ML17157C448
| ML17157C448 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 08/11/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17157C447 | List: |
| References | |
| NUDOCS 9308300313 | |
| Download: ML17157C448 (11) | |
Text
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Cy UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, O. C. 20555 RELATED TO AMENDMENT N0.12 TO FACILITY OPERATING LICENSE NO.
NPF-14 AMENDMENT NO.
8 TO FACILITY OPERATING LICENSE NO.
NPF-22 PENNSYLVANIA POWER 5 LIGHT COMPANY ALLEGHENY ELECTRIC COOPERATIVE INC.
SUS UEHANNA STEAM ELECTRIC STATION UNITS 1
AND 2 DOCKET NOS.
50-387 AND 388 O
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Op Vp
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION
- 1. 0 INTRODUCTION By letter dated May 4,
- 1993, and supplemented by letter dated July 15,
- 1993, the Pennsylvania Power and Light Company (PP8L or the licensee) submitted a
request for changes to the Susquehanna Steam Electric Station (SSES),
Units 1
and 2, Technical Specifications (TS).
The requested changes would revise the TSs to:
1.
Decrease the test frequency of the drywell-to-suppression chamber bypass test to coincide with the test frequency for the 10 CFR Part 50, Appendix J,
Integrated Leakage Rate Test (ILRT).
This test frequency would require that three low pressure bypass tests be conducted at 40+10-month intervals during each 10-year service period, and 2.
Require an additional surveillance test to measure the vacuum breaker leakage area,. for those outages for which the above drywell-to-suppression chamber bypass test is not scheduled.
The current Technical Specifications specify that a drywell-to-suppression chamber bypass test be conducted at least once-per-18-months to verify an acceptable containment bypass effective leakage
- area, A/4K.
Substitution of the suppression chamber bypass test with the individual vacuum breaker bypass tests would result in a significant economic benefit to the licensee while maintaining adequate safety as discussed below.
The July 15, 1993, letter provided clarifying information that did not change the initial proposed no significant hazards consideration determination.
- 2. 0 DISCUSSION AND EVALUATION 2.1 Pro osed TS Chan e
The current Technical Specification, 4.6.2.l.d, states that:
"The suppression chamber shall be demonstrated OPERABLE:
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At least once per 18 months by conducting a drywell-to-suppression chamber bypass leak test at an initial differential pressure of at least 4.3 psi and verifying that the A/4K calculated from the measured leakage is within the specified limit [10K of 0.0535 ft ].
If any drywell-to-suppression chamber bypass leak test fails to meet the specified limit, the test schedule for subsequent tests shall be reviewed and approved by the Commission.
If two consecutive tests fail to meet the specified limit, a test shall be performed at least every 9 months until two consecutive tests meet the specified limit, at which time the 18 month test schedule may be resumed."
The proposed TS, 4.6.2. l.d and e, would state:
"The suppression chamber shall be demonstrated OPERABLE:
d.
By conducting a drywell-to-suppression chamber bypass leak test at an initial differential pressure of at least 4.3 psi and verifying that the A//K calculated from the measured leakage is within the
~ specified limit.
The bypass leak test shall be conducted at 40 + 10 month intervals during shutdown, during each 10-year service period.
If any drywell-to-suppression chamber bypass leak test fails to meet
'the specified limit, the test schedule for subsequent tests shall be reviewed and approved by the Commission.
If two consecutive tests fail to meet the specified limit, a test shall be performed at least every 18 months until two consecutive tests meet the specified limit, at which time the above test schedule may be resumed.
e.
By conducting a leakage test on the drywell-to-suppression chamber vacuum breakers at a differential pressure of at least 4.3 psi and verifying that the total leakage area A/(k) contributed by all vacuum breakers is less than or equal to 30M of the specified limit and the leakage area for an individual set of vacuum breakers is less than or equal to 12X of the specified limit.
The vacuum breaker leakage test shall be conducted during each refueling outage for which the drywell-to-suppression chamber bypass leak test in Specification 4.6.2. I.d is not conducted."
2.2 SSES M rk II Pressure Su ression Containment Desi n
SSES incorporates a Mark II containment with the drywell located over the suppression chamber and separated by a diaphragm slab.
The suppression chamber contains a pool of water having a depth that varies between 22'nd 24'uring normal operation.
Eighty-seven downcomers and 16 main steam safety/relief (SRV) discharge lines penetrate the diaphragm slab and terminate at a pre-designed submergence within the pool.
During a loss of coolant accident (LOCA), the containment design directs steam from the 'drywell to the suppression pool via the downcomers to limit the maximum containment pressure response to less than the design pressure of 53 psig.
The effectiveness of the SSES pressure suppression containment design requires that leakage pathways from the drywell to the suppression chamber airspace be minimized.
Steam that enters the suppression pool airspace through leak paths will bypass the suppression pool and can result in a rapid increase in containment pressure depending on the size of the bypass flow area.
2.3 Basis for Current TS Re uirements The licensee's architect/engineer calculated the containment pressure response based on a conservative design bypass flow area of A/1K equal to 0.0535 ft'7.7in').
The analysis assumed a small break LOCA with a differential pressure between the drywell and suppression chamber airspace equal to the downcomer submergence.
The analysis showed that it takes approximately 27 minutes from the onset of the LOCA to reach the containment design pressure of 53 psig.
The steam bypass analysis results were evaluated by the NRC and reported in Supplement No.
3 to the SSES Safety Evaluation Report.
The criteria for the staff review were the requirements stipulated in the Standard Review Plan (SRP),
Appendix I of Section 6.2. 1. l.c, "Steam Bypass for Hark II Containments."
The staff concluded that SSES's steam bypass capability is
- adequate, since the operator has sufficient time to actuate the suppression chamber sprays prior to reaching the containment design pressure based on a
design bypass area equal to 0.0535 ft.
TS 3.6.2. l.b conservatively specifies a maximum allowable bypass area of 10X of the design value of 0.0535 ft.
The TS limit provides an additional factor of 10 safety margin above the conservatisms taken in the steam bypass analysis.
The drywell-to-suppression chamber bypass test required by TS 4.6.2. l.d verifies that the actual bypass flow area is less than or equal to the TS limit.
The bypass leakage test ensures that degradation in the measured bypass area is identified and corrected to ensure containment integrity during LOCA events.
The design value for leakage area is determined by analyzing a spectrum of LOCA break sizes.
For each break size there is a limiting leakage area.
In determining the limiting leakage
- area, credit is taken for the capability of operators to initiate drywell and suppression pool sprays after a period of time sufficient for them to realize that there is a significant bypass leakage flow.
The effect of suppression pool bypass on containment pressure response is greatest with small breaks.
The design value of 0.0535 ft for SSES represents the maximum leakage area that can be tolerated for that break size that is most limiting with respect to suppression pool bypass.
2.4 Potential for B ass Leaka e
Several potential bypass leakage pathways exist:
Leakage through the diaphragm floor penetrations (SRV discharge line and downcomers),
Cracks in the diaphragm floor/liner plate, Cracks in the downcomers that pass through the suppression pool
- airspace, Valve seat leakage in the five sets of drywell-to-suppression chamber containment vacuum breakers, and Seat leakage of isolation valves in piping connecting the drywell and the suppression chamber air space.
Each potential flow pathway was evaluated by the licensee with respect to the potential for suppression pool bypass leakage.
Dia hra m Floor The diaphragm floor is a reinforced concrete slab approximately 3.5 feet thick.
The drywell-side surface of the diaphragm slab is capped with a I/4-inch thick carbon steel liner plate.
The liner plate is constructed of carbon steel plates welded together to form a continuous steel membrane.
Non-destructive testing and vacuum box testing were performed on the welds at the liner plate seams to verify acceptable weld quality, structural integrity and leaktightness.
The liner plate is protected against corrosion and deterioration by a safety-related epoxy coating.
The diaphragm slab and liner plate provide a barrier against the potential for current and future bypass leakage from the drywell to the suppression pool airspace via the diaphragm floor.
The diaphragm floor liner plate is designed, constructed and coated to safety-related quality assurance requirements.
In addition, it is designed for all postulated loading conditions, including seismic, hydrodynamic,
- pressure, temperature and other loads.
Dia hra m Floor Penetrations The downcomers and SRV discharge lines penetrate through the diaphragm slab and terminate in the suppression pool.
The downcomers are 24-inch diameter, seamless carbon steel, galvanized pipes with 3/8-inch wall thickness.
A 42-inch diameter steel ring plate is welded to the outside of the downcomers.
The downcomer/ring plate assemblies are embedded in the diaphragm slab with the top surface of the ring plate flush with the drywell side diaphragm slab.
The diaphragm floor liner plate is installed to provide a minimum I-inch overlap around the ring plate.
All connections are welded to form a continuous steel membrane between the liner plate and downcomer penetrations.
The SRV discharge lines are routed through welded flued heads at the diaphragm floor.
The flued head design and construction are similar to the downcomer penetrations and also provide a continuous steel barrier between the liner plate and SRV discharge line flued heads.
Both the downcomer penetration assemblies and SRV discharge line flued heads are designed and constructed to safety-related quality assurance requirements.
In addition, they are designed for all postulated loading conditions, including seismic, hydrodynamic,
- pressure, temperature and other loads.
Downcomers Located in the Su ression Pool Airs ace The downcomers are constructed of 24-inch diameter seamless piping with a 3/8-inch wall thickness.
The downcomers were designed to the requirements of ASME Section III, NB-3600 for all postulated loading conditions, including seismic, hydrodynamic,
- pressure, temperature and other loads.
In addition, although not required by the ASME code, a fatigue analysis was performed for the portion of the downcomers located in the suppression chamber airspace.
The fatigue analysis was completed to address the staff concern for potential bypass leakage due to fatigue induced downcomer failures in the suppression pool airspace.
The fatigue analysis considered a conservative number of SRV and LOCA chugging load cycles.
The fatigue analysis confirmed that the downcomers will maintain their structural integrity for all postulated loading conditions.
These conservative design requirements ensure that the downcomers will not contribute to the non-vacuum breaker bypass leakage.
Vacuum Breakers There are five sets of drywell-to-suppression pool vacuum breakers in the SSES containment.
They are located in the suppression pool airspace on branch "T" connections off the drywell vent downcomers.
Each set consists of two 24-inch Anderson-Greenwood check valves in series.
The valve design utilizes an elastomer diaphragm at the valve seat to enhance leaktightness.
Position indication is provided in the control room for each valve.
Multi le Isolation Valve Seat Leaka e
There are several potential bypass flow paths between the drywell and suppression chamber air spaces via piping systems external to the containment.
All flow paths have multiple in-series containment isolation valves.
The piping systems include (a) vent and purge lines (two paths, each containing 18-inch and 24-inch diameter isolation valves),
(b) containment spray lines (two paths each containing 12-inch and 6-inch diameter valves),
(c) nitrogen pressurization lines (one I-inch line),
(d)
Hz and 0
analyzer lines (four I-inch lines),
(e)
ILRT instrumentation lines (one Cinch line),
and (f) containment instrument gas lines (one flow path containing I-inch and 3-inch piping).
The licensee analyzed the potential for suppression pool bypass through these piping paths.
For the vent and purge lines, the technical specifications limit the allowable leakage for each penetration to 0.05L Conservatively assuming that both penetrations in both pathways leak at the TS limit, the maximum additional equivalent leakage area contributed by the vent and purge lines is ZA/4(
0.00105in The remaining external piping flow paths do not have individual TS leakage limits.
Leakage of their isolation valves is included in the total limitation of 0.6L, for all local leak rate test results taken together.
Conservatively assuming that these valves together leak at the bounding value of 0.6L, these valves contribute an additional hA/4K =
0.00629in The sum of 0.50105in plus 0.00629in
- 0.00739in which js a very small fraction of the design maximum equivalent leakage area of 7.7in The facility TS therefore ensure that potential contributions to suppression pool bypass leakage through external piping paths between the drywell and suppression chamber are accommodated by the margins provided for vacuum breaker leakage.
2.5 0 erational Ex erience The licensee provided a discussion of past results of suppression pool bypass testing at SSES.
Past testing has been performed following vacuum breaker maintenance and is therefore indicative of leakage of the non-vacuum breaker (i.e., passive) leakage sources such as the floor and penetrations.
Based on the data from previous tests, bypass leakage through the floor and floor penetrations is consistently very low and it can be concluded that any significant pool bypass leakage under LOCA conditions would likely be via vacuum breaker disk leakage.
The staff conducted a search of the licensing event report files to determine if Anderson-Greenwood check valves in vacuum breaker service have a
significant failure history.
Of 87 events reported for Anderson-Greenwood equipment failures, only one (50-387/83-098) related to Anderson-Greenwood check valves in vacuum breaker service.
That event report was the of position indication failure only.
There were no reports of as-found leakage test results which would have resulted in suppression pool inoperability.
2.6 Substitution of Vacuum Breaker Leaka e Tests for Su ression Pool B
ass Test Durin Non-Inte rated Leaka e Rate Test ILRT Outa es Analyses of the design and construction of potential leakage paths and of -the historical results of suppression pool bypass tests, as discussed in 2.4 and 2.5 above, indicate that the drywell-to-suppression pool vacuum breakers constitute the only significant potential bypass leakage path.
Based on this finding, the staff finds that there is sufficient basis to allow individual vacuum breaker leakage tests to substitute for the suppression pool bypass test during non-ILRT outages.
This conclusion reflects findings of use of conservative margins with respect to vacuum breaker leakage test acceptance
- criteria, and assurance that the passive containment structure, liner and penetrations are not likely to deteriorate between normal suppression pool bypass tests.
The staff acknowledges that requiring the performance of a complete normal suppression pool bypass test on a schedule consistent with the ILRT assures that potential degradation due to corrosion of the drywell liner or downcomer piping in the suppression pool airspace would be detected in a timely manner.
The staff also acknowledges that (a) limiting total vacuum breaker leakage to 30X of the total leakage limit (which itself is 10X of the design capability),
and (b) limiting individual vacuum breaker leakage to 12X of the specified limit, provide large, conservative margins.
The staff therefore focused its review on how the proposed vacuum breaker tests would be performed and the data evaluated.
2.7 Pro osed Vacuum Breaker Leaka e Test The licensee intends to demonstrate compliance with the proposed TS 4.6.2. l.e by measuring and summing the A/VK for each of the five vacuum breaker sets.
The combined A/4K would be limited to 0.231in
[(30X)(10X)(0.0535ft )
(144in /ft )
= 0.231in ] corresponding to a leakage rate of 5.8 scfm.
In additio~, the individual A/4K for each vacuum breaker set would be limited to 0.09 in
[(12X)(lOX)(0.0535)(144) 0.0924in ] equivalent to 22.3 scfm.
To perform the test, the downcomer entrance in the drywell will be sealed with an inflatable plug or gasketed plate.
An air injection/flow measuring device will be attached to a threaded fitting installed downstream of the vacuum breaker disks.
Air flow rate into the downcomer through the flow measuring device will be adjusted to stabilize the pressure in the downcomer at 4.3 psig.
The required air flow is the leakage makeup rate.
A/4K will be determined by means of the following formula:
(s) (O) ~r (2.05) (Pd) (60) (28,317)
I
~ Zd~
acr~
where:
Td Drywell air temperature Pd Upstream (drywell) pressure (psia)
Pw = Wetwell pressure (psia)
A/4K = Vacuum breaker effective leakage area in square inches Leakage flow rate as measured by flow device in standard cubic centimeters per min S
- air density at standard conditions (0.0763 ¹/ft )
The above equation is a variation of the "homogeneous frozen flow equation" utilized in performing the licensing basis suppression pool bypass calculations (Ref: Licensee letter dated September 3,
1981 and SSES Safety Evaluation Report Supplement 3).
This proposed vacuum breaker surveillance test methodology provides an acceptable means of ensuring compliance with the A/IK limitation.
- 2. 8
~Summa r The staff has reviewed the information provided by the licensee in support of an application for amendment and has concluded that individual vacuum breaker leakage tests are an acceptable alternative to an integrated suppression pool bypass test during outages for which a Type A containment integrated leak rate test is not conducted.
This conclusion is based on the licensee analyses of potential suppression pool bypass leakage paths.
The analysis demonstrated that vacuum breakers are the predominant potential source of leakage and 'that the leakage for the other sources is conservatively accommodated by the margins included in the proposed TS.
The proposed TS changes are therefore acceptable.
- 3. 0 STATE CONSULTATION In accordance with the Commission's regulations, the Pennsylvania State official was notified of the proposed issuance of the amendments.
The State official had no comments.
- 4. 0 ENVIRONMENTAL CONSIDERATION The amendments change surveillance requirements.
The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released
- offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure.
The Commission has previously issued a
proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (58 FR 32390).
Accordingly, the amendments meet eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.
5.
~ON LU I The Commission has concluded, based on the considerations discussed
- above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed
- manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
W.
Long Date:
August ll, 1993
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