ML20149K488
| ML20149K488 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1986-011-00 | |
| Download: ML20149K488 (5) | |
Text
PRECURSOR DESCRIPTION SHEET LER No.: 269/86-011 Event
Description:
Emergency condenser cooling system is unavailable Date of Event: October 1, 1986 Plant: Oconee 1, 2, and 3 EVENT DESCRIPTION Sequence The design basis of the ECCW system is to provide water to the con-denser for the removal of decay heat during a loss of all ac power event (station blackout). The station blackout scenario is limiting in that CCW siphon flow through the main condenser is used to remove decay heat.
Decay heat is transferred to the main condenser via the turbine bypass valves. Feedwater is delivered to the SGs via the turbine-driven EFW pumps.
During performance a load shed test on Unit 2 during refueling, the low-pressure service-water system pumps were found to have failed. A load shed of nonessential loads is initiated when emergency power is required via the underground feeder from Keowee through transformer CT-4. The load shed protects this power path from overload. When the load shed test was initiated, the condenser circulating water pumps were deenergized. Normally, this causes the gravity flow system to align automatically and to allow the flow of water from the intake structure through the condenser and discharging to the Keowee tailrace into Lake Hartwell. The elevation difference and a siphon effect are used to cause the condenser circulating water to continue to flow. For this test, the condenser gravity drain to the Keowee tailrace was blocked because it was not part of the test. The pumps had started initially on loss of load to provide condenser cooling but began to cavitate after I h.
CCW flow was restored by restarting a CCW pump, and the plant was restored to its normal powered condition without any plant damage or system upsets. Before the occurrence, two low-pressure service-water pumps were operating with "13,000 gal/mmn per pump. The low-pressure service-water pumps are supplied from the CCW crossover header, which was being supplied from Unit 2 at the time.
Event Identifier: 269/86-011 D-47
In the evening of October 1, 1986, the test was repeated; but this time the gravity drain feature was also tested. The results were the same with the loss of low-pressure service-water flow. The U.S. Nuclear Regulatory Commission (NRC), Region II, was advised of these results late in the evening, and NRC concurred that Units 1 and 3 could continue to operate until the test data could be fully evaluated. Units 1 and 3 were at 100% power. At 0900 on October 2, 1986, evaluation of the tests revealed that the operation of this design feature (the CCW siphon flow) was questionable for Units 1 and 3 and that this resulted in inoper-ability of the low-pressure service-water systems for Oconee. As a result, an orderly shutdown of the two operating units was begun as required by Technical Specification 3.3.7. Both units reached cold shutdown conditions by October 3, 1986. An investigation determined that the ECCW system (gravity flow system) was not working. The lake level was lower than normal, and the low-pressure service-water pump housings were exposed. Because the housings were not qualified for this duty, air leaked in and caused pump cavitation and loss of the condenser syphon. All three units were affected.
The root cause of this incident is the inadequate design and test-ing of the ECCW system. This led to a failure of the ECCW system to perform its intended function as described in the FSAR under all assumed conditions. Inadequate original design evaluation of the ECCW system and the lower-than-normal lake level of Keowee are contributing factors to the cause of this incident.
Corrective Action Numerous design and procedure changes were made (see LER pp. 4-5).
Plant/Event Data Systems Involved:
AFW and SG emergency condenser cooling Components and Failure Modes Involved:
CCW pumps - failed in testing Component Unavailability Duration: Assumed 120 d Plant Operating Mode: 1 (100% power)
Discovery Method: Testing Reactor Age: 13.5, 12.9, and 12.1 years, respectively Plant Type: PWR Comments Because the system is shared by all units at the station, all three units were affected. AFW is affected given that an EPS failure has occurred.
Event Identifier: 269/86-011 D-48
MODELING CONSIDERATIONS AND DECISIONS Initiators Modeled and Initiator Nonrecovery Estimate Postulated LOOP Base case nonrecovery Branches Impacted and Branch Nonrecovery Estimate AFW and EPS 1.0 No recovery assumed possible in the short term Plant Models Utilized PWR plant Class D Event Identifier: 269/86-011 D-49
CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 269/86-011 Event
Description:
Station Emerqency Condenser Coolinq System is Unavailable Event Date: 10/1/86 Plant: Oconee I UNAVAILABILITY, DURATION= 2880 NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 5.2E-03 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CV LOOP 5.9E-08 Total 5.9E-OB CD LOOP 1.I1E-05 Total i.IE-05 ATWS LOOP 0.0E+00 Total 0.OE+00 DOMINANT SEQUENCES End State: CD Conditional Probability: 1.2E-05 218 LOOP -RT/LOOP EMERS.POWER AFW/EMERB, POWER SEQUENCE CONDITIONAL PROBABILITIES Sequence End State Prob N Rec*a 21B LOOP -RT/LOOP EMERB.POWER AFW/EMER6.POWER CD 1.2E-05 3. IE-O0 Event Identifier: 269/86-011 D-50
- dominant sequence for end state
- non-recovery credit for edited case Note: For unavailabilities. conditional probability values are differential values which reflectlthe added risk due to failures associated with an event. Parenthetical values indicate a reduction in risk compared to a similar period without the existinq failures.
SEQUENCE MODEL: c:\asp\newmodel\pwrdtree.cmp BRANCH MODEL: c:\asp\newmodel\oconee.txt PROBABILITY FILE: c:\asp\newmodel\pwr b.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail TRANS 4.8E-04 1.OE+00 LOOP 4.6E-06 3.9E-01 LOCA 2.4E-06 4.3E-01 RT 2.BE-04 1.2E-01 RT/LOOP O.OE+00 I,OE+00 EMERW.POWER 2.9E-03 B.OE-0J AFW 3.BE-04 2.6E-01 AFW/EMER6.POWER 5.OE-02 > 1.OE+00 3.4E-01 ) 1.OE+00 Branch Model: 1.DF.1 Train I Cond Prob: 5.OE-02 ) Unavailable MFW 2.DE-Ol 3.4E-01 PORV.OR.SRV.CHALL B.OE-02 1.OE+00 PORV.OR.SRV.RESEAT I.OE-02 5.0E-02 PORVOR.SRV.RESEAT/EMERS.PONER I.OE-02 1.0E+00 SS.RELEAS.TERM 1.5E-02 3.4E-01 SS.RELEAS. TERM/-MFW 1.5E-02 3.4E-01 HP] 3.OE-04 8.4E-01 HPI(F/B) 3.0E-04 B,4E-01 4.0E-02 HPR/-HPI 1.5E-04 I.0E+00 4.OE-02 86.DEPRESS 3.6E-02 1.OE+00 COND/MFW 1.OE+00 3.4E-01 LPI/HPI 1.5E-04 3.4E-01 LPR/-HPI.HPR 6.7E-01 I.OE+00 LPR/HPI 1.5E-04 I.OE+00
- branch model file
- f forced Austin 09-11-1987 Event Identifier: 269/86-011 D-51