ML20147A525
| ML20147A525 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1980-029-00 | |
| Download: ML20147A525 (5) | |
Text
PRECURSOR DESCRIPTION AND DATA NSIC Accession Number:
158860 Date:
April 30, 1980
Title:
Loss of Two Essential Buses and Loss of Decay Heat Removal Capability at Davis-Besse 1 The failure sequence was:
- 1.
The reactor was in cold shutdown in preparation for refueling with the following equipment/system status:
- a.
the head was detensioned but not removed (water level below the vessel flange)
- b.
decay heat was being removed using decay heat pump No.
2
- c.
decay heat pump No.
1 was out of service for maintenance with its associated piping drained
- d.
the manway covers on the top of the steam generators had been removed.
- 2.
The unit electrical lineup had been revised in preparation for work on buses "A" and "C".
Buses E2 and F2 were supplied from breaker HBBF2.
Essential distribution panels YI and Y3 were on their alternate feed (YBR) which is supplied by F2.
- 3.
The ground fault relay on breaker HBBF2 actuated (possibly due to vibration caused by construction personnel in the switchgear room) and tripped the breaker.
- 4.
This deenergized essential distribution panels Y1 and Y3,. which resulted in full SFAS actuation in levels 1 through 5.
- 5.
The SFAS actuation isolated the RCS letdown line and caused the suction of decay heat removal pump No.
2 to transfer to the emergency sump.
During the time the BWST outlet valves and emergency sump outlet valves were stroking, water gravity flowed into the emergency sump (approximately 1500 gallons).
The decay heat pump was injecting BWST water into the RCS and increased RCS inventory approximately 3500 gallons.
(The high pressure injection pumps and containment spray pump breakers had been racked out as required and hence did not actuate).
- 6.
The closing of the BWST outlet valve caused the decay heat pump to draw suction from the emergency sump which resulted in air being drawn into the pump suction.
The pump was shut down to stop the injection and to prevent pump damage due to loss of suction.
- 7.
The emergency sump valves were closed and power was removed from their operators.
Decay heat removal loop No.
2 was refilled from the BWST, vented, and returned to service.
The electrical lineup was restored with buses E2 and F2 separated.
- 8.
Decay heat removal was unavailable for approximately 2-1/2 hours.
During that time interval, reactor coolant temperature increased from 90'F to 170'F.
Corrective action:
Plant procedures have been revised to ensure power is removed from the emergency sump valves during mode 5 and mode 6 operation.
The instrument ac system procedure was revised to allow inverters to be supplied from the dc bus when the normal feed for the regulated rectifiers from motor control centers E12A or F12A are to be deenergized.
Design purpose of failed system or component:
The uninterruptable buses provide a continuous source of power to control and instrumentation circuitry which cannot tolerate short term power interruptions.
Reactor in cold Unit electrical Ground fault Deenergized dis-Emergency sump Operator Emergency shutdown with lineup revised relay on tribution panels valves not trips sump head detensioned, in preparation breaker HBBF2 Yl and Y3 cause locked out.
decay valves steam generator for maintenance, actuates due level 1-15 SFAS Stroking emer-heat closed and manway covers Buses E2 and F2 to vibration, actuation, which gency sump and pump power removed, and DH supplied by tripping transfers suction BWST valves removed pump #1 out of breaker HBBF2.
breaker from RCS to emer-results in from service for main-Essential dis-gency sump gravity flow operation.
tenance with its tribution panels of BSWT water Decay heat loop drained (DH Y1 and Y3 on into emergency loop #2 pump #2 in opera-alternate feed sump.
Open refilled tion) from F2 emergency sump from BWST, valves and vented, and closed BWST returned to valve result in service air being drawn into pump suction Potential Severe Core Damage No Possible -
time period to core damage is long.
Train fl could possibly have been returned to service.
If
- not, throttled HPI pumps could possibly have been used for core cooling Possible -
failure to trip pumps may have resulted in pump failure.
If so, and if train
- 1 could not have been returned to service, then throttled HPI pumps could possibly have been used for core cooling.
The time period to core damage is long and other mitigation paths could provide core cooling No No No NSIC 158860 - Actual Occurrence for Loss of Two Essential Busses and Loss of Decay Heat Removal Capability at Davis-Besse 1
Unit in Cold Shutdown DH Drop Line Valves Operator DH Pump Operator DH Loop
- 2 DH Loop
- 1 Other Means of with Head Detensioned, Shut.
Stroking Trips DR Continues Restores Refilled Returned to DH Removal Pro-SG Manways Open,
- 1 DH Emergency Sump and Pump to Oper-Valve and OH Service Prior vided (Throttled)
Train Drained, Unit BWST Valves Results ate with Alignment Removal to Core Damage RPI Pumps, etc.)
Electrical Lineup in Gravity Flow of Air in Resumed Revised for Mainte-RWST Water into Pump nance.
Breaker Ground Emergency Sump.
Suction Fault Relay Actuates Open emergency sump due to Vibration, Valves and Closed Tripping Breaker, BWST Valve Result Resulting in Level in Air Being Drawn 1-5 SFAS Actuation into DH Pump Suction Potential Sequence Severe No.
Core Damage No I
No No 2
3 Yes No Yes No No No Yes 4
No 10 11 12 Yes NSIC 158860 -
Sequence of Interest for Loss of Two Essential Buses and Loss of Decay Heat Removal Capability at Davis-Besse 1
CATEGORIZATION OF ACCIDENT SEQUENCE PRECURSORS NSIC ACCESSION NUMBER:
158860 LER NO.:
80-029 DATE OF LER:
April 30, 1980 DATE OF EVENT:
April 19, 1980 SYSTEM INVOLVED:
Decay heat removal system, essential power system COMPONENT INVOLVED:
Essential bus, decay heat pump CAUSE:
Tripped breaker resulted in loss of two essential buses and loss of decay heat removal SEQUENCE OF INTEREST:
Loss of two essential buses while in cold shutdown ACTUAL OCCURRENCE:
Loss of two essential buses and loss of decay heat removal capability REACTOR NAME:
Davis-Besse 1 DOCKET NUMBER:
50-346 REACTOR TYPE:
PWR DESIGN ELECTRICAL RATING:
906 MWe REACTOR AGE:
2.7 years VENDOR:
Babcock & Wilcox ARCHITECT-ENGINEERS:
Bechtel OPERATORS:
Toledo Edison LOCATION:
21 miles east of Toledo, Ohio DURATION:
N/A PLANT OPERATING CONDITION:
Cold shutdown TYPE OF FAILURE:
Made inoperable DISCOVERY METHOD:
Operational event COMMENT: