ML20147A544
| ML20147A544 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1989-001-00 | |
| Download: ML20147A544 (7) | |
Text
B-7 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No: 249/89-001 Event
Description:
Loss of offsite power with unavailable LPCI and degraded HPCI and CCSW Date of Event: March 25, 1989 Plant: Dresden 3 Summary Dresden 3 was at 89% of rated power on March 25, 1989, when the plant experienced a loss of offsite power (LOOP). Subsequently, the plant lost two divisions of the low-pressure coolant injection (LPCI) system and the instrument air (LA) system, as well as one division of the containment cooling service water (CCSW) system for over an hour.
In addition, LIPCI was degraded because of a partially completed manual initiation sequence. Condensate was used to provide makeup water to the isolation condenser (IC), which was being used to control reactor pressure following the LOOP, resulting in low level contamination of the area around the IC vent. The conditional core damage probability estimated for this event is 1.3 x 1i-5. The relative significance of this event compared with other postulated events at Dresden 3 is shown below.
LER 249/89-001 1E-7 1E-6 1E-5 IE-4 1E-3 1E-2 36Oh HPCIJLOF "~+J[3Ih
+ IC HPCI LO L.precursor cutoff Event Description The LOOP at Dresden 3 began with the failure of a ground capacitor in the A phase of power circuit breaker (PCB) 8-15. This resulted in a phase-to-ground fault that did not clear. Local breaker backup logic automatically tripped breakers 8-9 and 11-14, and isolated line 8014 at the Pontiac substation. This isolated the internal fault in breaker 8-
B-8 15 and de-energized Unit 3 reserve auxiliary transformer (RAT) 32. Automatic transfer of bus 32 from RAT 32 to unit auxiliary transformer (UAT) 31 was delayed for 14 s because of dirty contacts on a breaker 152-320 1 position switch (the breaker was last maintained in 1982).
This resulted in trip of a main feed pump and recirculation pump. When the bus switching was completed (14 s later), the standby reactor feed pump automatically started and began to rapidly refill the reactor vessel. The reactor vessel level increase could not be controlled, and an automatic feed pump and turbine trip occurred at +55" (instrument zero reference). The reactor scrammed on turbine stop valve closure. This resulted in loss of power to UAT 31 and a loss of power to the safety-related AC buses. The LOOP lasted until offsite power was restored approximately 7 h and 22 min later, and all the emergency diesel generators (EDGs) were unloaded and secured.
The ECCS Division (Div) I & II EDG started immediately upon loss of voltage to their respective buses, and their essential loads were sequenced on as required. However, the undervoltage condition on the 480-V ECCS Div 11 bus for 7 s was sensed as a valid condition for the Div I & HI ECCS Motor Control Centers (MCCs) 38-7/93-7, which then attempted to transfer to the ECCS Div I 480-V bus. The normal supply breakers tripped open, but one of the alternate supply breakers (252-3872) failed to close because of a sticking linkage (the breaker was not included in the preventive maintenance program).
Consequently, buses 38-7/39-7 were lost. These buses supply motive power to the Div I
& 11 LPCI injection valves, which, as a result of the failure to transfer, were not able to open if required.
HPCI was manually initiated 45 min into the event, primarily for pressure control.
Following adjustment of HPCI flow using the manual flow controller, the operator did not continue with the remainder of the manual initiation procedure. Because of this, two valves, which would have reoriented without operator intervention on auto initiation, were not manually operated as required. This resulted in no HPCI lube oil cooling.
Following annunciation of HPCI high-pressure bearing oil drain high temperature, the operators reviewed the HPCI system alignment, discovered the incorrectly positioned valves, and reoriented them. The HPCI system subsequently tripped on high reactor water level (in part because of operator attention to the unavailability of lube oil cooling).
Following HIPCI pump trip, the turning gear motor was found failed (this did not fail HPCI). The HPCI turbine was manually turned for 4.3 h.
When an attempt was made to backfeed bus 34 from Div 11 EDG-powered bus 34-1, breaker 152-3403 would not remain closed (a result of a defective undervoltage relay).
B-9 This breaker was finally closed about 3.5 h into the event; as a result, ECCS Div II CCSW pumps were not available for this time. This rendered one train of LPCI (containment cooling mode) unavailable. The other divisional (Div I) CCSW pumps were successfully powered by backfeeding their bus from the swing EDG, and the LPCI pump in that division was started for torus cooling.
The IA system was discovered lost when the main turbine turning gear disengaged and tripped off approximately 15 min after the LOOP. Additionally, to prevent depressurization, the Unit 2 IA system had to be isolated from the Unit 3 IA system. The Unit 3 IA system was restored about 3.5 h later when it was cross-connected to the Unit 1 IA system.
The IC was initiated three times. The first time, mildly contaminated condensate was used for makeup because the LOOP had disabled the normal demnineralized water supply.
The second use of the IC began with demnineralized water as the sole source of makeup water; however, the demand was greater than the supply and it was decided to supplement the supply with condensate. The third initiation of the IC was done with only demnineralized water as the makeup. The use of condensate resulted in approximately 250,000 ft 2 of contamination to the grounds and structures near the IC vent. The contamination levels ranged from 25-60,000 dpm/lOO cm 2 . The release was estimated to be less than 0.01% of the 10CFR20 Appendix I Quarterly Objectives.
Additional Event-Related Information Division I and 11 480-V AC MCCs 38-7 and 39-7 are tied together to form a common bus. One of the purposes of this common bus is to provide a dual source of power to the LPCI injection valves. Its normal power supply is from ECCS Div 11480-V bus 39, and its alternate supply is from ECCS Div I 480-V bus 38. If the normal supply is lost, automatic transfer between the normal and alternate feeders ensures continuous power.
The transfer occurs for lost voltage or undervoltage conditions on the bus. Normally, a undervoltage condition must exist for 15 s for it to be considered valid. This is so the EDGs have time to come up to speed and restore bus voltage to its normal voltage.
Following the LOOP, the EDGs were at rated speed, and loads were sequenced onto the generators within 7 s; however, the normal supply breakers tripped open anyway, and the alternate supply breakers did not close, causing a loss of power to the MCC.
The IA systems for Units 1, 2, and 3 may be cross-connected so that any unit's IA system may supply any other unit's IA system. In addition, the service air (SA) system may be used as a backup to the IA system for any unit.
B-10 The IC supply and makeup comes from the demnineralized water system pumps. The motor-operated valve (MOV) that controls this makeup water supply was de-energized during this event because its power supply MCC is designed to automatically trip during undervoltage conditions to limit loads on the EDGs. When power was restored to the MOV, the steaming rate of the IC was higher than the normal makeup system could supply. The backup used in this instance was from the condensate system. An alternate supply is from the fire protection system using the service water or fire pumps.
Previously, the utility had determined that when condensate was used for makeup, relatively low levels of radioactive contamination in the condensate tended to concentrate in the isolation condenser shell; then, under high steaming conditions, the carryover would slightly contaminate the area just below the IC vent to atmosphere.
ASP Modeling Assumptions and Approach The event has been modeled as a plant-centered LOOP with (1) both trains of LPCI unavailable because of lack of power on MCCs 38-7/39-7 (p(non-recovery) = 1.0), (2) one train of LPCI (CC) available due to inability to backfeed bus 34 (no change in non-recovery estimate), and (3) JJPCI unavailable because of misaligned lube oil cooling valves (p(non-recovery) = 0. 12)
Analysis Results The conditional probability of severe core damage for this event is 1.3 x 1i-5. The dominant sequence associated with the event (highlighted on the following event tree),
involves failure of an SRV to close following the LOOP, failure to recover ITPCI, and failure to depressurize using ADS. Note that the shutdown cooling system on Dresden is separate from LPCI, and redundant capability existed for decay heat removal (event with failed LPCI and degraded LPCI(CC).
The combined impact of the multiple system failures experienced during the event may not have been fully captured by the model, and hence the estimated conditional probability may not be conservative.
B-li SH.UT REC SRV SRVI-C.- PI CR. RV-I LPCS LPC.. O (CC EQ END DOWN CA CLJ D RDE)
MOR) OTER NO STATE OK 41 COREDAMAGE 06 06 42 COREDAMAGE 06(
OK 43 COREDA.MAGE OK OK 44 COREDAMAGE OK 45 COREDAMAGE 0K 46 COREDAMAGE 4, COREDAMAGE 48 COREDAMAGE OK 49 COREDAMAGE 0.
50 COREDAMAGE OK 52 COREDAMAGE OK 53 COREDAMAGE 54 COREDAMAGE 55 COREDAMAGE OKK OK 56 CORE DAMAGE OK O,K 57 CORE DAMAGE OK OK 59 CORE DAMAGE 06 OK 59 CORE DAMAGE 06CR DMG CS COREDAMAGE 62 COREDAMAGE 63 COREDAMAGE OK 06CR DMG 64 COREDAMAGE 6 COREDMG 06 06 66 COREDAMAGE 67 COREDAMAGE 84 CORE DAMAGE 97 ATWS Dominant core damage sequence for LER 249/89-001
B-12 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 249/89-001 Event
Description:
LOOP with degraded NPCI, unavail LPCI and 1 train of CCSW Event Date: 03/25/89 Plant: Dresden 3 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 3. OE-01 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD LOOP 1.3E-05 Total 1. 3E-05 ATWS LOOP 9.2E-06 Total 9. 2E-06 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
55 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scraxe srv.close CD 7.5SE-06 2 .6SE-02 NPCI srv.ads 65 LOOP erserg.power -rx.shutdown/ep -EP.REC srv.chall/loop.-scrars CO 1. 6E-06 2 . 9E-02
-srv.close isol.cond HPCI 67 LOOP emerg.power -rx.shutdown/ep -EP.REC srv.chail/loop.-scram CD 1.4E-06 2. 9E-02 srv.close HPCI 84 LOOP erserg.power -rx.shutdown/ep EP.REC CD 9. 6E-07 2. 4E-01 52 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scram srv.close CD 4. OE-07 1 .2E-02 NPCI -srv.ads lpcs LPCI -sdc firewater 41 LOOP -erserg.power -rx.shutdown srv.chall/loop.-scram -srv.close CD 3.5E-07 3.l1E-02 isol.cond -NPCI sdc LPCI(CC) 49 LOOP -emerg.power -rx.shutdown srv.chail/loop.-scrars srv.cloae CD 2. 9E-07 3 . IE-02
-NPCI sdc LPCI(CC) 98 LOOP -emerg.power rx.shutdown ATWS 9.OE-O6 3.OE-Ol
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
41 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scrarn -srv.close CD 3. 5E-07 3 . 1E-02 isol.cond -NPCI sdc LPCI(CC) 49 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scram srv.close CD 2. 9E-07 3. IE-02
-NPCI sdc LPCI(CC) 52 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scram srv.close CD 4 . OE-07 1. 2E-02 NPCI -srv.ads lpcs LPCI -sdc firewater 55 LOOP -erserg.power -rx.shutdown srv.chall/loop.-scram srv.close CO 7.5SE-06 2. GE-02 NPCI srv.ads 98 LOOP -emerg.power rx.shutdown ATWS 9. OE-06 3. OE-01 65 LOOP emerg.power -rx.shutdown/ep -EP.REC srv.chall/loop.-scrars CD I. 6E-06 2. 9E-02
-srv.close isol.cond HPCI 67 LOOP emerg.power -rx.shutdown/ep -EP.REC srv.chall/loop.-acram CD 1. 4E-06 2 . 9E-02 srv.close HPCI 84 LOOP emerg.power -rx.shutdown/ep EP.REC CD 9. 6E-07 2 .4E-Ol
"* non-recovery credit for edited case Event Identifier: 249/89-001
B-13 SEQUENCE MODEL: c:\asp\1989\bwrbseal .cmp BRANCH MODEL: c: \asp\1989\dresden.sll PROBABILITY FILE: c: \asp\1989\bwr csll .pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 3. 4E-04 l.OE+00 LOOP 1.6E-05 > 1.6E-05 3.6E-01 > 3.0E-0l Branch Model: INITOR Initiator Freg: 1.6E-05 loca, 3.3E-06 5. OE-0l rx. shutdown 3. GE-05 1.0GE+00 rx. shutdown/ep 3.5E-04 1. OE+00 pcs/trans 1.7E-01 1. OE+00 srv.chall/trans .-scram 1. OE+00 1. OE+00 srv.chall/loop.-scram 1. OE+00 1. OE+00 srv. close 1. 6E-02 1. OE+00 emerQ.power 2.9E-03 8.OE-01 EP.REC 6.EE-02 > 1.4E-03 1. OE+00 Branch Model: l.OF.l Train 1 Cond Prob: 6.6E-02 > 1.4E-03 fw/pcs .trans 2.9E-01 3.4E-01 HPC I 2.9E-02 > 1.OE+00 7.OE-0l > 1.2E-01 Branch Model: l.OF.l Train 1 Cond Prob: 2.9E-02 > Failed isol .cond 2.OE-02 1.OE+00 crd 1.OE-02 1. OE+00 1 .OE-02 s rv -ads 3.7E-03 7.1lE-01 1. OE-02 lpcs 2 .OE-03 3.4 E-01 LPC I l.OE-03 > 1.OE+00 7.1E-01 > l.OE+00 Branch Model: l.OF.2 Train 1 Cond Prob: 1. OE-02 Failed Train 2 Cond Prob: 1. OE-0l1 Failed sdc 2.9E-03 3.4E-01 l.OE-03 LPCI (CC) 1.OE-03 1. OE-01 3.4E-01 Branch Model: l.OF.2 Train 1 Cond Prob: 1.OE-02 > Failed Train 2 Cond Prob: 1.OE-01 lpci (cc) /lpci 1. OE+ 00 l.OE+00 lpci (cc) /-lpci 0. OE+00 l.OE+00 firewater l.OE+00 1. 0E+00 2 .OE-0 3
- branch model file
- forced Minarick 06-15-1990 17:42:29 Event Identifier: 249/89-001