ML22301A167
| ML22301A167 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 02/05/2022 |
| From: | Kenneth Kolaczyk, Jared Nadel Duke Energy Carolinas, NRC/NRR/DRO/IRAB |
| To: | |
| References | |
| Download: ML22301A167 (28) | |
Text
Oconee Trip and Notification of Unusual Event on February 5, 2022 Presented by:
Jared Nadel, Oconee SRI
Oconee Nuclear Plant Facts 3 Unit site of Babcock and Wilcox (B&W) Pressurized Water Reactors (2568MWt)
B&W plants have once through steam generators (straight tube, produce superheat, low feedwater inventory at full power)
The B&W integrated control system (ICS) uses process inputs from both the primary and secondary systems to control Tave, reactor power, feedwater flow, and turbine steam pressure, (and much more) automatically from very low power (~3%) to full power.
The Emergency Feedwater (EFW) Pumps (aka Aux Feedwater) are not normally required on a routine trip and do not auto-start on a plant trip if main feedwater pumps remain available
0343 0345 0348 0350 0357 0419 0811 Timeline of Events February 5, 2022 All 4 RCP Breakers open simultaneously
-> RPS RCP/Flux Ratio Trip -> Rx Trip First fire alarm in West Penetration Room Second fire alarm in West Penetration Room NOUE Declared (fire >15 min)
Fire brigade reports no flame/fire, but cannot determine source of smoke NOUE declaration is exited 2RIA-45, Aux building vent stack radiation alarm spikes
6th @0617 6th @1800 7th @1900 8th @0323 8th @0630 10th @1015 11th @1800 Timeline of Events February 2022 RCPs are restarted to prevent natural circulation cooldown to mode 4 24/7 NRC onsite coverage begins U2 startup approved by licensee after cause identified 2RC-67 PRZ safety valve declared inoperable (unit still in mode 3)
Licensee decides to shutdown to mode 5 to replace safety valve RCS draining for 2RC-67 removal results in Yellow risk U2 startup is approved (second time)
13th @1625 14th @1000 14th @1106 14th @1300 15th @0700 15th @1400 21st @0937 Timeline of Events February 2022 U2 enters mode 2 Temperature transient at 3% power causes the ICS to drive rods in, taking Rx subcritical (mode 3)
U2 starts up after CRDM replacement and enters mode 1 Licensee decides to shutdown to mode 5 to replace CRDM for stuck rod After manually inserting the stuck rod, it cannot be withdrawn past 19%
During rod insertion on shutdown, group 1 rod 5 becomes stuck at 97%
A fault in the MFP speed controller causes loss of SG level and auto-start of EFW pumps during mode 3 heat up (not reported under 50.72)
21st @2207 22nd @1300 22nd 26th 26th @2342 27th @0039 28th Timeline of Events February 2022 U2 in mode 2 Comprehensive review of plant equipment, procedures, and maintenance occurs with a focus on eliminating and mitigating single point vulnerabilities and risk and preventing another U2 trip Fleet independent review team is stood up to monitor U2 recovery and startup U2 in mode 1 U2 at 100%
power U2 manual Rx trip due to lowering SG level caused by feed reg valve going closed
CT3 3T SU N
3TB SU N
3TA M
M M
M 3A1 3B2 3A2 3B1 Unit 3 Reactor Coolant Pumps CT1 1T SU N
1TB SU N
1TA M
M M
M 1A1 1B2 1A2 1B1 Unit 1 Reactor Coolant Pumps 2T CT2 SU N
2TB SU N
2TA M
M M
M 2A1 2B2 2A2 2B1 Unit 2 Reactor Coolant Pumps 6900V Auxiliary Power System
Oconee OTSG Feed Flow Paths Diagram Green - Flow path at power Blue - Flow path after Rx Trip with RCPs Yellow - Flow path after Rx trip without RCPs
Group 1 Rod 5 Lead Screw with Foreign Material (setscrew)
Group 1 Rod 5 CRDM foreign material damage
U2 Trip February 21, 2022
U2 Trip February 21, 2022
Event Response by the Numbers 0 - Special Inspections 1 - Notification of Unusual Event 1 - LIV - 50.72 violation for missing the 4hr RPS actuation reporting requirement by minutes 2 - Plant Trips (one automatic, one manual, neither was complicated) 2 - Shutdowns to mode 5 2 - Automatic actuations of EFW on low steam generator level (dryout protection) 4 - Green/SL IV NCVs -
Inadequate coatings in West Penetration Room (NOUE on 2/5/22)
Failure to respond to main feed pump speed controller alarm (2/21/22 manual trip)
Two examples of 50.72 failure to report for EFW dryout protection actuations (on 2/13/22 and 2/21/22) 4 - Additional inspectors sent to the site for coverage help (plus more assisting remotely) 23 - Forced outage length in days 27 - Potential PDs investigated by the residents
Lessons Learned
- Single point vulnerability (SPV) program weaknesses: Fuse was an identified SPV; Reviewed only once; Not mitigated due solely to cost considerations
- Use of technology: Custom MS Teams Channel; Hybrid resident office response; MCR event communication; The power of too much information
- Licensee specific sensitivities: New site management with short time in role, Rapid restart cadence; The power of NRC influence;
Questions?