ML20149K464
| ML20149K464 | |
| Person / Time | |
|---|---|
| Site: | McGuire  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1989-004-00 | |
| Download: ML20149K464 (5) | |
Text
B-308 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No: 369/89-004 Event
Description:
Steam generator tube rupture Date of Event: March 7, 1989 Plant: McGuire Unit 1 Summary A steam generator tube rupture with a flow rate of -540 gpm occurred at McGuire 1. Th'e charging pump suctions were aligned to the RWST, the affected steam generator was isolated, and the reactor coolant system was depressurized below the steam generator relief valve set point, terminating flow through the break. The conditional core damage probability estimated for the event is 7.7 x 10-4. The relative significance of this event compared with other postulated events at McGuire is shown below.
1E-7 1E-6 IE-5 1E-4 \116-3 IE-2 Trip MM AFW L] 360 h AFW L360 hEP Event Description McGuire Unit 1 was operating at 100% power. Reactor coolant system leakage calculated on March 6, 1989, resulted in 0.377 gpm identified leakage and 0.199 gpmn unidentified leakage. A primary to secondary leak on the "B" steam generator (SG) had been detected in January 1989 and was being monitored. The leak was approximately 10-15 gal/d. On March 7, 1989, at 2338, the "B" steam line radiation monitor alarmed and would not reset. The "B" SG feedwater flow decreased while the narrow range level indication remained relatively constant. In addition, pressurizer level was decreasing.
The operators recognized this incident as a steam generator tube break. Power reductions were immediately started; the reactor was manually tripped at 2346. It was later estimated that flow though the tube rupture averaged approximately 540 gpm.
B-307 The operators swapped the suctions of the charging pumps to the RWST, isolated the "B" SG, and initiated RCS cooldown and depressurization. By 0025 on March 8, 1989, the pressure in the "B" SG and the RCS were equ alized. This terminated reactor coolant flow out the break.
At 1015, cooldown of the "B" SG was initiated. Hot shutdown was entered at 1025.
Both trains of RHR were operating by 1640, and cold shutdown was entered at 1744.
Based on a visual inspection after the event, the tube rupture was caused by a 3.48 in. by 0.2 in. (maximum) axial break in tube 18-25. At the time the LER was 'written, the cause of the break was believed to be corrosion-assisted cracking.
Activation of the Technical Support Center (TSC) to assist in recovery from the tube rupture was delayed because of freezing rain and ice buildup on the roads. In addition, the utility stated that discussions with NRC personnel at critical times interfered with the TSC emergency coordinator's management of the recovery.
An unplanned radioactive release occurred as a result of the steam generator tube rupture.
The release resulted from a steam release from the "A, ". .C," and "D" steam generator power-operated relief valves and from the condensate steam air ejector discharge to the unit vent. The total unplanned release averaged over the 2-h period from 2340 on March 7, 1989, to 0140 on March 8, 1989, was 12.72 Ci (133 Xe equivalent) and 5.24 x 10 -4 Ci (1311j equivalent).
ASP Modeling Assumptions and Approach The event has been modeled as a steam generator tube rupture. Information concernin g the model used for the analysis is included with the documentation for LER 338/89-005.
Analysis Results The core damage probability for this event is estimated to be 7.7 x 10O4 Two sequences dominate this risk estimate: (1) successful AFW and HPI following the tube rupture with operator failure to depressurize the RCS below the SG relief valve setpoint, and (2) failure of HPI. The dominant sequence for this event is highlighted on the following event tree.
B-309 STGR FIT AFW MF J RGS Depressurized Below SG RV Ruptured SG Isolated RCS InlCold Shutdown Prior to RWST End state Seq.
No.
Setpoint Depletion 0OK OK CD 101 CD 102 CO 103 OK OK CO 104 CD 105 CD 106 CD 107 ATWS 108 Dominant core damage sequence for LER 369/89-004
B-310 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 369/89-004 Event
Description:
Steam generator tube rupture Event Date: 03/07/8 9 Plant: PNR SGTR INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES SGTR 1. OE+00 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD SGTR 7. 7E-04 Total 7 . 7E-04 ATWS SGTR 3.4E-05 Total 3.4E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
102 sgtr -rt -afw -hpi rcs.depr<sg.rv.setpoint CD 4 .1E-04 1. OE+00 103 sgtr -rt -afw hpi CD 2. 5E-04 8 .4E-01 101 sgtr -ft -afw -hpi -rcs.deprcsg.rv.setpoint ruptured.sg.isol r CD 1. OE-04 1 . E+00 cs.cold.prior.to.rwst .depl 108 sgtr rt ATWS 3.4E-05 1.2E-01
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
101 sgtr -ft. -afw -hpi -rcs.depr<sg.rv.setpoint ruptured.sg.isol r CD l.OE-04 1. OE+400 cs.cold.prior.to. rwst .depl 102 sgtr -ft -afw -hpi rca.depr<sg.rv.setpoint CD 4 . IE-04 1 .OE+00 103 sgtr -ft -afw hpi CD 2. 5E-04 8 .4E-01 108 sgtr ft ATWS 3. 4E-05 1. 2E-01
- non-recovery credit for edited case SEQUENCE MODEL: a:\1989\PWRSGTR.CMP BRANCH MODEL: a:\1989\PWRSGTR.NEW PROBABILITY FILE: a:\1989\PWRBSL1.PRO No Recovery Limit BRANCH FREOUENCIES/PROBABILITIES Branch System Non-Rec ov Opr Fail sgtr 5.OE-03 1.OE400 rt 2.8E-04 1.2E-01 afw 3.9E-04 2.6E-01 mfw 2.OE-01 3.4E-01 hpi 3.OE-04 8.4E-01 ruptured.sg.isoi l.OE-02 l.OE+00 rcs.depr<sg.rv.setpoint 1.OE-05 1.OE+00 4 .OE-04 Event Identifier: 369/89-004
B-311 rcs.cold.prior.to.rwst.depl l.OE-02 l.OE+00 4.OE-04
- branch model file
- forced Minarick 06-17-1990 14:28:55 Event Identifier: 369/89-004