ML20150C842
| ML20150C842 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 03/11/1988 |
| From: | NRC OFFICE OF SPECIAL PROJECTS |
| To: | |
| Shared Package | |
| ML20127A683 | List:
|
| References | |
| NUDOCS 8803220341 | |
| Download: ML20150C842 (3) | |
Text
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k UNITED STATES
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g NUCLEAR REGULATORY COMMISSION j
WASHINGTON. O. C. 20555 1,
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SAFETY EVALUATION REPORT BY THE OFFICE OF SPECIAL PROJECTS EMPLOYEE CONCERN ELEMENT REPORT 229.S(B)
"CONTROL AIR SYSTEM ADEOUACY FOLLOWING PIPE BREAK" TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR POWER PLANT, UNITS 1 AND 2 00CXET NOS. 50-327 AND 50-328 1.
SUBJECT Category:
Engineering (2,000)
Subcategory:
22905 Element:
Control Air System Adequacy Following Pipe Break Employee Concern:
IN-85-348-002 The concern, "Control air system does not appear to have sufficient volume to assure functionality if the system should experience a guilliotine air line break.
Individual specifically requested a description of the maximum system volume available and of appropriate backup systens," was first investigated and found to be not valid for Watts Bar Nuclear Plant (WBN).
Due to the similarity of the WBN coepressed air system design with the.Sequoyah (SCN) air system, it was decided by TVA to investigate the concern for applicability to SQN.
The concern does not specif Auxiliary Control Air (ACA)y whether a guillotire break was to occur in the or Station Control and Service Air-(SCSA) as an initiating event, or was to occur in either the ACA or SCSA subsequent to some other initiatirg event.
The TVA evaluation considered both cases.
II.
EVALUATION As clarified by letter of February 1988, the SCSA system fo, Units 1 and 2 now contains four air compressers and three air receivers.
A fffth (610 SCFM at 100 PSIG) air compressor is planned.
Any two of the four SCSA compressors can satisfy all control air recuirements during normal two unit operating cordi-tions.
The safety-related ACA system consists of two completely redundant
-J sets of ccrpressed air supply equipnent and associated supply piping.
It serves all safety-related plant cceponents whose active functions require control air.
A TVA calculation is referenced which states that one ACA ccmpressor (64.9 SCFM) under limiting conditions can supply both units with recuired shutdown air flew with 20 SCFM to spare.
I 8803220341 800311 DR ADOCK.050 8
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' The ACA air ecmpressor is in a standby condition during normal plant operation.
The ACA receivers are normally charged from the cross-connected SCSA.
Cn indication of low pressure in the SCSA, the ACA compressors are autcmatically On further decrene in system pressure, both trains of the ACA are started.
automatically isolated from the SCSA.
Guillotine Breaks Originatine in the Air System "High energy" piping is defined as that which normally operates at pressure above 275 PSIG or a temperature above 200'F, "Moderate Energy" piping is that which operates at a pressure and temperature below these values.
It is required that both "guillotine breaks" and "critical cracks" be assumed (separately) in high energy piping but that only "critical cracks" be assumea l
in mcderate energy piping.
A concurrent Single Active failure is assumed concurrently with the breaks except when the break occurs in a dual purpose moderate energy system; that is, a safety grade moderate energy system that is used during normal operation and to mitigate the consequences of an accident.
Reference NUREG-0800, SRP Section 3.6.1.
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Air System Pice Break as an Iritiatina Event For the r,a,4crity of the SCSA and ACA piping the design pressure is 105 PSIG and the design temperature is 100*F.
The only exception is the air line from compressor discharge to the Feat exchanger, for which the r.axinum design temperature is 260*F.
The portion of the ACA between the compressor and the heat exchangers is not i
pressurized during normal plant operation; as such
't falls into a category exerpt frem postulated piping failures.
Therefore, cnly the SCSA cempressor discharge lines to the af ter coolers require cuillotine pipe break assurptions.
However, a guillotine break in a SCSA compressor discharge line will not cause i
loss of system function as each of the compressors are separated from the main system headers by check valves.
Each break would not interrupt norral plant I
eperation or the function of the air systen.
We may ccaclude therefore that a guillotine break in the centrol air system is not a valid initiating event to cause operational upset.
Pioe Break as a Subsecuent Event Another interpretation considered by TVA was a contrel air failure subsequent to some other initi. ting event.
A "single failure" and a passive failure were t
each censidered relative to postulated initiating events leading to plant sbut-down.
Functionality to achieve safa shutdown was corsie red assured through the use of the redundant ACA train.
Pipe Break Caused by the Initiatina Event By this we mean breaks in the air system resulting frcm a high energy break iritiating event.
As a result of NRC field evaluetiens to assess compliance with separation criteria, six nonconformance recorts were issued in 1981 identifyina inadequate separatier of these high enerny lires and the essertial (ACA) headers both inside and outside containment.
A review of two potential interactionswasdocurentedincalculationnurberfjEB 810811274, RI.
The
- review concluded that the interactions were acceptable, because ACA services lost were not required to accomplish safe shutdown following the particuler-pipe breaks within range of the ACA.
The analysis did not account for a concurrent single failure in the unaffected train of'the ACA, since this portion of the ACA is a Dual Purpose Moderate Energy System; and therefore, is exempt from a Single Active Failure.
Furthermore, the components of the ACA inside containment are designed to be single active failure proof while the components outside containmert are accessible to the operators. We may conclude, therefore, that a single active failure following a critical crack in the ACA is beyond the design basis of the plant.
!!I. CONCLUSION 1.
Guillotine breaks in the air system need not be considered as an initiating event.
2.
The volume of one str receiver and the flow from one of the air compressors is adequate to support plant shutdown.
The loss of all six corpressors need not be postulated.
3.
The present air system design at Sequoyah is acceptable since it satisfies the guidelines of NUREG-0800, SRP Section 3.6.1.
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