ML20059E041
| ML20059E041 | |
| Person / Time | |
|---|---|
| Site: | Fermi  |
| Issue date: | 01/04/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059E040 | List: |
| References | |
| NUDOCS 9401100223 | |
| Download: ML20059E041 (4) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT N0.c6 TO FACILITY OPERATING LICENSE NO. NPF-43 DETROIT EDISON COMPANY FERMI-2 DOCKET NO. 50-341
1.0 INTRODUCTION
By letter dated March 23. 1993, the Detroit Edison Company (Deco or the licensee) requested an :,mendment to the Technical Specifications (TS) appended to Facility Operating License No. NPF-43 for Fermi-2. The proposed amendment would revise the TS surveillance requirements for the suppression chamber (wetwell) to drywell vacuum breakers (VBs).
The licensee's application states that approval of the application would enhance safety due to the fact that the nitrogen-actuated stroking devices used to test the VBs are prone to failure in a manner that can cause the VBs to fail open. An open VB renders the suppression chamber inoperable due-to the creation of a suppression pool steam bypass pathway.
2.0 DISCUSSION AND EVALUATION 2.1 Backaround Information and Discussion Vacuum Breaker Desian:
The VB is a swing-check device having a 20-inch flapper or " pallet" which self actuates to open under a differential pressure of 0.5 psi.
The VB vendor is GPE Controls. The VBs are located in the suppression chamber air space attached to the drywell vent system.
Resilient seals are used in the pallet-seat interface. Magnetic latches are provided to prevent chattering and vibration. The VBs were modified (strengthened) as a result of a finding during Full Scale Test Facility tests that the VBs are subject to chugging and condensation oscillation loads (Ref: Generic Letter 83-08, " Modification of Vacuum Breakers on Mark I Containments").
Unlike later BWR series facilities, at Fermi-2, a Mark I facility, the drywell VBs are not installed in series-pair configurations for single-failure capability.
Failure of any one of the 12 VBs creates a bypass pathway.
Each VB is fitted with a pneumatically powered stroke-test actuator enabling control room personnel to remotely stroke it as a demonstration of operability.
Redundant safety-grade limit switches are provided to indicate when the pallet is in the fully closed position. Another limit switch (not redundant or safety-grade) is provided to indicate when the pallet is in the fully open position.
In order that a VB not be rendered inoperable by testing, the test actuator is intentionally designed to have insufficient power to hold the pallet open against LOCA dynamic forces. However, the test actuators are considered non-safety-related devices and have not been tested under LOCA stresses. There are at least three recorded cases, one at Fermi-2, 9401100223 940104 7 PDR ADOCK 050003417' P
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, where a VB failed to reclose during testing. These failuros were due to test actuator problems.
In the Fermi-2 case, the actuator jammed preventing VB reclosure.
Drywell VBs are relatively simple devices. Not counting its test actuator and limit switches, each VB has only a single moving part. A VB is capable of performing its blowdown function (to prevent suppression pool bypass), and post blowdown function (to relieve differential pressure), with no active e
power source.
In its normal position it is already closed and is thus capable -
of performing the blowdown safety function with no physical movement.
Basis for Current Stroke Test Frecuency: A monthly frequency has been specified for drywell V3s since the earliest suppression pool containment facilities were licensed. The Inservice Testing requirements of ASME Section XI have historically specified a 3-month stroke test interval for vacuum breakers and other valves. However, the staff has maintained that the drywell VB stroke testing frequency should continue to be at monthly intervals, due to th-
'ortant safety function, inaccessibility, significant number of failures i ve been reported, and harsh environment in which they are located. howeve, in at least two safety evaluations for TS amendments (i.e., Browns Ferry 1/2 and Vermont Yankee) the staff has accepted the ASME Section XI requirements for a 3-month vacuum breaker stroke test fregeency.
Flow-Checkina and V guum Relief Functions: The purpose of the drywell VBs is to protect the containment drywell from excessive negative pressures.
Should a loss-of-coolant accioent (LOCA) occur, most of the noncondensible gases in the drywell would be blown into the suppression chamber as the drywell became pressurized with steam.
Subsequent to the blowdown, a vacuum would form in the drywell as the steam condensed. A severe negative pressure could also occur due to an inadvertent spray actuation during normal operation.
If the vacuum were not relieved, the drywell could buckle (implode). The conteinment design negative pressure is 2.0 psid.
If 9 of the 12 VBs are operable (i.e.,
free to open and pass rated flow from the suppression chamhee to the drywell under reverse differential pressure), the noncondensible gases in the suppression chamber can pass back into the drywell to restore pressure.
Another function of the VBs is to minimize the elevation of the water column in the vent system during normal operation. This is necessary to assure that vent clearing loads are not exceeded during a LOCA. Accordingly, a TS Limiting Condition of Operation (LCO) for the vacuum relief function requires at least 10 VBs to be capable of opening under 0.5 psid pressure differential.
Now that testing will be performed during plant shutdown, the licensee-proposes to change this LCO to require all 12 VBs to be capable of vacuum relief to ensure the repair of inoperable valves prior to startup.
If a VB is open, a suppression pool bypass path is present. During a LOCA, steam from the drywell could bypass the submerged vents in the suppression pool.
This could result in containment overpressurization beyond its structural capability (if wetwell spray was not manually initiated promptly).
The maximum allowable amount of equivalent bypass leakage area is that of a 7-inch diameter line. An LC0/ action statement is provided for the vacuum relief function requiring all VBs to be closed or a shutdown begun within
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. 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
If position indicators are inoperable, closure of all VBs can be verified during power operation by a low pressure decay test on the drywell with nitrogen. Repair of a drywell VB requires that the drywell and suppression chamber be deinerted to permit entry.
During post-LOCA recovery operations, the VBs would serve to maintain desired flow paths during recombiner operation or nitrogen vent / purge operations.
2.2 Evaluathn The staff has reviewed the licensee's proposed TS changes and determined that there is sufficient technical basis for approval. The staff's approval is based on the following findings:
1.
A Nuclear Plant Reliability Data System search and telephone survey of similar plants by the licensee indicates that although there has been a considerable number of cases of failures of VBs to reclose during stroke testing, for GPE Controls VBs, there is no history of failures to open during testing.
2.
With respect to the vacuum relief safety function, the drywell vacuum relief system has triple failure redunoancy. Only 9 of 12 VBs are needed for vacuum relief for the limiting negative pressure event (i.e., 4 VBs must fail to open before the safety function would be compromised). This provides an extremely high level of assurance that the drywell is protected from buckling and thus supports extended surveillance intervalc.
3.
It is critical that all VBs be closed during the blowdown phase of a LOCA. However, the VBs are normally closed. No motion is needed for them to perform their reverse-flow (flow-check) safety function.
During the blowdown phase of a LOCA, the VBs are thus " passive" devices.
Regulatory criteria do not postulate passive failures during the short-term phase of a LOCA.
4.
Each VB has redundant safety-grade position indicators for the CLOSED position. There is thus a high degree of assurance that an open VB would not go undetected during plant operation and that no steam bypass pathway would exist due to an open VB should a LOCA occur.
5.
Unlike typical swing-check valves for which ASME Section XI authorizes a 3-month stroke test interval (or testing during cold shutdown if impractical to test at pezer), the drywell VBs are located in an inert environment which minimizes corrosion. Also unlike typical VBs, the VBs are provided with magnetic latching to minimize vibrational wear.
i These factors further support extended surveillance intervals.
Based on the considerations des:.ribed in Section 2.2 above, the staff has determined that, for Femi-2, an extended interval for cycling each drywell vacuum breaker thrcogh at least one cycle of full travel is acceptable. The extension from a monthly cycling to cycling on a cold shutdown frequency provides an adequate means to assess the operational readiness of the vacuum
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3 i 1 breakers. The requirements for verifying the opening.setpoint.on a refueling outage frequency are maintained. Together, these surveillance requirements- -
ensure an acceptable level of quality and safety and are therefore acceptable.
j The Fermi 2 inservice testing program was developed to the 1980 Edition, with i
addenda through the Winter 1980 Addenda. Per.IWV-3521 and IWV-3522, full-stroke exercising of check ~ valves is required every 3 months, unless j
exercising is impractical during power operations. For the reasons given in.
the licensee's justification for the TS change, and as approved by the NRC, a-l determination was made that exercising the vacuum breakers during power
-j operations is impractical. Therefore, the licensee may document a " cold-shutdown justification" in its inservice testing program.. As indicated in response to Question 102 of the " Minutes of the Public Meetings on Generic-Letter 89-04," NRC review of cold shutdown justifications' is not required.
t Per telephone conversation with the licensee, the licensee has indicated-its intention to document the cold shutdown justification prior to. implementation.
of this TS change.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Michigan State official was notified of the proposed issuance of the amendment.
The State official had no comments.
4.0 ENVIRONMENTAL-CONSIDERATION The amendment changes requirements with respect to the installation or use of i
a facility component. located within the restricted area as defined in 10 CFR Part 20 and the related surveillance requirements. The staff has determined j
that the amendment involves no significant increase in the amounts, and no -
i significant change in the types, of any effluents that may be released l
offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure.
The Commission has previously issued a t
proposed find %g that the amendment involves no significant hazards consideration and there has been no public comment.on such finding (58 FR 30191).
Accordingly, the' amendment meets the eligibility criteria'for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR' 1
51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
5.0 CONCLUSION
I The staff 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 amendment will not be inimical to the common defense and security-or to the health and safety of the public.
H Principal Contributor:
W. Long Date:
January 4 199a l