05000298/LER-1992-019-01, :on 921230,design Deficiency W/Ac Electrical Sys Confirmed When Single Nonaccident Event Could Result in Common Mode Failure of Both Dgs.Permanent Electrical Jumper Installed in Each Breaker Trip Circuit
| ML20128B881 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 01/28/1993 |
| From: | Reeves D NEBRASKA PUBLIC POWER DISTRICT |
| To: | |
| Shared Package | |
| ML20128B866 | List:
|
| References | |
| LER-92-019-01, LER-92-19-1, NUDOCS 9302030201 | |
| Download: ML20128B881 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2981992019R01 - NRC Website | |
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o, o.i On December 30, 1992, a design deficiency with the AC Electrical System was confirmed wherein a single non-accident event, such as an earthquake, tornado, or missile, could have potentially resulted in a common mode failure of both Diesel Genetators (DGs).
This design deficiency was found as a result of the Design Basis Reconstitution Program for t.he AC Electrical System.
At 10:07 am, a Limiting Ccndition for Operation was entered per Technical Specification 3.9.B.l.b.2 and 3.5.F.2, a Notification of Unusual Event was declared, and a power reduction to 25 percent of rated was initiated. At the time, the plant was in operation at full power.
The identified deficiency involved the use of non-safety related interlocks as open and close permissives in the control circuitry for safety related power supply breakers to the 4160V 1F and 1G safety related buses.
An Operability Evaluation of the adverse effects of the non-safety related interlocks indicated that the safety function of the DGs could potentially be affected by one breaker trip circuit in each division (IFA in Division I and 1GB in Division II).
The condition has existed for thn life of the plant and is considered to have been an original design deficiency.
A permanent electrical jumper was installed in each breaker trip circuit, as corrective action, bypassing the non-safety related interlocks located in the non-safety related Switchgear Room.
The jumpers reconfigured the breaker 1FA and ICB trip circuits so that the conductors to the safety related trip interlocks are located entirely wit'nin Class IS Structures.
An evaluation of the trip circuit configurations with the existing non safety related interlocks will be performed.
Modifications will be implemented, if appropriate, to enhance the electrical separation of the bienkers.
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Event Description
on December 30,1992, at 10:07 am, following review of a design deficiency by the Station Operations Review Committee and on-duty Shift Supervisor, both Diesel Generators (DCs) were declared inoperable and a Notification of Unusual Event was declared.
The design deficiency involved a single non accident Design Basis Event (DBE), such as an earthquake, a tornado, or a missile which could potentially cause a common mode failure of both DCs.
In accordance with paragraphs 3.9.B.1.b.2 and 3.5.F.2 of the a load reduction to 25 percent of rated was-Technical Specifications, initiated at 10:30 am.
The deficiency was discovered during a design basis review of the AC Electrical System, conducted as a part of the Design Basis Reconstitution Program. During the course of the review, it was determined that non safety related interlocks were used as open and close circuit permissives in the control circuitry for safe.ty related power supply breakers to the 4160V 1F and 1G safety related buses.
An Operability Evaluation of the adverse effects of the non-safety related interlocks indicated that the safety function of the DCs could potencia11y be affected by one breaker trip circuit in each division (IFA in Division 1 and 1GB in Division II).
The trip control circuits for safety related tie-breakers 1FA and 1GB are routed outside the safety related Switchgear Room to non-safety related interlocks located in the non safety related Switchgear Room, then back to the safety related Switchgear Room to the breaker IFA and 1GB trip circuits for undervoltage trip contacts and Control Room indication and control (refer to Figures 1, 2, and 3).
As a result, a DBE resulting in damage to the non-safety related Switchgear Room could have caused a discontinuity in the. iip circuits, preventing the breakers from opening.
In the vvent of a coincident loss of off-site power, this would have prevented the DC cutput breakers (EGl/EG2) from automatically closing and powering the buses upon startup of the DG units, because 4160V breakers 1 FA and 1 GB would not trip.
This would result in station blackout (SBO).
B.
riane status The plant was in normal full power operation at the time this condition was discovered.
The condition has axisted for the life of the piant.
C.
Basis for Report An event or condition alone that could have prevented the fulfillicent of the safety function of the Diesel Generators, reportable in accordance with 10GFR50.73(a)(2)(v).
With both DCs cormidered inoperable, a power reduction to 25 percent of rated was initiated. Additionally, due to loss of both DGs, a Notification of Unusual Event was declared.
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Cause
Design.
As previously noted, this condition hac existed for the life of the plant.
Apparently during initial design and during prior design change activitics associated with the AC Elen?rical Systen, the potential impact that a DBE could have on thu
- .fety related interlock:
installed in the trit circuits for 4160: breakers 1FA ' and 1GB was doc recognized.
E.
Safetv Sinni ficance i
l As a result of the design deficiency, both DC's could have been prevented from performing their safety function of automatically powering 4160V safety related buses IF and 1G.
This design deficiency only has safety significance in the highly unlikely event that a Design Basis Event' occurs, causing the trip circuits for the safety related to non-safety related bus tie breakers to fail, concurrent with a loss of of f site power during power operation. As a result, a SB0 condition would occur with the DGs operating but not londed.
According to the Diesel Generator Design Basis Document, the DGs are capable of running without Sc.rvice Water for up to 2 minutes at full lead.
It is estimated that the units - could _
operate in an unloaded condition for 10 to 15 minutes without Service Water, proildin6 aufficient time for Operator intervention.
Existing Opera ting Procedures provida guidance on evaluating the extent of equipment malfunctions during a SBO and identify actions to be taken to terminate and/or cope with the event. The emergency procedure.for loss of all AC power identifies actions to be taken to restore power to the station.
- lhese include
li '
Dispatcher notification of the condition. An Operating Instruction-has been developed to expedite restoration of off-site power.:o the i
station.
2)
Startup and loading of the Diesel Generators.
Upon restoration of AC power to the safety related buses, the Service Wat ar Systein-would automatically start, restering normal cooling to the DGs.
The RCIC and liPCI Systems are both individually capable of providing.
adequate cooling water to the reactor core during c.nd after a SBO for the.
time periods required to restore AC power to the station.
In accordance with SBO analyses, the RCIC System can provide the required makeup water to the reactor core for 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />; 11PCI can provide the required makeup -
water to the core for up to I hour.
F.
Safety 7mr11 cations The plant was in operation at full power, the condition assumed to be the worst case initial condition for a SB0 event.
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Corrective Action
A permanent electrical jumper was installed in each breaker trip circuit,
- - l bypassing the non-safety related interlocks located in the non-safety related Switchgear Room. The jumpers reconfigured the breaker 1FA and 1GB trip circuits so that the conductors to the safety related trip interlocks are located entirely within Class IS Structures.
At 2:43 pm, with plant power reducted to approximately-500 MW, jumpers were installed in the 1Ft.
Breaker trip circuit, enabling DG #1 to be declared operable. This placed the plant in a 7 day LCO per Technical Specification 3.5 F.1.
Shortly thereafter, the Notification of Unusual Event was terminated.
Jumpers-i were installed in the 1GB trip circuit and at 3:54 pm, DC #2 was declared operable, restoring both on-site emergency power sources to normal.
An evaluation of the trip circuit configurations with the existing non-safety related interlocks will be performed.
Modifications _ will be implemented, if appropriate, to enhance the electrical separation of the breakers.
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