ML15014A084
| ML15014A084 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 01/14/2015 |
| From: | Jeffrey Mitman NRC/NRR/DRA |
| To: | |
| Weerakkody S, NRR/DRA | |
| References | |
| Download: ML15014A084 (22) | |
Text
Significance Determination Process Significance Determination Process Evaluation of ANO Unit 1 Stator Drop & Loss of SDC ANO Unit 1 Stator Drop & Loss of SDC EPRI Configuration Risk Management Forum g
g January 14, 2015 Jeff Mitman Senior Reliability and Risk Analyst US NRC US NRC Office of Nuclear Reactor Regulation Division of Risk Assessment Division of Risk Assessment
Overview On March 31st 2013 ANO Unit 1 (ANO1) lost shutdown cooling (SDC) during an outage
- Event was caused by dropping main generator stator (~525 tons)
- Event was caused by dropping main generator stator (~525 tons)
- Stator drop damaged beyond repair both non-safety 4.16kV safety buses which were normal and only path for offsite power to unit safety buses to unit safety buses
- EDG 1 auto started immediately as designed, EDG 2 auto started after short delay (~10 seconds)
- Delay due to outage alignment of DC power battery 2 was out of service
- Delay due to outage alignment of DC power, battery 2 was out of service, train 2 DC power was being supplied by a train 1 power battery charger Cavity was flooded 7days into refueling outage 7days into refueling outage
- Time to boil (TTB) ~12 hours
- Time to core un-covery (TTCU) ~ 4.8 days based on licensee calculation 2
calculation
Plant Conditions Prior to Event Plant Conditions Prior to Event Both trains of reactor shutdown cooling (SDC) were in service Plant electrical lineup was in a plant shutdown configuration to support maintenance and testing as follows:
- 4160 Volt non-safety bus A2 was de-energized
- Safety related 4160 Volt Buses A3 and A4 were cross tied and y
supplied power via Non-safety related 4160 Volt bus A1
- Train 2 (Green) battery D06 disconnected from D02 bus
- D04 battery charger supplied from Swing MCC B56 to provide D04 battery charger supplied from Swing MCC B56 to provide power to Green train DC bus D02.
- Therefore, train 2 DC was being supplied via train 1 AC and if train 1 AC failed so would train 2 DC (which is required for train 1 AC failed so would train 2 DC (which is required for train 2 EDG to start)
Both licensees PRA and NRCs SPAR system models modified to reflect these unusual lineups 3
reflect these unusual lineups
Mitigation Capability Status after Event
- Both emergency EDGs operating
- Note: EDG 2 did not start until EDG 1 started and restored DC control power to EDG 2 (it was delayed by ~10 seconds)
- SBO EDG was unavailable as its cabling into Unit 1 was SBO EDG was unavailable as its cabling into Unit 1 was severed by load drop
- After event initiation, following equipment was available i
l t i l
t d
assuming electrical power was restored:
- Both LPI/SDC pumps
- All three trains HPI
- Fire water unavailable after load drop due to severance of fire water header - fire water primary source for B5b 4
- Gravity feed unavailable as water level in BWST was lower than water level in flooded RCS
5
Loss of Offsite Power (LOOP) ET Loss of Offsite Power (LOOP) ET IE-M6-LOOP LOOP Event Occurs during Mode 6 EPS EMERGENCY POWER AVAILABLE OPR-72H OPERATOR FAILS TO RECOVER OFFSITE POWER IN 72 HOURS LORHR-D DIAGNOSIS LOSS OF RHR/DHR BEFORE BOILING (11 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br />)
SDC-REC Recover RHR/SDC DURING SHUTDOWN before Boiling (11 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br />)
MINJ Gravity or Forced Feed (with AC power) after Loss of SDC/DHR G-FEED GRAVITY FEED (without AC Power) before TAF LPR-SD LOW PRESSURE RECIRCULATION during Shutdown REFILL BWST REFILL DGR-96H OPERATOR FAILS TO RECOVER EMERGENCY DIESEL IN 96 HOURS LTREC LATE RECOVERY OF SDC/DHR COOLIING End State (Phase - CD)
IE M6 LOOP EPS OPR 72H LORHR D SDC REC MINJ G FEED LPR SD REFILL DGR 96H LTREC 1
OK 2
OK 3
OK LTREC-6D 6 Days 4
CD 4E-4 5
OK LTREC-5D 5 Days 6
CD 7
OK 4 Days 8
CD 9
OK 2E-5 10 OK LTREC-6D 6 Days 11 CD 12 OK C
5 Days 13 CD LTREC-5D 14 OK 4 Days 15 CD 9.15E-3 Undeveloped Branch as Probability is 0.0 16 OK 100%
0% (Undeveloped branch as given failure) 17 OK O
8 100%
100 %
18 OK 4E-2 4 Days 19 CD
CCDP Risk Results without Mi i i
C di Mitigation Credit LOOP CCDP Results Point Estimate Point Estimate Sequence Using Revised Time to Core Uncovery 4
1.6E-05 6
2.1E-08 8
3 3E 07 8
3.3E-07 11 1.0E-07 13 4.3E-09 15 7 2E 09 A
l ld b t
th 8 d ft h td th f
15 7.2E-09 19 2.4E-04 Total 2.6E-04 9
Any releases would be greater than 8 days after shutdown, therefore no LERF input These results are after crediting licensees revised TTB and TTCU
Dominant Sequence Dominant Sequence
- Sequence 19 is the major contributor
- Sequence 4 secondary contributor
- Initiating event occurred (frequency = 1.0)
- Failure to recovery offsite power: 1.0 (initially no credit for mitigation)
- Failure of gravity feed: 1 0
- Failure of gravity feed: 1.0
- Gravity feed would not work because level in the cavity was higher then level in BWST
- Failure to recover EDGs in four days: ~4E 2
- Failure to recover EDGs in four days: ~4E-2
- Based on standard INL analysis of industry data on offsite power recovery event 10
Time/Recovery Considerations Time/Recovery Considerations Shortest time to core uncovery was about 4.8 day If EDGs or SDC/DHR fail there was significant time to recover
- These recoveries where explicitly modeled based on well established data and associated models that are built into SAPHIRE
- These recoveries were quantified Additionally, other mitigating strategies could and would be tried (e.g., B.5.b)
- Appropriately quantifying these methods is difficult -
quantification was not performed as part of this analysis
- It is NRCs position that none of current human reliability p
y analysis (HRA) methods were intended to quantify these types of actions
- If SPAR-H were used (it was not) it would yield results in failure 11
(
)
y probabilities in range of 0.1 to 0.5
Licensee Recovery Approaches Licensee Recovery Approaches Proposed three methods to recover power from switchyard to 4160V safety buses
- Cable from startup 1 transformer to bus 1A3
- Cable from alternate SBO diesel generator breaker to bus 1A3
- Cable from Unit 2 non-vital switchgear to bus 1A3 Proposed single method to recover forced flow makeup capability
- 480V cable between B51 and B21 buses to power boric acid p
recirculation pump (P66) and spent fuel pool pump (P40A)
- As there were would be no problems with injection other than lack of electrical power, this looks like power recovery at 480V vs. 4160V Even though licensee never fully developed methods - they appear viable
- This is supported by licensees ability to complete cabling between i h d
d f
b i
4 4 h 12 switchyard and safety buses in 4.4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />
Licensees Recovery Analysis Licensee s Recovery Analysis Licensee believes that recovery can be modeled using HRA Divided human error probabilities (HEPs) into 3 components p
(
)
p
- Cognitive/decision making - used standard HRA (EPRI HRA Calculator) methods
- Execution - used standard HRA methods
- Design development - no standard tools applicable, therefore, used EPRI SHARP1 (TR-101711, 1992)
Standard resources (in HRA terminology: performance shaping factors) available:
available:
- Time, personnel, cable, cues, etc.
- However, two were not: procedures & training Because of lack of procedures and training all current HRA would conclude p
g probability of failure is very high
- Human factors (HF) analysis would conclude actions not feasible This is why licensee used EPRIs SHARP1 methodology - its only method that 13 allows recovery without procedures and training
- SHARP1 rarely used - not included in EPRIs own HRA Calculator
Licensees SHARP1 Analysis of D
i D
l Design Development Structure is from EPRI SHARP1 Values circled in red are licensees interpretation/implementation of EPRI method 14 interpretation/implementation of EPRI method
Licensees Recovery Results Licensee s Recovery Results Licensees Power Recovery FT Li
lt 3 1E 2 Licensee s Power Recovery FT
- Licensees results: 3.1E-2
- Dominated by SHARP1 results of 3E-2 results of 3E 2
- Identical approach for recovery of forced flow
NRCs Current Regulatory Position C
di f R
S i
on Credit for Recovery Strategies RIS 2008-15 NRC Staff Position on Crediting Mitigating Strategies Implemented in Response to Security Orders in Risk-Informed Implemented in Response to Security Orders in Risk-Informed Licensing Actions & in SDP, ADAMS Ascension ML080630025 Supplies guidance on when & how to apply credit in SDP Incorporation of manual actions special equipment operation or other Incorporation of manual actions, special equipment operation, or other non-standard actions into risk assessment needs to meet current consensus PRA standards, as endorsed by RG 1.200 guidance
- Manual actions must be included in plant procedures Manual actions must be included in plant procedures
- Staff be trained to perform actions Licensee did not and does not have strategies (equipment, procedures or training) to cover these scenarios or training) to cover these scenarios Therefore, per RIS no credit is warranted Previous flooding SDP gave no credit for multi-day scenario However given that this is four day sequence is this 16 However, given that this is four day sequence is this realistic/appropriate?
NRC Recovery Probability C
l i
Conclusion Input: Over 4 days available for recovery, therefore, some credit is warranted There is basis for no credit in standard HRA and HF when no procedures and training: RIS 2008-15: Without procedures and training l
k t
d d
d dibilit any recovery lacks standard assurances and credibility EPRIs SHARP1 method is stretch
- Only method that credits no procedures and training
- No development since 1992 and apparently no use since then NRC believes a ~90% chance of success
- Based on engineering experience and judgment
- Insights from licensees SPARP1 analysis
- Given no procedures & no training
- Adequate supplies and personnel 17 Adequate supplies and personnel
- 3 to 4 day to complete task with 4 to 5 days time available
Flooding Sensitivity Case Flooding Sensitivity Case Internal flooding did occur due to fire header rupture S
did l
i LPI/SDC/DHR l
Some water did accumulate in LPI/SDC/DHR pump vault but did not impact pumps Added basic event to corresponding system models to add probability of losing train due to flooding -
p y
g g
probabilities of 0.1 and 1.0 were tested 18 Note 1: Model was actually run by setting probability to True
Conclusion Conclusion
- If mitigating strategies were not credited If mitigating strategies were not credited, finding was Red
- However NRC did credit mitigating
- However, NRC did credit mitigating strategies despite challenges Fi l d t i
ti lt d i Y ll
- Final determination resulted in a Yellow finding 19
Backup Slides Unit 1 Backup Slides Unit 1 20
Electrical Layout Prior to Event
Dominant HRA Results Dominant HRA Results Human Error Event Description Time Needed Time Available Mean Diagnosis HEP Mean Action HEP Total Mean HEP Event HEP HEP HEP SD-XHE-D-LOSDC Operator Fails to Diagnose Loss of SDC before boiling 5
minutes 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> 2E-5 n/a 2E-5 Operator Fails to Recover 30 SD-XHE-XL-LOSDC Loss of SDC before Boiling 30 minutes 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> n/a 4E-4 4E-4 SD-XHE-XL-MINJ Operator Fails to Inject (AC power available) before Level Reaches TAF 30 minutes 3 days n/a 2E-5 2E-5 before Level Reaches TAF SD-XHE-XL-LPR Operator Fails to Initiate Low Pressure Recirc 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> 4.8 days 2E-5 2E-4 2.2E-4 SD-XHE-XM-BWST Operator Fails to Refill BWST during Shutdown 10 hour1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> 4.8 days n/a 2E-5 2E-5 DCP-XHE-XM-DD11D12 Operator Action to Align 125VDC Panel D11 to Feed 125VDC Panel D21 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> 4 hours 2E-3 2E-3 4E-3 22