ML20247K250
| ML20247K250 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 05/25/1989 |
| From: | Larson C NORTHERN STATES POWER CO. |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| IEB-88-011, IEB-88-11, NUDOCS 8906010252 | |
| Download: ML20247K250 (8) | |
Text
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e-r f-Northem States Power Company f
414 NicoNet Mall Minneapohs, Minnesota 55401 Telephone (612) 330 5500 May 25, 1989 NRC Bulletin 88-11 Director of Nuclear Reactor Regulation U S Nuclear Regulatory Commission Attn:
Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PIANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Pressurizer Surge Line Thermal Stratification Justification for Continued Operation In our February 24, 1989 response to NRC Bulletin 88-11 " Pressurizer Surge Line Thermal Stratification" we proposed an alternative schedule to that requested in Bulletin'88-11. This request was based on our involvement in the Westinghouse Owners Group (WOG) program for resolution of Ett11etin 88-11 and included a commitment to provide a Justification for Continued Operation (JCO) to the NRC Staff to assure that the plant safety would not be compromised during the two year period required to complete the detailed qualification analysis.
The JC0 regarding pressurizer surge line thermal stratification is attached.
In our February 24, 1989 response we committed to provide this JC0 within four months of the receipt of Bulletin 88-11.
However, as a result of an April 11, 1989 meeting between the WOG and the NRC Staff, the JC0 submittal date required by the Bulletin was revised to May 31, 1989. The technical basis in support of this JCO is covered in the bounding evaluation presented in Westinghouse Topical Report WCAP-12277/12278 scheduled for submittal to the NRC on June 15, 1989.
Please contact us if you have any questions related to this JCO.
r C E Larso Vice Presi ent Nuclear Generation c: Regional Administrator III, NRC Senior Resident Inspector, NRC NRR Project Manager, NRC 8906010252 890525 N
DR ADOCK 05000282 PNU l t 1
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UNITED STATES NUCLEAR REGULATORY COMMISSION' Q
NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO. 50-282 50-306 t
Pressurizer Surge Line Thermal Stratification Justification for Continued Operation Northern States Power.' Company, a' Minnesota corporation, with this letter
! 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.
Piping inside diameter B.
' Piping slope (average)
C.
Entrance angle of hot leg nozzle D.
Presence of mid-line vertical riser Theso 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 PWR's.
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 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 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.
Structura] Effects Significant parameters Which can influence the structural effects of stratification are:
a.
Location and design of rigid supports and pipe whip restraints l-b.
Pipe layout geometry and size c.
Type and location of piping components Although the material and fabrication technique for Westinghouse surge lines are reasonably consistent and of high quality, the design pararoters 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 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 l.
d 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 layouts, pipe sizes, support and restraint designs and piping components. Although the full range of variation in these parameters has not been evaluared, experience gained from these evaluations indicates that further evaluations will not result in a more limiting configuration than those already evaluated.
l C.
Operatine 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 Lhe 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 0
coolant system (RCS) hot leg of more than 300 F.
In many cases, individual plant operating procedures and technical specifications provide j
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 l
respect to Westinghouse PWRs.
I D.
Pine Stress and Remainine 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 l
gross placticity and to prevent excessive fatigue loadings in the pipe.
The basic characteristic of a secondary stress is that it is self limsting; thus, a failure from a single application of a secondary loading
]
is not expected.
1 For the stratification issue, the potential effects of excessivo displacements are investigated through a detailed visual inspection of the i
surge line during the walkdown required per Bulletin 88-11 action 1.a.
This inspection will be conducted on Prairie Island Unit 1 at the next
]
cold shutdown exceeding seven days. The next planned outage on Unit 1 is in Jar.uary 1990. The Unit 2 surge line was inspected during the April 1989 outage and no indications of problems were found.
(See also "F.
Inspection History").
l
9, i
i-
)
I Ths 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 j
1 excess of a 40 yr. plant-life. Review of plant specific fatigue j
calculations indicates that the surge line fatigue life is primarily I
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-k with the worst case number of heatup-cooldown cycles (75, at a different
'i 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 th fatigue life has been used at any Westinghouse plant to date.
.j 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 i
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.
F.
Inspection History The NDE inspection history at Prairie Island Units 1 & 2, as well as all other domestic Westinghouse designed PWR's, have not revealed any service induced degradation in the surge line piping that has been attributed.to thermal stratification.
Northern States Power Company (NSP) will perform a visual inspection l
(ASME,Section XI, VT-3) of the Prairie Island Unit 1 pressurizer surge l
line during the next available cold shutdown, exceeding 7 days duration.
The next planned refueling for Unit 1 is scheduled for January 15, 1990.
During the September 1988 outage, all welds on the Unit 1 pressurizer surge line were examined by surface and volumetric NDE methods. No relevant indications were found.
NSP performed a visual inspection (ASME,Section XI, VT-3) of the entire j-Prairie Island Unit 2 pressurizer surge line on April 3, 1989.
The l
results of this inspection did not indicate any gross discernible distress or structural damage, including piping, pipe supports, pipe whip restraints, and anchor bolts. All of the Unit 2 pressurizer surge line welds were examined in April 1989 and no relevant indications were found.
ifn...i Summarv'of Conclusions From WOG Program Based'on information assembled on surge lines for all domestic Westinghouse PWR's fand evaluation of that information in conjunction with plant-specific and other interim evaluation results,-the WOG concludes that:
A high degree of confidencefexiats that further evaluation will confirm o
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.
Through wall crack propagation is highly unlikely, however, " leak-before-o break" would permit a safe and orderly shutdown if a through wall leak should develop.-
NDE inspection history demonstrates the present day integrity of o
Westinghouse PWR pressurizer surge lines.
While additional monitoring, analyses, and surveys of operating procedures o
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, Northern States Power believes it is acceptable for the Prairie Island Nuclear Plant to continue power operation for at least ten additional heatup-cooldown cycles. Northern States Power has committed to address the requirements of Bulletin 88-11 by January 1991.
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