lObtain monitoring data, as necessary The bulletin encourages licensees to perform actions b) and c) above through ec.'lective efforts with other plants.. In October 1988, Commonwealth Edison Company (Edison) 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 analyces on Westinghouse-PWR surge lines. These detailed analyses included definition of revised thermal transients (including stratification) and evaluations of pipe stress, fatigue usage factor, th3rmal 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.

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As of Maren 1989, plant specific analyses have been performed on five (5) domestic Westinghouse PWRs. In addition, twelve (12) 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.

WG_PJmgIMLStatus 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, and operating history 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 version, 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 (AT > 1000F) 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 (overage)

C. Entrance angle of hot leg noz-de D. Presence of mid-line verticat riser These parameters were used in a grouping evaluation which resulted in the definition of ten (10) monitoring groups corresponding to various combinations of these parameters at Westinghouse PWRs. Approximately 40% of the plants, including Zion Station, are in one group for which a large amount of monitoring data has already been received and for which the enveJoping thermal transients, discussed above, are applicable. The remaining 60% of Westinghouse PWRs aro divided among the other nine additional groups (with Byron and Braidwood Stations included in the same monitoring group). Although l 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.

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B. BLtucturbl_ELignLg Significant parameters which can influence the structural effects of

! stratification are

- location and design of rigid supports and pipe whip restraints,

- pipe layout geometry and size, and

- 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.

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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 i additional plants (interim evaluation is in progress on six more plants as of j 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 indicate that further evaluations will not result in a more limiting configuration than those already evaluated.

C. Opgrating 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 cf more than 3000F. In many cases, individual plant operating procedures and technical specifications provide limits on this value. Based on information currently available to the WOG, a high confidence ,

exists that the steam bubble mode heatup, assumed to date, is conservative I with respect to Westinghouse PWRs.

D. Eipg_Sittsg._andlemaining Liff 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.

, o Stratification ~1n 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, for five of the six Edison PWR units, through the visual inspection of the pressurizer surge line as requested by Action 1.a of the bulletin. The visual inspections, performed for Byron Station Unit 2, Braidwood Station Units 1 and 2, and Zion Station Units 1 and 2, have revealed no discernable distress or structural damage in the pressurizer surge lines.

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 year plant life. Review of plant specific fatigue calculations indicate 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 cese years of operation (28.5 years) in combination with the worst case number of heatup-cooldown cycles (75, not at an Edison 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-cooldewn 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 f rene needed to resolve the stratification issue.

E. htDk-D%iQER:DMk 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.

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F. Inspection History The Non-Destructive Examination (NDE) inspection history at Byron, Braidwood, and Zion Stations has not revealed any service induced degradation in the surge line' piping that has been attributed to thermal stratification.

As indicated previously, Edison has performed the visual inspection of the pressurizer surge line requested by Action 1.a for Byron Station Unit l

2, Braidwood Station Units 1 and 2, and Zion Station Units 1 and 2. The l inspections revealed that no discernable distress or structural damage in the l

pressurizer surge line was evident.

S ummRIy_O L_Cancl usiQns._Er.pm_1!DG._E tag rm 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 precadures.

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 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 i 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 gratification indicates that Westinghouse PW0s may be safely operated while additional data is obtained. 1 i

Qretall Concinninn Based on the above discussions, Edison believes it is acceptable for each Byron, Braidwood, and Zion Unit to continue power operation for at least ten (10) additional heatup-cooldown cycles. Edison has committed to address the requirements of Bulletin 88-11 by the end of December 1990. )

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