ML20080M153
| ML20080M153 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 02/21/2020 |
| From: | John Hanna NRC/RGN-III |
| To: | Office of Nuclear Regulatory Research |
| T. Aird | |
| Shared Package | |
| ML20080M135 | List:
|
| References | |
| Download: ML20080M153 (31) | |
Text
Risk and Operational Insights of St. Lucie Flooding Event PFHA Workshop Rockville, MD February 21, 2020 John Hanna Senior Reactor Analyst USNRC, Region III Office
2 Topics Covered
- Description of the event, especially how rainwater infiltrated the Reactor Auxiliary Building (RAB)
- Performance Deficiency and associated violation assessed by the NRC
- Detailed risk evaluation performed
- Plant operating states evaluated
- Initiating Event frequencies used
- Submergence of in-plant components
- Remaining mitigation
- Operational Insights
3 January 9, 2014 Event
- Extreme localized rainfall at the St. Lucie site
5+ (2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />), 6.5+ (4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />), 7.3 (24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />)
- Blocked pipes in storm drain basin caused backup into Component Cooling Water (CCW) open pit
- Flood waters entered non safety-related electrical conduits in a pipe tunnel
- Missing flood seals in conduits allowed water to enter Reactor Auxiliary Building (RAB)
- Total of 50,000 gallons (190,000 liters) entered RAB
- Both units remained at 100% power and no safety-related equipment was affected during the event
4 Root Cause - Storm Drain West Storm Water Basin East Storm Water Basin South Overflow Water Basin South East Overflow Water Basin 36 (100%)
24 (75%)
5 Root Cause - Storm Drain
-0.5
7 Root Cause - RAB U1
8 RAB (-0.5) on Jan 9, 2014
9 Root Cause - RAB U1
10 Hydraulic Paths - RAB U1 HCVs 4x drain lines (parallel) into ECCS room sumps (2x in series) minor water leakage even with closed drains valves 1B/1C Drain Tank Pumps impacted early during flood
11 Performance Deficiency
- Licensing bases states RAB protected against flooding at
+19.5 above mean low water (MLW), PMP = 47.1
- Units 1 & 2 Near Term Task Force flooding walkdowns stated RAB is protected against external flood
RAB U1 had significant flood via degraded conduits
RAB U2 had minor leakage at piping boots
- Failure to ensure that all below grade conduits that enter U1 and U2 RABs were sealed to prevent water ingress
- Self-revealing violation of Title 10 Code of Federal Regulations Part 50, App. B, Criterion III, Design Control
- Degraded flood protection existed since original plant construction (i.e., SDP full exposure time of 1 year)
12 Risk Analysis - operating states Initiating Events considered At-power, localized rain event At-power (initially), hurricane coastal surge (Cat 1-3)
At-power (initially), hurricane coastal surge (Cat 4-5)
Refueling Outage, localized rain event Refueling Outage, hurricane-induced coastal surge Pipe rupture in ECCS Tunnel (internal flooding)
Event/Scenarios considered Drain valves Open/Closed, TRANS Drain valves Open/Closed, LOOP
13 Precipitation Data
- Precipitation frequency from NOAA Atlas 14 @ St. Lucie based on a 24-hour duration storm
14 Frequency - Rain/Hurricane
- Based on available historical hurricane data from NOAA All Categories ~ 0.125/yr Above Cat 3 ~ 0.053/yr
15 Frequency - Int. Flooding Licensee provided list of piping in ECCS Pipe Tunnel Area Available pipe rupture frequencies in the range of 6E-6/year to less than 1E-6/year Additional mitigation expected to be at least 0.1 Not a significant CDF contributor
16 Affected Components-Rain
- Based on licensees site hydraulic model coupled with a plant flooding model (precipitation elevation SSCs)
17 Risk Assessment
18 Risk Analysis Approach Split fraction for plant operational states from available data Failure to close drain valves treated in NRC, licensee analyses
Includes HEP screening value of 1E-2 in licensee analysis
Similar value obtained using generic data, estimating CCF
Success/failure due to cycling of valves not considered Split fraction of LOOP/non-LOOP obtained from available data
Mostly insensitive to various splits (e.g., 99/1, 95/5, 90/10)
LOOP assumed for Category 4 and 5 hurricanes Calculated CCDP values for TRANS/LOOP depend on SSCs
Results from SPAR model in the range of E-4/year to E-6/year
Licensee values lower (e.g., additional CST refill credit)
Credit for additional mitigation in NRC analysis
Significant change from full credit (low white) to no credit for specific sequences (yellow/red threshold)
19 Risk Insights CDF was the dominant item of merit; risk was initially above 1E-5/year, but lowered due to qualitative/quantitative factors Exposure time was capped at 1 year per our process, however the perform. deficiency had elevated risk for > 1yr. historically Initiating event frequency was quite high for an external flooding (and particularly a FLEX-related) finding/violation, e.g., E-2/year Simplistic modeling of drain valves either open or shut, but not intermediate/indeterminate states Assumption of core damage when safe and stable not achieved at 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> was a driver (Aux Feedwater for decay heat removal important)
20 Operational Insights Maintenance of non-safety related structures, systems &
components (in this case storm drains, removal of vegetation) can have risk significant impacts Operators may need to go outside of existing procedures/guidance in order to mitigate a flood (HCV valves)
Location of Control Power Transformers in AC breakers can be very physically low and if submerged Loss of DC may result During refueling outage flood barriers may be impaired Low leakage reactor coolant pump seals important for station blackout (Extended Loss of AC Power scenarios)
21 Questions/Comments Any comments or questions?
22 Backup Slides BACKUP SLIDES
- NRC Inspection Report 05000335/389/2014-009, Preliminary White Finding and Apparent Violations
- Licensee Event Report 50-335/2014-001, Internal RAB Flooding During Heavy Rain Due to Degraded Conduits Lacking Internal Flood Barriers
- NUREG/CR-5820 Consequences of the Loss of the Residual Heat Removal Systems in Pressurized Water Reactors
- WCAP -17601-P, Reactor Coolant System Response to the Extended Loss of AC Power Event for Westinghouse, Combustion Engineering and Babcock &
Wilcox NSSS Designs 23 Additional Info. Resources
24 Drainage Detail
25 Penetration Details
26 Fault Tree Modeling HCV
27 Rain, At-power (NRC)
28 Rain, Other POS (NRC)
29 Hurricane, Mode 3 (NRC)
30 Hurricane, Other POS (NRC)
31 ICCW Pipe Break (NRC)