ML20161A162
| ML20161A162 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 06/09/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1983-007-00 | |
| Download: ML20161A162 (6) | |
Text
B.3-1 B.3 LER No. 250/83-007 Event
Description:
Three Auxiliary Feedwater Pumps Unavailable Date of Event: April 14, 1983 through April 19, 1983 Plant: Turkey Point 3 B.3.1 Summary Manual valves in the steam supply lines to the B and C auxiliary feedwater (AFW) pump turbines were found to be closed on April 19, 1983. Since AFW pump A was out of service at the time, all of the AFW pumps were unavailable. The increase in core damage probability, or importance, over the duration of the event is 5.5 x I0". The base-case core damage probability (CDP) over the duration of the event is 1.5 x 10', resulting in an estimated conditional core damage probability (CCDP) of 5.6 x 10-5.
B.3.2 Event Description On April 19, 1983 with the unit at full power, manual valves 3-084A and 3-086B on the steam supply lines to the B and C auxiliary feedwater pump turbines, respectively, were found to be closed. This rendered the B and C pumps inoperable. Since AFW pump A was out of service at the time, all of the AFW pumps were unavailable. The cause of the event was determined to be human error in tagging the valves and lack of independent verification of the tag locations and valve positions. The two manual valves were immediately locked open and AFW pumps B and C were returned to service within an hour.
B.3.3 Additional Event-Related Information Because of modifications to the A AFW pump and the common redundant steam supply piping, manual valves 001 B, 002B, 001 C, and 002C in the AFW pump turbine steam supply lines were closed on March 26, 1983.
Valves 001B and 002B are in series in the steam supply line from steam generator 3A to the AFW pump B turbine. Valves 001C and 002C supply the AFW pump C turbine. On April 11, 1983, valves 001 B and 001C were opened to perform a hydro test, and they were supposed to be reclosed on April 14. On April 19, valve 001B was found closed, but with its clearance tag removed. The tag was found on valve 3-084A, which was also closed. Valve 001C was found open and its tag was located on valve 3-086B, which was closed. Thus, steam supplies were isolated to all AFW pump turbines.
The plant has a standby steam generator feedwater (SSGFW) system consisting of two 100% capacity motor-driven pumps. This system is shared with Turkey Point 4. Although the SSGFW system is not safety related, it is powered from multiple onsite and offsite sources.
B.3.4 Modeling Assumptions It is assumed that valves 3-084A and 3-086B were mistakenly closed on April 14, 1983. With valves 001B, 002B, 001C, and 002C closed continuously since April 14, and Unit 4 in refueling, there was no steam available for AFW pumps B and C for at least five days or 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br />. AFW pump A was out of service during LER No. 250/83-007
B.3-2 this period so all three AFW pumps were unavailable. Therefore, all three trains of AFW were modeled as unavailable for five days. The errors which caused the steam supply valves to be incorrectly closed would have to have been restored locally. This restoration would be complicated by the mislocated tags. To reflect this situation the AFW nonrecovery probability was increased to 0.55.
The failure probability for the SSGFW system was estimated as described in the analysis of LER 251/92-007 in Precursors To Potential Severe Core Damage Accidents: 1992 A Status Report, NUREG/CR-4674, ORNL/NOAC-232, Vol. 18. This system requires one of the two pumps to operate and realignment of one valve to be successful. An operator failure rate of 0.01 was assumed. Since the SSGFW system is placed into service prior to attempting feed and bleed, the operator failure rate for initiating feed-and-bleed was increased to 0.2, consistent with the Turkey Point probabilistic risk assessment (PRA). The SSGFW system failure probability, 0.011, was calculated as:
(PMPA x PMPB) + VLVI + OPR
= (0.01 x 0.01) + 0.0004 + 0.01
= 0.011 This value was incorporated into the model by modifying the nonrecovery probability of the main feedwater (MFW). Transient, loss-of-offsite power (LOOP), loss-of-coolant accident (LOCA), and steam generator tube rupture (SGTR) were used as potential initiators in the unavailability analysis.
B.3.5 Analysis Results The increase in core damage probability over the duration of the event is 5.5 x 10"'. The base-case CDP (not shown in calculation) is 1.5 x 10', resulting in an estimated CCDP of 5.6 x 10'. The contributions of the postulated LOCA and SGTR initiators are negligible compared to those due to a transient or LOOP. The dominant core damage sequence, shown in Figure B.3.1, involves a transient, successful reactor trip, failure of AFW, failure of main feedwater, and failure of feed and bleed.
LER No. 250/83-007
B.3-3 w C')
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Figure B.3.1 Dominant core damage sequence for LER 250/83-007 LER No. 250/83-007
B.3-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 250/83-007 Event
Description:
Three AFW pumps unavailable due to tagging error Event Date: 4/14/83 - 4/19/83 Plant: Turkey Point 3 UNAVAILABILITY. DURATION= 120 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.1E-01 LOOP 1.4E-03 LOCA 1.6E-04 SGTR 2.OE-04 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 3.OE-05 LOOP 2.5E-05 LOCA 3.9E-08 SGTR 2.2E-07 Total 5.5E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
120 trans -rt AFW mfw feed.bleed CD 2.7E-05 6.OE-03 215 loop -rt(loop) -ep AFW -offsite.pwr.rec/-ep.and.afw feed.bleed CD 2.1E-05 9.3E-02
/loop 508 trans rt -prim.press.limited AFW/ATWS CD 3.OE-06 1.OE-01 239 loop -rt(loop) ep AFW/EP CD 1.9E-06 8.3E-02 219 loop -rt(loop) -ep AFW offsite.pwr.rec/-ep.and.afw feed.bleed CD 1.6E-06 9.3E-02
/loop 214 loop -rt(loop) -ep AFW -offsite.pwr.rec/-ep.and.afw -feed.bleed CD 6.7E-07 9.3E-02
/loop recov.sec.cool hpr 226 loop -rt(loop) ep -AFW/EP porv.chall/sbo -porv.reseat/ep seal CD C 3.OE-07 ) 6.8E-02
.loca offsite.pwr.rec/seal.loca
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
120 trans -rt AFW mfw feed.bleed CD 2.7E-05 6.OE-03 508 trans rt -prim.press.limited AFW/ATWS CD 3.OE-06 I.0E-01 214 loop -rt(loop) -ep AFW -offsite.pwr.rec/-ep.and.afw -feed.bleed CD 6.7E-07 9.3E-02
/loop recov.sec.cool hpr 215 loop -rt(loop) -ep AFW -offsite.pwr.rec/-ep.and.afw feed.bleed CD 2.1E-05 9.3E-02
/loop 219 loop -rt(loop) -ep AFW offsite.pwr.rec/-ep.and.afw feed.bleed CD 1.6E-06 9.3E-02 LER No. 250/83-007
B.3-5
/loop 226 loop -rt(loop) ep -AFW/EP porv.chall/sbo -porv.reseat/ep seal CD ( 3.OE-07 ) 6.8E-02
.loca offsite.pwr.rec/seal.loca 239 loop -rt(loop) ep AFW/EP CD 1.9E-06 8.3E-02
- non-recovery credit for edited case Note: For unavailabilities. 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.
SEQUENCE MODEL: c:\asp\models\tpoint82.cmp BRANCH MODEL: c:\asp\models\tpoint3.82 PROBABILITY FILE: c:\asp\models\pwr8283.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 9.5E-04 1.0E+00 loop 6.7E-05 1.7E-01 loca 2.4E-06 5.4E -01 sgtr 1.6E-06 1.OE+0O rt 2.8E-04 iDOE-01 rt(loop) O.OE+O0 1.OE+Q0 AFW 1.5E-03 > 1.OE+O0 4.5E-01 > 5.5E-01 Branch Model: 1.OF.3 Train 1 Cond Prob: 5.OE-02 > Unavailable Train 2 Cond Prob: IOE-01 > Unavailable Train 3 Cond Prob: 3.OE-01 > Unavailable AFW/ATWS 1.2E-02 > 1.OE+O0 1.OE+00 Branch Model: 2. OF.3 Train 1 Cond Prob: 5.OE-02 > Unavailable Train 2 Cond Prob: 1.0E-01 > Unavailable Train 3 Cond Prob: 3.OE-01 > Unavailable AFW/EP 1.5E-03 > 1.OE+O0 4.5E-01 > 5.5E-01 Branch Model: 1.OF.3 Train 1 Cond Prob: 5.OE-02 > Unavailable Train 2 Cond Prob: i1,E-OI > Unavailable Train 3 Cond Prob: 3.OE-O1 > Unavailable mfw 1.9E-01 1.1E-02 porv.chall 4.OE-02 1.QE+00 porv. chal 1/afw 1.OE+0O 1.OE+00 porv.chall/loop 1.0E-OI 1.OE+00 porv.chall/sbo 1.OE+00 1.OE+00 porv. reseat 2.OE-02 1.1E-02 porv. reseat/ep 2.OE-02 1.0E+00 srv.reseat(atws) I.OE-01 1.OE+00 hpi 7.5E-04 8.9E-01 feed.bleed 2.1E-02 1.OE+00 2.O0E -01 feed.bleed/loop 2.1E-02 1.OE+O0 1.OE-02 emrg .boration O.OE+O0 1.OE+00 1.QE-02 recov .sec. cool 2.OE-O1 1.OE+00 recov.sec.cool/offsite.pwr 3.4E-01 1.OE+00 rcs.cooldown 3.OE-03 1.OE+00 1.OE-03 rhr 2.2E-02 7.OE-02 1.OE-03 rhr.and.hpr 1.0E-03 1.OE+00 1.OE-03 hpr 4.OE-03 1.0E+00 1.OE-03 ep 2.9E-03 8.9E-01 seal.loca 2.6E-01 1.OE+00 LER No. 250/83-007
B.3-6 offsite.pwr.rec/-ep.and.-afw 2.4E-01 1.OE+O0 offsite.pwr.rec/-ep.and.afw 7.1E-02 1.OE÷O0 offsite.pwr.rec/seal.loca 6.2E-01 1.OE+O0 offsite.pwr.rec/-seal.loca 7.6E-02 1.OE+O0 sg.iso.and.rcs.cooldown 1.OE-02 1.OE-O1 rcscool.below.rhr 3.OE-03 I.OE+O0 3.OE-03 prim.press.limited 8.8E-03 l.OE*O0
- branch model file
- forced I - I LER No. 250/83-007