ML20207P941
| ML20207P941 | |
| Person / Time | |
|---|---|
| Site: | Washington Public Power Supply System |
| Issue date: | 01/13/1987 |
| From: | Sorensen G WASHINGTON PUBLIC POWER SUPPLY SYSTEM |
| To: | Stolz J Office of Nuclear Reactor Regulation |
| References | |
| RTR-NUREG-1061, RTR-NUREG-CR-4290 GO1-87-0006, GO1-87-6, NUDOCS 8701200438 | |
| Download: ML20207P941 (5) | |
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WASHINGTON PUBLIC POWER SUPPLY SYSTEM P.O. Box 968
- 3000 George Washington Way
- Richland, Washington 99352 January 13, 1987 G01-87-0006 Docket No. 50-460 Director of Nuclear Reactor Regulatior, Attention: John G. Stolz, Director PWR Poject Directorate No. 6 Division of PWR Licensing-B U. S. Nuclear Regulatory Commission Washington, D. C.
20555
Subject:
NUCLEAR PROJECT NO. 1 DECOUPLING 0F SEISMIC AND PIPE BREAK EVENTS
Reference:
UEWP-86-0091, letter, dated July 28, 1986, "TS Leak Before Break, Phase II Steam Line Break Studies" The Supf y System hereby requests an exemption, pursuant to 10CFR50.12(a), from l
General Design Criteria 2 to decouple seismic and postulated pipe breaks from plant design structural basis.
The NRC Piping Review Committee has recommended that the event combination of earthquake and pipe rupture in high energy piping shculd be excluded from Westinghouse and Combustion Engineering reactor systems (NUREG 1061, Volume 4, Recommendation 1).
At that time, the NRC sponsored study was underway at the Lawrence Livermore National Laboratory (LLNL) on Babcock & Wilcox reactor designs.
Now this study is complete (NUREG/CR-4290, Volume 2) and the results show that combination of safe shutdown earthquake and pipe break events should be excluded from the structural design basis.
The. double ended guillotine break (DEGB) and safe shutdown earthquake (SSE) load combination represents severe design requirements leading to massive supports on piping and their components.
This requirement has led to high design cost, stiffer piping systems, interference with normal plant operation and added radiation exposure to plant maintenance personnel.
No analytical, physical or technical evidence supports a casual relationship between pipe break and earthquake.
The LLNL's study (NUREG/CR-4290, Volume 2) has demonstrated that earthquakes are extremely unlikely to induce pipe break.
The postulated pipe breaks and seismic load combination requirement should be deleted from the WNP-1 plant design structural basis.
This exclusion of load combinations would not compromise plant safety or structural design.
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Docket No. 50-460 Ms. Elinor G. Adensam Page Two The attachment summarizes the exemption request.
A technical justification and summary of potential benefit is also provided in the attachment.
WNP-1 will still be designed for individual events.
The implementation of decoupling seismic and pipe break event combination will have a positive effect on the safety and reliability of WNP-1.
The piping and equipment would be more accessible and inspectable, thereby reducing occupational radiation exposures, costs, and the likelihood of undetected defects.
The ultimate objective is to improve the thermal flexibility of piping, reduce outage time, and increase plant safety and reliability.
Your prompt and favorable action on this request will be appreciatsi.
Once approved for application, the new criteria will be incorporated in a future FSAR revision.
J Pursuant to 10CFR170.12, please find a check in the amount of $150.00.
b s
G. C.
- rensen, Manager, Regulatory Programs GCS:LCO:pd Attachments cc:
N. D. Amaria, UE&C (898)
J. B. Martin, NRC, Walnut Creek, CA E. Revell, BPA (399)
N. Reynolds, BLCPR NRC Document Control Desk, Washington, D. C.
FDCC
Docket No. 50-460 Attachment SUPPLY SYSTEM WNP-1 DEC0UPLING OF SEISMIC AND PIPE BREAK EVENTS Exemption Request Pursuant to 10CFR50.12(a), the Supply System hereby applies for an exemption'from the provisions of 10CFR Part 50, Appendix A, to decouple seismic and postulated pipe break events from plant design structural basis. The requested exemption is based upon NRC sponsored work performed by the Lawrence Livermore National Laboratory (Reference 1).
The Code of Federal Regulation Title 10, Part 50, Appendix A, General Design Criteria 2 requires that structures, systems, and components important to the safety of nuclear plants be designed to withstand individual and combined effects caused by normal operations, by extreme natural phenomena, and by postulated accident conditions.
The U.S.
NRC staff, through various standards such as regulatory guides (R.G.1.48 and R.G.1.67), branch technical positions, and the Standard Review Plan (SRP 3.9.3), has specified how these effects are to be considered in the design of safety related structures, systems and components.
The requirements include combining seismic safe shutdown earthquake and postulated pipe break events. There is no technical or experience data available to establish the degree of dependence between earthquake and pipe breaks.
This requirement has led to superfluous and also massive supports for plant piping and equipment.
These supports have little or no beneficial function for normal operation of the plant.
In fact, inevitably they reduce overall reliability during normal operation. The seismic and postulated pipe break event combination requirement has led to high design cost, interference with normal plant operation and added radiation exposure to plant maintenance personnel.
A well-developed rational basis for considering concurrent earthquake and pipe break loads in the design basis has never been established.
In the early 1960's, the double-ended guillotine rupture of reactor coolant loop piping was postulated for containment sizing and emergency core cooling system (ECCS) performance.
Later this pipe rupture was combined with earthquake and applied to containment structural design and subsequently to the design of other plant features, including nuclear reactor piping and its support systems.
The evolution of seismic design requirements over the last two decades has led to increases in seismic stresses.
Likewise, large increases in the calculation of pipe rupture loads have taken place since the 1960's, thus designing to meet the requirements of this event combination has become progressively more difficult.
Field evaluations of piping at conventional power plants and petrochemical facilities indicate that ruptures in the type af piping found in nuclear power plants in general, do not occur during severe earthquakes.
Moreover, recent probabilistic assessments demonstrate that, for the particular case of PWR primary system piping, pipe rupture is extremely unlikely under any transient condition, including earthquakes (Reference 1).
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Attachment Docket No. 50-460 Supply System, WNP-1 Decoupling of Seismic and Pipe Break Events Page Two The NRC Piping Review Committee recommended (Recommendation A-4, Reference 3) that the event combination of earthquake and pipe rupture in high energy piping should be excluded on B&W nuclear plants if the Lawrence Livermore National Laboratory's (LLNL) study indicates so.
The LLNL's study (Reference 1) is complete now, and the results show that earthquakes are extremely unlikely to induce pipe break.
The LLNL's study evaluated indirectly induced double ended guillotine breaks (DEGB) of the Reactor Coolant Line piping due to causes such as structural
- failures, missiles, electrical
- failures, and transient events caused by earthquakes.
By convolving the WNP-1 seismic hazard curves with the fragility curves of the equipment supports, the probability of indirect DEGB was calculated (Section 2.5.6, Refer 1).
The median probability of indirect DEGB was calculated as 1.8x10~ynceper reactor-y p r, with the 10 pprcent to 90 percent 4.4x10'gve probability (interval) on subject DEGB being 3.6x10 per reactor-year to per reactor-year.
The sensitivity of the results to the seismic hazard prediction was examined by convolving the fragility curves with the generic seismic hazard curves developed for the eastern and midwestern United Stptes.
The resulting median probability of DEGB was determined to be 2.1x10~
per peactor-yearandthe10percentand90percentsubjectivepropbilityintervalon DEC3 was found to be 4.2x10~g per reactor-year to 3.3x10 per reactor-year.
For the purposes of comparison, the median probability of igirect DEGB for the lowest capacity Westinghouse reactor estimated was 3.3x10 per reactor-year withthelgpercentto90percentsubjectiveprobabilityintervalbeing2.3x10~7 to 2.3x10~ per reactor-year.
The results show the independence between seismic and pipe break events.
The Supply System believes that current knowledge and experience supports the conclusion that exclusion of SSE and postulated pipr break event combination is technically justified. The Supply System requests th exemption, with respect to the plant structural design basis, excluding the combination of SSE and postulated pipe breaks for the mechanical design of components, piping and their supports.
This load comoination exclusion would not compromise plant safety.
WNP-1 will still be designed for individual events.
The implementation of changes will have a posi'.ive effect on the safety and reliability of WNP-1.
Benefit Summary Implementation of the exemption will have the following benefit on the structural design for WNP Unit 1:
1.
Will reduce the cost of hardware and its safety analyses.
2.
Will reduce congestion in the plant, making piping and equipment more accessible and inspectable.
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. Attachment Docket 50-460 Supply System, WNP-1 Decoupling of Seismic and Pipe Break Events Page Three Benefit-Summary (con't)
Will reduce' occupational radiation exposure during inspection-and
-3.
. maintenance.
4.
Will reduce undetected defects.
5.
Will create more flexible piping and will reduce thermal stresses in piping and equipment nozzle.
6.
Will reduce likelihood of improper installation.
.7.
Will result in a-large increase in calculated safety margin of components, piping, and related supports.
8.
In the event that seismic hazard is increased or design modifications are performed, adequate margins may still be shown to exist without undertaking any othar plant modification.
9.
Will improve overall plant safety and reliability.
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References 1.
Ravindra M.K., Campbell R.D., etc., " Probability of Pipe Failure in the Reactor. Coolant Loops of - Babcock & Wilcox PWR Plants - Guillotine Break Indirectly Induced by Earthquakes", NUREG/CR-4290, Volume 2, July 1985.
2.
U.S. NRC " Report of the U.S. Nuclear Regulatory Commission Piping Review Committee - Evaluation of Other Loads and Load Combinations", NUREG-1061, Volume 4, December 1984.
3.
U.S. NRC " Report of the U.S. Nuclear Regulatory Commission Piping Review Committee Summary - Conclusions and Recommendations", NUREG-1061, Volume 5, April 1985.
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