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Regarding Concurrent Loss of Station Auxiliary Transformer 1X-04 and 480V Safeguards Bus 1B-04
	ML091480413
Person / Time
Site:	Point Beach
Issue date:	06/19/2009
From:	; Florida Power & Light Energy Point Beach
To:	; NRC/RES/DRA
	Hunter C, 251-7575 RES/DRA
Shared Package
ML091480404	List: ML091480413;
References
	LER 08-001
	Download: ML091480413 (22)
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Final Precursor Analysis Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Point Beach, Unit 1 Concurrent Loss of Station Auxiliary Transformer 1X-04 and 480V Safeguards Bus 1B-04.

Event Date: 01/15/2008 LER: 266/2008-001 IR: 50-266/2008-07 CDP = 7x10-6 EVENT

SUMMARY

Event Description. Unit 1 experienced a lockout and loss of Low Voltage Station Auxiliary Transformer 1X-04 (offsite power supply to the Unit 1 4.16 kV safeguards busses) at 1404, on January 15, 2008. The unit concurrently experienced a loss of 480V Safeguards Bus 1B-04. All four emergency diesel generators (EDGs) responded as designed. As a result of loss of Bus 1B-04, the normal letdown valve closed and the operator controlled the pressurizer level by aligning the excess letdown line and utilized minimum charging flow.

Safeguards Bus 1B-04 was recovered approximately 6.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months after it was lost. On recovering Safeguards Bus 1B-04, normal letdown could not immediately be established, owing to difficulties with Letdown Isolation Valve 1CV-371A. In compliance with Technical Specification requirements, Unit 1 shutdown commenced on January 16, 2008 at 1549, and Unit 1 was in Mode 3 at 1948 on January 16, 2008.

Further event information is provided in References 1 and 2.

Cause. The licensee concluded that the cause of the cable failure from the 1X-04 Transformer to Buses 1A-03 and 1A-04 was a direct fault to ground of the B5 cable for the feed to Bus 1A-03 and that this fault was caused by long-term water induced degradation of the cables outer jacket, shield, and insulation. The licensees root cause team concluded that the organization failed to assign the appropriate prioritization to address the issue of submerged power cables.

The spurious actuation of the 50G relay of Breaker 1A52-84 was likely due to high frequency transients caused by the repeated grounding of cabling associated with the low side of the 1X-04 Transformer. As a result, the licensees root cause team concluded that the settings for this relay were too conservative. In addition, laboratory testing determined that the relay would actuate on primary current spike duration less than the existing time delay setting.

Additional Event Details. The sequence of key events is provided in Appendix A.

Recovery Opportunities. Safeguards Bus 1B-04 was recovered at 2049 on January 15 (approximately 6.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months after it was lost). The 6.5 hour5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months concurrent loss of Station Auxiliary Transformer 1X-04 and 480V Safeguards Bus 1B-04 is the event window for this analysis.

Analysis Rules. The Accident Sequence Precursor (ASP) Program uses Significance Determination Process (SDP) results for degraded conditions when available. However, the ASP Program performs independent initiating event analysis when an initiator occurs and a condition analysis when there are no performance deficiencies identified for a particular event.

In addition, the ASP Program analyzes separate degraded conditions that were present during 1

LER 266/08-001 the same period and similar degraded conditions on an individual system or component that had different performance deficiencies.

Three GREEN findings have been identified for this event and are described in Reference 2.

Since there was no plant trip and separate performance deficiencies were evaluated by the SDP, this analysis focuses solely on the risk due to concurrent loss of Station Auxiliary Transformer 1X-04 and 480V Safeguards Bus 1B-04.

ANALYSIS RESULTS Importance1 The importance for this event is 7x10-6. The results of an uncertainty assessment on the importance are summarized below.

5%

Mean 95%

Point Beach 1 1.3x10-6 6.5x10-6 1.8x10-5 The ASP Program acceptance threshold is an importance of 1x10-6.

Dominant Sequences The dominant accident sequences, TRANS Sequences 02-06-05, 02-08-05, and 02-10-05, contribute to 63% (1.3x10-6 per sequence) of the total internal events importance. The sequences involve a loss of reactor coolant pump seal cooling and subsequent seal loss-of-coolant accident (LOCA), failure to rapidly depressurize the secondary, failure to depressurize the reactor coolant system, and failure of high-pressure recirculation.

The dominant sequences are shown in Figures B-1 and B-2 of Appendix B. The events and important component failures in TRANS Sequences 02-06-05, 02-08-05, and 02-10-05 are:

The reactor trips, auxiliary feedwater is successful, power-operated relief valves (PORV) correctly reseat (if opened),

reactor coolant pump seal cooling is lost, operators successfully trip the RCPs, rapid secondary depressurization fails, reactor coolant pump seal LOCA initiates (o-ring extrusion of Stage 1 and/or Stage 2 seals),

feedwater is successful, high-pressure injection is successful, secondary side cooldown is successful, reactor depressurization fails, and high-pressure recirculation fails.

1 For a conditional assessment, the parameter of interest is the measure of importance. This value is obtained subtracting the baseline core damage probability (CDP) from the conditional core damage probability (CCDP).

2

LER 266/08-001 Results Tables The conditional probabilities for the dominant sequences are shown in Table 1.

The event tree sequence logics for the dominant sequences are presented in Table 2a.

Table 2b defines the nomenclature used in Table 2a.

The most important cutsets for the dominant sequences are listed in Table 3.

Definitions and probabilities for modified or dominant basic events are provided in Table 4.

MODELING ASSUMPTIONS Analysis Type The Revision 3-Plus (Change 3.45) of the Point Beach 1 and 2 Standardized Plant Analysis Risk (SPAR) model [Ref. 3] created in June 2008 was used for this assessment.

This event was modeled as a Unit 1 concurrent loss of Station Auxiliary Transformer 1X-04 and 480V Safeguards Bus 1B-04.

Unique Design Features Point Beach has a somewhat unique dependence on instrument air and a unique dependence of AFW on operator action. Manual action (basic event AFW-XHE-XM-MINGAG) is needed in order to preserve AFW function. This circumstance drives a relatively high SPAR model result for baseline CDF for this plant.

Modeling Assumptions Summary Key Modeling Assumptions. This event is analyzed using the GEM module in SAPHIRE. The risk increase that this ASP analysis focuses is due to three event factors:

(1) the increased probability of a reactor trip (i.e., failure of operators to initiate excess letdown and minimize charging flow, (2) the loss of Station Auxiliary Transformer 1X-04, and (3) concurrent loss of 480V Safeguards Bus 1B-04. The time window for this risk increase is 6.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months . This event analysis is conditional analysis; however, the GEM initiating event module was utilized because the conditional probability of reactor trip cannot be converted into a frequency. The base CDP for this short time window is considered negligible when compared the CCDP; therefore, the importance equals the CCDP for this analysis.

Basic Event Probability Changes Table 4 provides all the basic events that were modified to reflect the best estimate of the conditions during the event. The basis for these changes is provided below:

ACP-BAC-LP-1B04 set to TRUE. This basic event represents the Safeguards Bus 1B04 that failed during the event and was unavailable for 6.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months . This event was set to TRUE.

ACP-TFM-FC-1X04 set to TRUE. This basic event represents the Station Auxiliary Transformer 1X04 that failed during the event; therefore, this event was set to TRUE.

3

LER 266/08-001 ACP-XHE-XM-B089 set to 0.65. This basic event represents operators failing to align power to Busses B08 and B09 per abnormal operating procedures. In many of dominant cutsets, this operator action would follow ACP-XHE-XM-2A031A03 (operators fail to transfer power from Bus 2A03 to Bus 1A03) in the postulated accident sequence. After a review of the applicable sequences and cutsets, the staff determined that dependency needed to be addressed for ACP-XHE-XM-B089. Based on the dependency matrix provided in Reference 4, it was determined that this event was highly dependent (same crew, close time, different locations) on ACP-XHE-XM-2A031A03. Therefore, the failure probability of ACP-XHE-XM-B089 was calculated to be 0.65 using the dependency formula [Ref. 4].

IE-TRANS set to 2x10-2. In this analysis, IE-TRANS does not represent the reactor trip frequency. Rather, it represents the conditional probability of operators failing to establish excess letdown and minimize charging to flow to control pressurizer level and preclude a trip. This probability was calculated using the SPAR-H method [Ref. 4] and details of this evaluation are provided in Appendix C. All other initiating event frequencies were set to zero due to their very small probabilities during this short time window (i.e., the 6.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months of concurrent unavailability of the Station Auxiliary Transformer 1X-04 and Safeguards Bus 1B-04).

REFERENCES

1. LER 266/08-001 Rev. 0, Manual Reactor Shutdown Required by Technical Specification LCO 3.8.1 AC Sources-Operating Not Met, March 16, 2008.

2. U.S. Nuclear Regulatory Commission, Point Beach Nuclear Plant-NRC Special Inspection Report 05000266/2008007, April 21, 2008.

3. Idaho National Laboratory, Standardized Plant Analysis Risk Model for Point Beach 1 and 2, Revision 3 Plus (Change 3.45), June 2008.

4. Idaho National Laboratory, The SPAR-H Human Reliability Analysis Method, NUREG/CR-6883, August 2005.

4

LER 266/08-001 Table 1. Conditional core damage probabilities of the dominating sequences.

Event Tree Name Sequence Number CCDP1 Contribution (%)

TRANS 02-06-05 1.3E-006 21.3 TRANS 02-08-05 1.3E-006 21.3 TRANS 02-09-05 1.3E-006 21.3 TRANS 20 7.5E-007 12.3 TRANS 02-07-05 6.5E-007 10.7 TRANS 02-10-05 6.5E-007 10.7 Total (all sequences)2 6.1E-006 100

1. Values are point estimates.

2. Total CCDP includes all sequences (including those not shown in this table).

Table 2a. Event tree sequence logic for dominating sequences.

Event Tree Name Sequence Number Logic

(/ denotes success; see Table 2b for top event names)

TRANS 02-06-05

/RPS /AFW /PORV LOSC /RCPT RSD /BP1 /O1

/BP2 O2 /FW /HPI /SSC PZR HPR TRANS 02-08-05

/RPS /AFW /PORV LOSC /RCPT RSD /BP1 O1

/BP2 /O2 /FW /HPI /SSC PZR HPR TRANS 02-09-05

/RPS /AFW /PORV LOSC /RCPT RSD /BP1 O1

/BP2 O2 /FW /HPI /SSC PZR HPR TRANS 20

/RPS AFW MFW FAB TRANS 02-07-05

/RPS /AFW /PORV LOSC /RCPT RSD /BP1 /O1 BP2 /FW /HPI /SSC PZR HPR TRANS 02-10-05

/RPS /AFW /PORV LOSC /RCPT RSD /BP1 O1 BP2 /FW /HPI /SSC PZR HPR Table 2b. Definitions of top events listed in Table 2a.

Top Event Definition AFW AUXILIARY FEEDWATER BP1 RCP SEAL STAGE 1 INTEGRITY BP2 RCP SEAL STAGE 2 INTEGRITY FAB FEED AND BLEED FW FEEDWATER (AFW or MFW)

HPI HIGH PRESSURE INJECTION HPR HIGH PRESSURE RECIRCULATION LOSC LOSS OF SEAL COOLING MFW MAIN FEEDWATER O1 RCP SEAL STAGE 1 INTEGRITY O2 RCP SEAL STAGE 2 INTEGRITY PORV PORV/SRVs ARE CLOSED PZR RCS DEPRESS FOR LPI/RHR RCPT REACTOR COOLANT PUMPS TRIPPED RPS REACTOR TRIP RSD RAPID SECONDARY DEPRESSURIZATION SSC SECONDARY SIDE COOLDOWN 5

LER 266/08-001 Table 3. Conditional cutsets for the dominant sequences.

CCDP Percent Contribution Minimum Cutsets (of basic events)

TRANS, Sequence 02-06-05 3.4E-007 26.0

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2

/RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 ACP-CRB-CC-57 ACP-XHE-XM-B0809 7.8E-008 6.06

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2

/RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 EPS-DGN-FR-G01 EPS-DGN-FR-G02 ACP-XHE-XM-B0809 7.8E-008 6.01 RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2

/RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 ACP-CRB-CC-57 CVC-XHE-XM-112B 6.5E-008 5.02 EPS-XHE-XM-G02-1A05 /RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2

/RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 EPS-DGN-FR-G01 ACP-XHE-XM-B0809 5.8E-008 4.50 ACP-TFM-FC-1X13

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2

/RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-B0809 1.3E-006 100 Total (all cutsets)1 TRANS, Sequence 02-08-05 3.4E-007 26.0

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1

/RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 ACP-CRB-CC-57 ACP-XHE-XM-B0809 7.8E-008 6.06

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1

/RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 EPS-DGN-FR-G01 EPS-DGN-FR-G02 ACP-XHE-XM-B0809 7.8E-008 6.01 RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1

/RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 ACP-CRB-CC-57 CVC-XHE-XM-112B 6.5E-008 5.02 EPS-XHE-XM-G02-1A05 /RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1

/RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 EPS-DGN-FR-G01 ACP-XHE-XM-B0809 5.8E-008 4.50 ACP-TFM-FC-1X13

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1

/RCS-MDP-LK-O2 ACP-XHE-XM-B0809 1.3E-006 100 Total (all cutsets)1 TRANS, Sequence 02-09-05 3.4E-007 26.0

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 ACP-CRB-CC-57 ACP-XHE-XM-B0809 7.8E-008 6.06

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 EPS-DGN-FR-G01 EPS-DGN-FR-G02 ACP-XHE-XM-B0809 7.8E-008 6.01 RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 ACP-CRB-CC-57 CVC-XHE-XM-112B 6.5E-008 5.02 EPS-XHE-XM-G02-1A05 /RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-2A031A03 EPS-DGN-FR-G01 ACP-XHE-XM-B0809 5.8E-008 4.50 ACP-TFM-FC-1X13

/RCS-MDP-LK-BP1

/RCS-MDP-LK-BP2 RCS-MDP-LK-O1 RCS-MDP-LK-O2 ACP-XHE-XM-B0809 1.3E-006 100 Total (all cutsets)1

1. Total CCDP includes all cutsets (including those not shown in this table).

6

LER 266/08-001 Table 3. Conditional cutsets for the dominant sequences (cont.).

CCDP Percent Contribution Minimum Cutsets (of basic events)

TRANS, Sequence 20 1.1E-007 14.5 IAS-MDC-CF-K2AB3ABR AFW-XHE-XM-MINGAG 8.6E-008 11.5 IAS-MDC-FR-K2A AFW-XHE-XM-MINGAG IAS-MDC-FR-K2B IAS-MDC-FR-K3B 5.3E-008 7.04 ACP-XHE-XM-2A031A03 AFW-XHE-XM-MINGAG ACP-CRB-CC-57 2.6E-008 3.52 FPS-EDP-TM-P35B ACP-XHE-XM-2A031A03 ACP-CRB-CC-57 7.5E-007 100 Total (all cutsets)1 TRANS, Sequence 02-07-05 1.7E-007 26.1