ML20135G972

Final ASP Analysis - Limerick 1 (LER 352-95-008)

ML20135G972
Person / Time
Site:	Limerick
Issue date:	05/14/2020
From:	Christopher Hunter NRC/RES/DRA/PRB
To:
Littlejohn J (301) 415-0428
References
LER 1995-008-00
Download: ML20135G972 (17)

Text

Annendix B LER No. 352/95-008 B.7ndx LER No. 352/95-008 Event

Description:

Safety/relief valve fails open, reactor scram, suppression pool strainer fails Date of Event: September 11, 1995 Plant: Limerick 1 B.7.1 Event Summary Limerick Unit 1 was manually scrammed from 100% power after a safety/relief valve (SRV) failed open.

Residual beat removal (RI-R) pump A was in the suppression pool cooling (SPC) mode of operation and was being used to remove beat from the suppression pool to compensate for various SRV steam leaks when an SRV failed open, forcing the manual scram. RHR pump A was secured and declared inoperable after oscillations in the pumip motor current and decreasing pump flow were observed. Subsequent examination revealed that the pump suction strainer bad become obstructed with debris from the suppression pool. The conditional core damage probability (CCDP) estimate for the one-year potential unavailability of the Emergency Core Cooling Systems (ECCS) dependent upon the suppression pool is 1.3 x 10'. This is an increase of 9.0 x 10 over the nominal core damage probability (CDP) of 4.0 x 10' for the same period. The CCDP for the actual transient event is 2.5 x 10'.

B.7.2 Event Description Limerick 1was operating at 100% power at 1245 on September 11, 1995, when SRV "M" failed open. When plant operators were unable to reclose the valve within 2 min, they manually scrammed the reactor in accordance with technical specification requirements. At the time of the SRV failure, RHR pump A was in service to remove heat from the suppression pool to compensate for various SRV steam leaks.

After the scram, operators aligned RHR B pump for SPC as well. At 1307, the pressure in the reactor had decreased from 6.930 to 2.83 MPa (1005 psig to 410 psig). Even though a closed indication was received for the "M" SRV, reactor pressure continued to decrease. Typically, Technical Specifications for boiling water reactors (BWRs) require a controlled depressurization if the temperature in the suppression pool exceeds 49'C (120' 0 F). In such a case, the cooldown rate is typically limited to less than 380 C/h (lOOTF/h).

During this event, however, the uncontrolled depressurization resulted in a cooldown rate of approximately 54 0 C/h (130'F/h) and the temperature in the suppression pool peaked at 51 OC (124 0 F).

At 1320, operators observed a decrease and fluctuations in flow from the A RHR pump as well as oscillations in its motor current. Operators, attributing these signs to suction strainer fouling, secured the A RI{R pump and declared it inoperable. After it was checked, the A pump was restarted but at a reduced flow rate of 7570 1/in (2000 gpm). No problems were observed so the flow rate was gradually increased to 32,170 l/m (8500 gpm) and no problems were observed. A pressure gauge located on the pump suction was observed to have a gradually lower reading, which was believed to be indicative of an increased pressure drop across the pump suction strainers located in the suppression pool (Ref. 2). At 0227 on September 12, 1995, reactor B.7-1 NUREG/CR4674, Vol. 23

LER No. 352/95-008 Appendix B pressure was reduced below 0.52 MPa (75 psig), with one loop of shutdown cooling in service. By 0430, the unit was in cold shutdown with a reactor coolant temperature of 9O0 C (1 94 0F).

B.7.3 Additional Event-Related Information SRV "M" was removed and sent to a laboratory for testing, where it was found to have been damaged by steam erosion of the pilot valve seat. Failure of the pilot valve caused a pressure differential across the SRV main disk, which resulted in spurious operation of the main SRV valve. The SRV was reported to have been leaking for more than a year before its failure. Four other SRVs were found to have seat damage and were also replaced.

During an inspection of the A RHR pump suction strainer assembly, a mat of brown, fibrous material and a sludge of oxide corrosion products were found covering most of the assembly. The sludge material was determined to have come from the suppression pool. Upon inspection, personnel discovered that most of the suction strainer assembly for the B RHR pump was covered with a thinner layer of the same material.

However, the B RHR pump ran normally during and after the event. The other strainers in the suppression pool for the pumps which were not employed during this event also had minor sludge accumulations.

Limerick concluded that the blowdown caused by the SRV opening did not significantly increase the rate of debris accumulation on the strainer. Approximately 635 kg (1,400 Ib) of debris (wet weight, dry weight would be less) were removed from the suppression pool. A similar amount of material had been removed previously from the Unit 2 suppression pool.

B.7.4 Modeling Assumptions Two assessments were required to analyze this event. First, a transient event assessment was performed to analyze the actual event. Second, a condition assessment was performed because of the prolonged potential unavailability of those ECCS systems which are dependent on the suppression pool.

Transient event assessment This event was modeled as a scram with one SRV failed open and one train of RHR unavailable in all modes except SDC because train A of RHR was declared inoperable and secured when debris from the suppression pool clogged its suction strainer. Similar debris was found on other strainers, and 635 kg (1,400 lb) of debris (wet weight) were later removed from the suppression pool. Reference 4 indicates that, under some circumstances, debris could have migrated and caused obstruction of additional pump strainers. This effect could depend on a number of factors, including the amount of suppression pool agitation caused by shock waves from SRV discharge; the amount of debris in the suppression pool; which specific pumps were placed in service; what flow rates were demanded; how long the pumps were operated, etc.

The potential for common-cause failure of all strainers was modeled by adding an additional basic event to the model for each appropriate system. The event "RHRSTRAINERS" was added to the suppression pool cooling models (SPC, SPC/L), the low pressure coolant injection models (LC1IL), and the containment spray system models (CSSIL). In addition, this event was added to the low pressure core spray system models (LCS/L),as core spray is also dependent upon the suppression pool for water. No change was made to the NUREG/CR4674, Vol. 23 B.7-2

Appendix B Appenix BLER No. 352/95-008 high pressure coolant injection (HPCI) or reactor core isolation cooling (RCIC) system models, which may take suction from the suppression pooi, because these systems also are provided with an alternate water supply from the condensate storage system.

The CCDP calculated for this event is dependent upon assumptions made regarding the likelihood that the foreign matter in the suppression pool could cause failure of additional ECGS pumps. Research cited in Reference 4 indicates that the debris concentrations present in the Limerick suppression pool 635 kg sludge/3,780 m3' suppression pool water volume x 1,008 kg/in 3 = 0.02% wt %sludge) were easily sufficient to obstruct multiple ECCS system strainers. Based on Reference 5, a common-cause strainer failure probability of 0.135 was used in the analysis because the A train of RHR had operated over an extended period for SPC, during which time the strainer was believed to have clogged gradually. A sensitivity analysis also was performed, assuming a common-cause strainer failure probability of 1.0.

Condition assessment In addition to the analysis of the reported transient event, an analysis was made of the prolonged potential unavailability of the EGCS systems that are dependent upon the suppression pool for water. The debris in the suppression pooi was assumed to have been present throughout the operating year (6,132 h, assuming a 70% availability), and it was assumed to have the potential to cause failure of LCI, LCIIL, LCS/L, SPCIL, and CSS/L. This event was modeled with one train of RHR unavailable because, during an actual demand, train A of RHR was declared inoperable and secured when debris from the suppression pool clogged its suction strainer. A common-cause strainer failure probability of 0.135 was used in this analysis (RHRSTRAINERS), and a sensitivity case was evaluated for a common-cause failure probability of 1.0.

Potential recovery of the power conversion system (PCS) was credited with event PGS-LONG, as it was in the transient assessment.

B.7.5 Analysis Results The CCDP estimate for the one-year potential unavailability of ECCS systems dependent upon the suppression pool is 1.3 x 10'~. This is an increase of 9.0 x 10 'over the nominal CDP for the same period of 4. 0 x 10~. The CCDP for the actual transient event is 2.5 x 10' In both cases, the dominant sequence, highlighted as sequence number 4 on the event tree in Fig.B.7. 1, involves

• the reactor successfully scrams,

" the PCS initially fails,

" RHR system fails,

• personnel fail to recover PCS in the long term, and

• containment venting fails.

Sequence number 4 is still the dominant sequence if a common-cause strainer failure probability of 1.0 is assumed (versus the 0.135 probability used for the actual event analysis). A CCDP of 7.1 x 10 with an importance of 6.7 x 10' is estimated for the long-term unavailability of the EGGS. The importance increased 7 times for this sensitivity analysis (from 9.0 x 10-6to 6.7 x 10-5). The GCDP for the sensitivity analysis for NUREG/CR-4674, Vol. 23 B.7-3 NUREG/CR-4674, Vol. 23

LER No. 352/95-008 AmDendix ADDendix B B LER No. 352/95-008 the transient event is 1.4 x 10 -', or an increase of about 6 times over the CCDP for the actual transient event of 2.5 x 1' It should be noted that main feedwater success coincident with PCS failure is possible in the Limerick model because some failures that render the PCS incapable of functioning as a sink for reactor decay heat do not render it incapable of supporting main feedwater (e.g., turbine trips or load rejections with failures of the turbine bypass valves).

Definitions and probabilities for selected basic events are shown in Table B.7. 1. Table B.7.2 describes the system names associated with the dominant sequences for both the condition assessment and the initiating event assessment. The conditional probabilities associated with the highest probability sequences for the condition assessment are shown in Table B.7.3. Table B.7.4 lists the sequence logic associated with the sequences listed in Table B.7.3. Minimal cut sets associated with the dominant sequences for the condition assessment are shown in Table B.7.5. The conditional probabilities associated with the highest probability sequences for the initiating event assessment are shown in Table B.7.6. Table B.7.7 lists the sequence logic associated with the sequences in Table B.7.6. Minimal cut sets associated with the dominant sequences for the initiating event assessment are shown in Table B.7.8.

B.7.6 References

1. LER 352/95-008 from PECO Energy to U.S. Nuclear Regulatory Commission, "Unusual Event and RPS Actuation When the Reactor was Manually Shutdown Due to the Inadvertent Opening of a Main Safety Relief Valve Caused by Pilot Valve Seat Leakage," October 10, 1995.

2. NRC Bulletin 95-02, "Unexpected Clogging of a Residual Heat Removal (RHR) Pump Strainer VWhle Operating in a Suppression Pool cooling Mode," U.S. Nuclear Regulatory Commission, October 17, 1995.

3. NRC Information Notice 95-47, "Unexpected Opening of a Safety/Relief Valve and Complications Involving Suppression Pool Cooling Strainer Blockage," U.S. Nuclear Regulatory Commission, October 4, 1995.

4. Zigler, et al., ParametricStudy of the Potentialfor BWR ECCS Strainer Blockage Due to LOCA GeneratedDebris, NUREG/CR-6224, Science and Engineering Associates for U.S. Nuclear Regulatory Commission, 1995.

5. Common-Cause Failure Data Collection and Analysis System, Vol. 6, "Common-Cause Failure Parameter Estimations," INEL-94/0064, Marshall and Rasmuson,, Idaho National Engineering Laboratory for U.S. Nuclear Regulatory Commission, 1995.

B.7-4 NUREGICR-4674, Vol.

NUREGICR-4674, Vol. 23 23 B.74

Annendix B Ffn 352/954I008 AD~~endLEB Fig. B.7. 1. Dominant core damage sequence for the initiating event assessment and the condition assessment for LER No. 352/95-008.

B.7-5 NLTREG/CR-4674, Vol. 23

LER No. 352/95-008 Appendix B Table B.7.1. Definitions and Probabilities for Selected Basic Events for LER No. 352/95-008 Modified Event Base Current for this name Description probability probability Type event IE-TRAN Transient Initiator 4.5 E-004 1.0 E+000 Yes' ADS-SRV-CC-VALVS Automatic Depressurization 3.7 E-003 3.7 E-003 No System (ADS) Valves Fail to Open ADS-XHE-XE-ERROR Operator Error Prevents 1.0 E-002 1.0 E-002 No Depressurization ADS-XHE-XE-NOREC Operator Fails to Recover ADS 7.1 E-00 1 7.1 E-00 1 No ADI-XJ-E-XE-ERROR Operator Fails to Inhibit ADS 1.0 E-002 1.0 E-002 No and Control Level CDS-SYS-VF-COND Condensate Hardware 3.4 E-O001 3.4 E-001I No Components Fail CDS-XHE-XE-NOREC Operator Fails to Recover 1.0 E+000 1.0 E+000 No Condensate CVS-XHE-XE-VENT Operator Fails to Vent 1.0 E-002 1.0 E-002 No Containment EPS-DGN-FC-DGC Diesel Generator Failure 1.9 E-002 1.9 E-002 No EPS-XH-E-XE-NOREC Operators Fail to Recover 5.0 E-00l1 5.0 E-001I No Electric Power System HCI-TDP-FC-TRAIN HPCI Train Level Failures 8.6 E-002 8.6 E-002 No HCI-XHE-XE-NOREC Operator Fails to Recover HPCI 7.0 E-001 7.0 E-001I No LCI-MOV-CC-LOOPB Low Pressure Coolant Injection 3.1 E-003 3.1 E-003 No (LPCI) Train B Injection Valves Fail to Open_______ _______

LCI-XHE-XE-NOREC Operator Fails to Recover LPCI 1.0 E+000 1.0 E+000 No LCS-XHE-XE-NOREC Operator Fails to Recover Low 1.0 E+000 1.0 E+000 No Pressure Core Spray System MFW-SYS-VF-FEEDW Main Feedwater System (MFW) 4.6 E-00 1 4.6 E-001 No Hardware Components Fail MFW-XH-E-XCE-NOREC Operators Fail to Recover MFW 3.4 E-00 1 3.4 E-001I No PCS-LONG Operators Fail to Recover the 3.9 E-00 1 3.9 E-00 I NEW No PCS in the Long Term PCS-SYS-VF-MISC PCS Hardware Components Fail 1.7 E-00 1 1.7 E-00 1 No rPCS-XHEXE-NOREC Operator Fails to Recover PCS 1 OE+000 1.0 E+000 No NUREG/CR-4674, Vol. 23 B.7-6

AnDendix B LER No. 352/95-008 Table B.7.1. Definitions and Probabilities for Selected Basic Events for LER No. 352/95-008 Modified Event Base Current for this name Description probability probability Type event PPR-SRV 1VLV One or Less SRVs Fail to Close 1.0 E-*000 1.0 E+000 TRUE Yee PPR-SRV-OO-2VLVS Two SRVs Fail to Close 2.0 E-003 0.0 E+000 FALSE Yes6 PPR-SRV-OO-3VLVS More Than Two SRVs Fail to 2.0 E-004 0.0 E+000 FALSE Yesb Close RCI-TDP-FC-TRAIN RCIC Train Component Failures 8.3 E-002 8.3 E-002 No RCI-XHE-XE-NOREC Operator Fails to Recover RCIC 7.0 E-00 1 7.0 E-001I No RI-R-MDP-CF-MDPS Common-Cause Failure of RHR 3.0 E-004 3.0 E-004 No Pumps RHR-MDP-FC-TRN'A RHR Train A Components Fail 3.8 E-003 1.0 E+000 TRUE Yes' RHR-MOV-00-BYPSB RHR Loop B Valve to Bypass 3.0 E-003 3.0 E-003 No Heat Excbangers Fails_____

RHRSTRAINERS Common-Cause Failure of All 0.0 E+000 1.4 E-001 NEW Yese Strainers RPS-NONREC Nonrecoverable Reactor 2.0 E-005 2.0 E-005 No Protection System (RPS) Trip System Failures RPS-SYS-FC-MECH Mechanical Failures of the RPS 1.0 E-005 1.0 E-005 No RRS-XHE-XE-ERROR Operator Fails to Trip the 1.0 E-002 1.0 E-002 No Recirculation Pumps SDC-MOV-CC-SUCT Shutdown Cooling System 6.0 E-003 6.0 E-003 No (SDC) Suction Valves Fail to Open____ _

SDC-XH-E-XE-ERROR Operator Fails to Align/Actuate 1.0 E-002 1.0 E-002 No the SDC SDC-XH-E-XE-NOREC Operator Fails to Recover the 1.0 E+000 1.0 E+000 No SDC SLC-CKV-CC-INJEC The Injection Check Valves in 2.0 E-004 2.0 E-004 No the Standby Liquid Control System (SLC) Fail______

SLC-EPV-CF-VALVS The Explosive Valves in the 2.6 E-004 2.6 E-004 No SLC Fail From Common Cause NUREG/CR-4674, Vol. 23 B.7-7 NUREG/CR-4674, Vol. 23

ADDendix B LER No. 352/95-008 Amni Table B.7.1. Definitions and Probabilities for Selected Basic Events for LER No. 352/95-008 Modified Event Base Current for this name Description probability probability Type event SLC-MDP-CF-MDPS The Motor-Driven Pumps in the 6.3 E-004 6.3 E-004 No SDC Fail From Common Cause SLC-XHE-XE-ERJIOR Operator Fails to Start/Control 1.0 E-002 1.0 E-002 No the SDC SLC-XHE-XE-NOREC Operator Fails to Recover SDC 1.0 E+000 1.0 E+000 No SRV One or Less SRVs Fail to Close 2.2 E-003 2.2 E-003 No SSW-MOV-CC-FLOOD Valve Fails to Open 6.1 E-003 6.1 E-003 No SSW-XHE-XE-ERROR Operator Fails to Align RHR 1.0 E-002 1.0 E-002 No Service Water SSW-XHE-XE-NOREC Operator Fails to Recover RHR 1.0 E+000 1.0 E+000 No Service Water

'Applicable to the initiating event assessment only.

bThe probability was set to 0.0 E+000 (FALSE) for the initiating event assessment. For the conditional event assessment, the base probability was not changed in the model.

'The base probability was changed for both the initiating event assessment and the conditional event assessment.

Vol. 23 B.7-8 NUREG/CR-4674, Vol. 23 B.7-8

AnDendix B LER No. 352/95-008 LER No. 352/95-008 Anoendix B Table B.7.2. System Names for LER No. 352/95-008 System name Logic AD 1 Failure to Inhibit ADS and Control Reactor Level ADS Automatic Depressurization Fails CDs Failure of the Condensate System CVS Containment (Suppression Pool) Venting EPS Emergency Power System Fails HCI HPCI Fails to Provide Sufficient Flow to the Reactor Vessel LCI LPCI Fails LCIL LPCI Fails During a LOOP LCS Low Pressure Core Spray (LPCS) Fails LCSL LPCS Fails During a LOOP MFW Failure of the MFW System P2 Two SRVs Fail to Close PCs PCS Fails RCI RCIC Fails to Provide Sufficient Flow to RCS RHRL RHR System Fails During a LOOP RHRPCS RHR System Fails RP 1 Reactor Shutdown Fails RPS RPS Fails RRS Recirculation Pump Trip SLC SLC System Fails SRV One or Less SRVs Fail to Close SSW RHR Service Water Makeup Fails SSWL RHR Service Water Makeup Fails During a LOOP B.7-9 B.7-9NUREG/CR-4674, Vol. 23

LER No. 352/95-008 ADDendix AnDendix B B LER No. 352/95-008 Table B.7.3. Sequence Conditional Probabilities for the Condition Assessment for LER No. 352/95-008 Conditional Event tree core Core damage Importance Percent name Sequence damage probability (CCDP-CDP) contribution" name probability (CDP)

(CCDP) _______________ __

TRANS 04 4.6 E-006 5.7 E-007 4.0 E-006 44.6 LOOP 03 1.9 E-006 2.4 E-007 1.7 E-006 19.1 TRANS 44 8.3 E-007 0.0 E+000 8.3 E-007 9.2 LOOP 20 7.2 E-007 0.0 E+000 7.2 E-007 7.9 TRANS 07 7.1 E-007 8.7 E-008 6.2 E-007 6.9 LOOP 34 4.1 E-007 0.0 E+000 4.1 E-007 4.5 TRANS 27 2.2 E-007 0.0 E+000 2.2 E-007 2.5 LOOP 06 1.1 E-007 1.4 E-008 1.0 E-007 1.1 Condition Assessment 1.3 E-005 4.0 E-006 9.0 E-006 ...

I Total (all sequences) ________________

8Percent contribution to the total imvnortance.

NUREG/CR-4674, Vol. 23 B.7-10

Appendix B Appenix No.

BLER352/95-008 Table B.7.4. Sequence Logic for Dominant Sequences for the Condition Assessment for LER 352/95-008 Event tree name Sequence name Logic TRANS 04 IRPS, PCS, /SRV, IMFW, RHRPCS, CVS LOOP 03 /RP1I, /EPS, /SRV, /HCI, RHRL, CVS TRANS 44 /RPS, PCS, P2, IEICI, CDS,

___________LCS, LCI, SSW LOOP 20 /RP 1, IEPS, /SRV, HCI, RCI,

___________/ADS, LCSL, LCI/L, SSWL TRANS 07 /RPS, PCS, /SRV, MFW,

______________________/HCI, RHRPCS, CVS LOOP 34 /RP 1, IEPS, P2, IHCI, LCSL,

____ ____ ____ ___ LCIL, SSWL TRANS 27 /R.PS, PCS, /SRV, MFW, HCI, RCI, /ADS, CDS, LCS, LCI, SSW LOOP 06 /RP1I, IEPS, /SRV, HCI, /RCI,

____ ____ ___ RHRL, CVS B.7-11 NUREG/CR-4674, Vol. 23 NLTREG/CR-4674, Vol. 23

LER No. 352/95-008 ADnendix B Table B.7.5. Conditional Cut Sets for Higher Probability Sequences for the Condition Assessment for LER No. 352/95-008 Cut set Percent Conditional no. contribution probability' Cut sets TRANS Sequence 04 4.6 E-00....6..........

1 54.5 2.5 E-006 PCS-LONG. PCS-SYS-VF-MISC, PCS-XHiE-XCE-NOREC, /SRV, SDC-XHE-XE-ERROR, RI-RSTRAINERS, CVS-XHiE-XE-VENT 2 32.4 1.5 E-006 PCS-LONG, PCS-SYS-VF-MISC. PCS-XHE-XE-NOREC, /SRV, RHRSTRAINERS, SDC-MOV-CC-SUCT, SDC-XHE..XE-NOREC, CVS-XHE-XE-VENT 3 12.0 5.6 E-007 PCS-LONG, PCS-SYS-VF-MISC. PCS-XHE-XE-NOREC, /SRV, RHR-MDP-CF-MDPS, SDC-XHE-XE-NOREC, CVS-XHiE-XE-VENT LOOP Sequence 03 1.9 E-006. ..

3OO 1eu nc 31.9 2. E-0067 ISV JI RM PC-D S IIS....C..................OR........

CVS-XHiE-XE-VENT 4 1.1 2.3 E-008 /SRV, RHR-MOV-OO-BYPSB, EPS-DGN-FC-DGC, EPS-XHE-XE-NOREC. SDC-XHE-XE-NOREC, CVS-XHE-XE-VENT TRANS Sequence 44 8.3 E-007 1 52.1 4.3 E-007 PCS-SYS-VF-MISC, PCS-XHiE-XE-NOREC, PPR-SRV-OO-2VLVS, CDS-SYS-VF-COND, CDS-XHiE-XE.NOREC, RI-RSTRAINERS, LCS-XHiE-XE-NOREC, LCI-XHE-XE-NOREC, SSW-XHE-XE-ERROR, 2 31.8 2.6 E-007 PCS-SYS-VF-MISC, PCS-XHiE-XE-NOREC, PPR-SRV-OO-2VLVS, CDS-SYS-VF-COND, CDS-XHiE-XE-NOREC, RI-RSTRAINERS, LCS-XHiE-XE-NOREC. LCI-XHiE-XE-NOREC. SSW.MOV-CC-FLOOD, SSW-XHiE-XE-NOREC

-3 16.1 -1.3 E-007 PCS-SYS-VF-MISC, PCS-XHE-XCE-NOREC, PPR-SRV-OO-2VLVS, CDS-SYS-VF-COND, CDS-XHiE-XE-NOREC. RHRSTRAINERS, LCS-XHE-.XE-NOREC, LCI.MOV.CC-LOOPB. LCI-XHE-XCE-NOREC, SSW-XHiE-XE-NOREC LOOP Sequence 20 7.2 E-007 .......................................

1 51.6 3.7 E-007 /SRV, I-CI-TDP-FC-TRAIN. HCI-XHiE-XE-NOREC,

__ __ _ ___RCI-TDP-FC-TRAIN. RCI-XHiE-XE-NOREC, RHRSTRAINERS, LCS-XHiE-XE-NOREC, LCI-XHE-XE-NOREC, SSW-XHE-XE-ERROR B.7-12 NUREG/CR-4674, Vol. Vol. 23 23 B.7-12

Annendix B LER No. 352/95-008 Table B.7.5. Conditional Cut Sets for Higher Probability Sequences for the Condition Assessment for LER No. 352/95-008 Cut set Percent Conditional no. contribution probability' Cut sets 2 31.5 2.3 E-007 /SRV. HCI-TDP-FC-TRAIN. I-CI-XHE-XE-NOREC, RCI-TDP-FC-TRAIN, RCI.XHiE-XE-NOREC, RI-RSTRAINERS, LCS-XHiE-XE-NOREC, LCI-XHE-XE.NOREC, SSW.MOV-CC-FLOOD, SSW-XHiE-XE-NOREC 3 16.0 1.2 E-007 /SRV, HCI-TDP-FC-TRAIN, HCI-XHiE-XE-NOREC, RCI-TDP-FC-TRAIN. RCI-XHE-XCE-NOREC, RHRSTRAINERS, LCS-XHE-XE-NOREC, LCI-MOV-CC-LOOPB, LCI-XHE-XCE-NOREC.

SSW-XHiE-XE-NOREC TRN Sequence 07 7. 1E-00~I7........... . . . . . . .

1 54.9 3.9 E-007 PCS-LONG, PCS-SYS-VF.MISC, PCS-.XHE-XE-NOREC, /SRV, MFW-SYS-VF-FEEDW, MF'W-XiE-XE.NOREC. RHRSTRAINERS.

SDC-.XHE-XE-ERROR, CVS-XHE-IXE-VENT 2 32.9 2.3 E-007 PCS-LONG, PCS-SYS-VF-MISC. PCS-XHE-XCE.NOREC. /SRV.

MFW-SYS-VF-FEEDW, MFW-XHE-XE-NOREC, RHRSTRAINERS,

______SDC-MOV-CC-SUCT, SDC.XHiE-XE.NOREC, CVS-XHiE-XE-VENT 3 12.2 8.7 E-008 PCs-.LONG, PCS-SYS-VF-MISC, PCS-XHiE-XE-NOREC, /SRV, MFW-SYS-VF-FEEDW, MFW-XHE-.XE-NOREC. RHR-MDP-CF-MDPS, SDC-XHE-XE-NOREC. CVS-.XHE-XE-VENT LOOP Sequence 34 4.1 E-007 1 52.1 2.1 E-007 PPR-SRV..OO-2VLVS, RHRSTRAINERS, LCS-XHE-XE-NOREC, LCI-XHiE-XE.NOREC, SSW-XHiE-XE-ERROR 2 31.8 1.3 E-007 PPR-SRV-OO-2VLVS, RHRSTRAINERS, LCS-XHiE-XE-NOREC, LCI.XHiE-XE-NOREC, SSW.MOV-CC-FLOOD, SSW-XHE-XE.NOREC 3 16.1 6.6 E-009 PPR-SRV-OO-2VLVS, RHRSTRAINERS, LCS-.XHE-XE.NOREC, LCI.MOV-CC.LOOPB, LCI-XHiE-XE.NOREC, SSW-XHE-XE.NOREC TRAN Sequence 27 2.3 E-007 1 52.1 1.2 E-007 PCS-SYS-VF-MISC, PCS-XHiE-XE.NOREC, /SRV.

MFW-SYS-VF-FEEDW, MFW-XHiE-XE-NOREC. HCI-TDP-FC-TRAIN.

HCI-XHiE-XE.NOREC, RCI.TDP-FC-TRAIN, RCI-XHE-XCE-NOREC.

CDS-SYS-VF-COND, CDS-.XHE-XE-NOREC, RHRSTRAINERS,

________________________LCS-XHiE-XE-NOREC, LCI-XHiE-XE-NOREC. SSW-XHE-XE-ERROR B.7-13 B.7-13NUREG/CR-4674, Vol. 23

ADnendix B LER No. 352/95-008AtenxB Table B.7.5. Conditional Cut Sets for Higher Probability Sequences for the Condition Assessment for LER No. 352/95-008 Cut set Percent Conditional no. contribution probabilitys Cut sets 2 31.8 7.3 E-008 PCS-SYS-VF-MISC, PCS-XH-E-XE-NOREC, /SRV, MFW-SYS-VF-FEEDW, MFW-XHiE-XE-NOREC. HCI-TDP-FC-TRAIN, HCl-XH-E-XE-NQREC. RCI.TDP-FC-TRAIN, RCI-XHE-XCE-NOREC, CDS-SYS-VF-COND. CDS-XHiE-XE-NOREC, RH-RSTRAINERS, LCS-XHE-XE-NOREC, LCI-XHE-XE-NOREC, SSW-MOV-CC-FLOOD, SSW-XHE-XE-NOREC 3 16.1 3.7 E-008 PCs-SYs-VF-MIsC, PCS-XH-E-XE.NOREC, /SRV, MFW-SYS-VF-FEEDW. MFW-XHE-IXE.NOREC, HCI-TDP-FC-TRAIN, HCI-XHE-XE-NOREC, RCI-TDP-FC-TRAIN, RCI-XHE-XE-NOREC, CDS-SYS-VF-COND, CDS-XHiE-XE-NOREC, RHRSTRAINERS, LCS-XHE-XE-NQREC, LCI-MOV-CC-LOOPB, LCI-XHE-XE-NOREC, SSW-XH-E-XE-NOREC LOOP Seqence.06...1.....

1 54.9.0 E-00 /SRV, CI-TDP-C-TRAI% H IX E E-O C...............

S ICV 2 32.9 3.6 SDC M O -C ..S.... .........XlIH ..

I- H E NXEN CS-...-..-...

R C...............

3 12.2 1.3 E-008 /SRV, HCI-TDP-FC-TRAIN, HCl-XH-E..XE-NOREC, RHR-MDP-CF-MDPS. SDC-.XHE..XE-NOREC. CVS-XHiE-XE-VENT Condition Assessment j1.3 E-005 Total__(all__sequences) I________________

a The conditional probability for each cut set is determined by multiplying the probability that the portion of the sequence that makes the precursor visible (e.g., the system with a failure is demanded) will occur during the duration of the event by the probabilities of the remaining basic events in the minimal cut set. This can be approximated by 1 - e", where p is determined by multiplying the expected number of initiators that occur during the duration of the event by the probabilities of the basic events in that minimal cut set. The expected number of initiators is given by At, where A is the frequency of the initiating event (given on a per-hour basis), and t is the duration time of the event (in this case, 6132 h). This approximation is conservative for precursors made visible by the initiating event.

The frequencies of interest for this event are: X,.= 4.57 x lOA/h, and X,,. = 1.29 x 10-'/h.

B.7-14 NUIREG/CR-4674, Vol.

NUREG/CR-4674, Vol. 2323 B.7-14

ADDendix B LER No. 352/95-008 Table B.7.6. Sequence Conditional Probabilities for the Initiating Event Assessment for LER No. 352/95-008 Conditional core Event tree damage Percent name Sequence name probability contribution (CCDP) ___ __

TRANS 04 1.6 E-006 65.3 TRANS 07 2.6 E-007 10.2 TRANS 80-15 2.2 E-007 8.7 TRANS 80-16 2.0 E-007 8.0 TRANS 27 8.2 E-008 3.2 TRANS 80-14 3.4 E-008 1.3 IE Assessment 2.5 E-006 Total (all sequences)

Table B.7.7. Sequence Logic for Dominant Sequences for the Initiating Event Assessment for LER 352/95-008 Event tree name Sequence name Logic TRANS 04 /RPS, PCS, /SRV, /MFW, RIIRPCS, cvS TRANS 07 /RPS, PCS, /SRV, MFW, /HCI,

___________RHRPCS, CVS TRANS 80-15 RPS, JRRS, SLC TRANS 80-16 RPS, RRS, TRANS 27 IRPS, PCS, /SRV, MEW, HCI, RCI,

___________ __________/ADS, CDS, LCS, LCI, SSW TRANS 80-14 RPS, IRRS, /SLC, PCS, AD I NUREG/CR-4674, Vol. 23 B.7-15 NUREG/CR4674, Vol. 23

LER No. 352/95-008 Appendix B Appendix B LER No. 352/95-008 Table B.7.8. Conditional Cut Sets for Higher Probability Sequences for the Initiating Event Assessment for LER No. 352/95-008 Cut set Percent Conditional no. contribution probability' Cut sets TRANS Sequence 04 1.6 E-006.....

1 53.7 8.9 E-007 PCS-LONG. PCS-SYS-VF-MISC, PCS-XHE-XE-NQREC. /SRV.

SDC-XHE-XE-ERROR, RHRSTRAINERS, CVS-XHE-XE-VENT 2 32.2 5.3 E-007 PCS-LONG, PCS-SYS-VF-MISC, PCS-XHE-XE-NOREC, /SRV, RHRSTRAINERS. SDC-MQV-CC-SUCT, SDC-XHE-XE-NOREC, CVS-XHE-XE-VENT F 3 11.9 1.9 E-007 PCS-LONG. PCS-SYS-VF-MISC, PCS-XHE-XE-NOREC. /SRV.

RHR-MDP-CF-MDPS, SDC-XHE-XCE-NOREC, CVS-XHE-XE-VENT TR A N S. Sequence.0.2.6.....

M..........-

YSV -F E W NO EC ................

TRANS Sequence8071 2.2 E-007 ...".::................:::..

2 53.6 1.2 E-007 RPS-NONRE C.SL-MDP-CF-MDPS, PSLCH-XHE-XENQREC,S 3 32.3 5.2 E-008 RPS-NONRE,PSL-EPSV-F-VALVS, PSLC-XE-XE-NOREC,SV 4 17 40 -00 RSD-NONC.SCKV-CC- S INJEC, LC-XHE-XE-NORECCSai-EVN TRN Seqenc 3016

1. E-0087 C-OG C-Y-FMSPSXE)ENRC SV 1 97.6 2.0 -007 RPS-NO REC, RRSXH-XaE-ERROR N Vol. 23 B.7-16 NLTREG/CR-4674, NUREG/CR-4674, Vol. 23 B.7-16

ADDendix B LER No.

LER No. 352/95-008 352/95-008 Appendix B Table B.7.8. Conditional Cut Sets for Higher Probability Sequences for the Initiating Event Assessment for LER No. 352/95-008 Cut set Percent Conditional no. contribution probability' Cut sets TRANS Sequence 27 8.2 E-008........

1 51.7 4.2 E-008 PCS-SYS-VIF-MISC. PCS-XHiE-XE-NOREC, /SRV, MFW-SYS-VF-FEEDW, MFW-.XHE-XE-NOREC, HCl-TDP-FC-TRAIN, HCI-XH-E-XE-NOREC, RCI-TDP-FC-TRAFN, RCI-XHE-XE-NOREC, CDS-SYS-VF-COND, CDS.XH-E-XE-NOREC, RH1-RSTRAINERS, LCS-XHE-XE-NOREC. LCI-XH-E..XE-NOREC, SSW-XHE-XE-ERROR 2 31.5 2.6 E-008 PCS-SYS-VF-MISC, PCS-XHE-XCE-NOREC, /SRV, MFW-SYS-VF-FEEDW, MFW-XHE-XE-NOREC, HCI-TDP-FC-TRA1N, HCI-XHE-XE-NOREC, RCI-TDP-FC-TRAIN, RCI-XHE-XCE-NOREC.

CDS-SYS-VF-COND. CDS-XHE-XE-NOREC. RHRSTRAINERS, LCS-XHE-XE-NOREC, LCI-XHE-XE-NOREC, SSW.MOV-CC-FLOOD, SSW-XHE-XE-NQREC 3 16.0 1.3 E-008 PCS-SYS-VF-MISC, PCS-XHE-XE-NOREC, /SRV.

MFW-SYS-VF-FEEDW, MFW-XH~E-XE-NOREC, HCI-TDP-FC-TRA1N, HCI-XHE-XE-NOREC, RCI-TDP-FC-TRAIN, RCI-XHE-XE-NOREC, CDS-SYS-VF-COND, CDS-XHE-XE-NOREC. RH-RSTRAJNERS, LCS-XHE-~XE-NOREC, LCI.MOV-CC-LOOPB, LCI-XHE-XE-NOREC, SSW-XHEF-XF-NORF.C TRANS Sequence 80-14 3.4 E-008 1 1 99.5 3.4 E-008 IRPS-NONREC.

IAfl1 PCS-SYS-VF-MISC.

)f-YMF.-YF--FRRO)R PCS-XHE..XE-NOREC, IE Assessment 2.5 E-006 Total_(all sequences) _______

a The conditional probability for each cut set is determined by multiplying the probability of the initiating event by tbe probabilities of the basic events in that minimal cut set. The probability of the initiating events are given in Table B.7. 1 and begin with the designator "IE."

The probabilities for the basic events also are given in Table B.7. 1.

NIJREG/CR-4674, Vol. 23 B.7-177 B.7-1 NUREG/CR-4674, Vol. 23