ML20147A537
| ML20147A537 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 08/16/1985 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1985-034-00 | |
| Download: ML20147A537 (5) | |
Text
PRECURSOR DESCRIPTION AND ANALYSIS LER No.:
237/85-034 Event
Description:
Scram and Isolation Condenser Failure During LOOP Date of Event:
August 16, 1985 Plant:
Dresden 2 EVENT DESCRIPTION Sequence During normal unit operation on August 16, 1985, at -0021 h, Dresden Unit 2 scrammed because of low reactor water level.
The scram resulted from a LOOP caused by a fault on the secondary side of Unit 1 reserve auxiliary transformer, TR12.
TRI2 primary side was being powered from the same 138-ky power source as the Unit 2 reserve auxiliary transformer, TR22.
When protective relaying sensed the fault, the Unit 2 offsite power source was isolated, and power to TR22 was lost.
Four-kV buses 22 and 24 did not automatically transfer power.
One of two running reactor feedpumps powered from bus 22 tripped on bus undervoltage, and the standby reactor feedpump also powered from bus 22 failed to start automatically because of the bus undervoltage.
The low-water-level scram and complete loss of power to the unit resulted.
Both emergency diesel generators automatically started and provided power to their respective loads.
The operator initiated the isolation condenser system to control reactor pressure.
Au attempt was made to provide a supply of clean demineralized water to the isolation condenser shell, but the supply valve, MO-2-4399-74, would not open because the power supply to the valve (MCC-29-3) had tripped because of bus undervoltage.
Realizing that the clean demineralized water pumps were also tripped because of loss of power supply (MCC 25-2), the reactor operator attempted to provide the backup source of water, contaminated condensate
- transfer, to the isolation condenser shell.
Valve MO-2-1301-10 on the condensate transfer supply line failed to open from the control room because of a bad control coil.' Equipment attendants were dispatched to open the valve manually, and makeup to the isolation condenser shell was supplied.
After the trip to MCC 29-3 had been reset and power restored to MCC 25-2, clean demineralized water was supplied to the isolation condenser, and the condensate transfer supply was secured.
Corrective Action Investigation of the 4-kV breaker control circuitry has shown that under the condition that prevailed, reserve feed breakers for buses 22 and 24 were not designed to automatically transfer power feed to the unit auxiliary transformer, TR21.
The circuitry has been modified and Event Identifier:
237/85-034 E-3 1
functionally tested to ensure that this event does not recur.
Event review resulted in recommending that more procedures be written for LOOP breaker circuitry modifications for auto transfers.
Plant/Event Data Systems Involved:
"A" channel reactor protection system (RPS), off site power -
phase short in reserve auxiliary transformer for Unit 1 Reserve auxiliary transformer -
protective circuitry and particular hookup Switchyard bus auto transfer-design and error during prior modification Components and Failure Modes Involved:
Process alarm printer signal temporarily lost in process of installing two new prime computers Communications system repeater -
loss of power OSEP telephones -
loss of power Condensate transfer valve auto operation -failed control coil Component Unavailability Duration:
NA Plant Operating Mode:
70% power Discovery Method:
Operational event Reactor Age:
15.7 years Plant Type:
BWR Comments None MODELING CONSIDERATIONS AND DECISIONS Initiators Modeled and Initiator Nonrecovery Estimate LOOP (postulated)
Base case Branches Impacted and Branch Nonrecovery Estimate Isolation condenser 0.34 Manually recoverable locally Plant Models Utilized BWR plant Class B Event Identifier:
237/85-034 E-32
CONDITIONAL CORE DAMAGE CALCULATIONS LER Number:
Event
Description:
Event Date:
Plant:
2307/Bc5-03.4 Scram and Isolation Condenser Failure During LOOP 8/16/85 Dresden 21 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator
- 3.
400E-01 Probability CV LOOP Total
- 7.
- 7.
- 3.
- 3.
- 7.
- 7.
107E-06 DOMINANT SEQUENCES End State:
CV Conditional Probability:
7.179E-06 222 LOOP -EMERG.PDWER SCRAM -SLC.OR.RODS HPCI -SRV.ADS -LPCS -SDC End State:
CD Conditional Probability:
1.4?OE-05 207 LOOP -EMERG.POWER -SCRAM SRV.CRALL/LOOP.-SCRAM -SRV.CLOSE ISOL.COND HPCI CR0 SRV.ADS End State:
AIWS Conditional Probability:
6.904E-06 236 LOOP -EMERG.POWER 'SCRAM SLC.OR.RODS Event Identifier: 237/85-034 E-33
SEQUENCE CONDITIONAL PROBABILITIES Sequence End State Seq. Prob Non-Recovtt 207 LOOP -EMERG.POWER -SCRAM SRV.CHALL/LOOP.-SCRAM -SRY.CLOSE IS CD 1.490E-05 I 2.40bE-013 OL.COND HPCI CRD SRV.ADS 213 LOOP -EMERB.POWER -SCRAM SRV.CHALL/LOOP.-SCRAM SRY.CLOSE HP CD J.4B2E-05 3.245E-02 CI SRV.ADS 222 LOOP -EMERG.POWER SCRAM -SLC.OR.RODS HPCI -SRV.ADS -LPCS -SD CV 7.179E-06 I I.7BbE-01 C
236 LOOP -EMERG.POWER SCRAM SLC.OR.RODS ATWS 6.904E-06 I 7.414E-02 2378 LOOP EMERS.POWER -SCRAM SRV.CHALL/LOOP.-SCRAM -SRV.CLOSE IS CD 9.416E-06 3J.360E-02 DL.COND HPCI 240 LOOP EMERB.PONER -SCRAM SRV.CHALLILOOP.-SCRAM SRY.CLOSE HP CD 4.646E-07 9.BB4E-02 CI I dominant sequence for end state It non-recovery credit for edited Ease Note.'
Conditional probability values are differential values which reflect the added risk due to failures associated with an event.
Parenthetical values indicate a reduction in risk compared to a similar period without the existing failures.
MODEL:
A:bwrbtree.cmp DATA:
A: drprob. cmp No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail TRANS 1.142E-03 1.000E+00 LOOP
- 1.
- 3.
400E-01 LOCA 3.250E-06 3.400E-01 SCRAM 4.100E-04 1.000E+00 SLC.OR.RODS l.000E-02 1.000E+00 4.OOOE-02 PCS/TRANS 1.700E-01 1.000E+00 PCS/LOCA l.000E+00 1.OOOE+00 SRV. CHALLfTRANS. -SCRAM 1.000OE+00 1.0OO0E+00' SRV.CHALL/TRANS.SCRAM 1.000E+00 1.OOOE+00 SRV.CHALL/LOOP.-SCRAM 1.000E+00 1.000E+00 SRV.CHALL/LOOP.SCRAM 1.000E+00 1.000OE+00 SRV.CLOSE 1.650E-02 1.000E+00 EMERG.POWER 2.850E-03 5.100E-01 FW/PCS.TRANS 2.900E-01 3.400E-01 FW1PCS.LOCA 4.OOOE-02 3.400E-01 HPCI 1.000E-01 5.700E-01, Event Identifier:
237/B5-03 -4 E-34
ISDL.C OND Branch Model:
I.0F.1 Train I Cond Prob:
CRD SRV. ADS CIJND/FW. PCS LPCS LPC I FIRENTR. DR.DTHER/LPC5.LPCI/TRA FIRENTR. DR.DTHER/LPCS. LPCI/LOO FIRE WTR.DR.BTHER/LPCS. LPCI/LDC SDC LPCI (CC)
I.ANO.V/LPCI 2.OOOE-02 > 1.0OOE+00 2.OOOE-02 > Failed I.000OE-02
- 6.
700E-03 1.00OE+00 2.000OE-013 1.0O0E-03
- 1.
OOOE+00 I.
O0OE+00 1.OOOE+00
- 2.
878E-03 1.000E-03 I.00OOE+00 1.000E+00' 1.OOOE+00 > 3.400E-01
- 1.
OOOE+Ot) 1.000E+00
- 3.
- 3. 400E-01
- 3. 400E-01 1.00OOE+00 1.00OE+00
- 1.
OOOE+OO
- 3.
- 3.
- 1.
OOOE+00
- 3.
- 4. OOOE-02
- 4. OOOE-02
$It forced Austin 08-12-1986 09:07: 42 Event Identifier: 237/B5-034 E-3 5