1. Sequoyah Nuclear Plant
Dual Unit Secondary Plant High
Dissolved Oxygen (DO) Levels

2. Sequoyah Facts
• Unit 1 began commercial operation July 1, 1981
• Unit 2 began commercial operation June 1, 1982
• Unit 1 is licensed for operation through 2020 and Unit 2
is licensed through 2021
• Application process and inspections are ongoing with the
Nuclear Regulatory Commission (NRC) to renew the
licenses on both units; extending operation to 2040 and
2041
• Each unit can produce more than 1,160 MW of electricity

3. Days Leading up to the Event
• Fukushima/FLEX “fast track” mods in progress.
• 3/19/15, Operations tagged out the demineralized
(DI) water header to support piping modification
work.
• 3/19/15 - 3/24/15, Operations isolated/tagged out
nitrogen to Condensate Storage Tanks (CSTs) a few
times for Fukushima Mod, resulting in DO >100 ppb
until restored and treated (Carbo, Hydrazine).
• 1st Quarter Training: Technicians taught to walkdown
Secondary Plant panels and make adjustments IAW
procedure to ensure proper instrument operation.

4. Timeline 3/24/15
• 13:30 Operations releases clearance to restore the
DI water header.
• 15:30 Dayshift Chemistry technicians leave site, with
the 12-hour rotational Shift Tech to cover the
remainder of the day (3 hours). Duty Chemistry
Supervisor has stepped away from the site.
• 16:00 Operations hangs clearance to remove
nitrogen from the Turbine Building for Fukushima
Mod.
• 16:00 Operations reports that there is a DI water
header leak, location unknown.

5. Timeline 3/24/15
• 16:05 Chemistry Shift Technician notices a
significant rise in the Condensate DO concentration
for Unit 1 with corresponding drop in hydrazine. Due
to the DO and hydrazine-to-oxygen ratio exceeding
limits per 0-TI-CEM-000-001.1, Action Levels 2 and 3
were entered, requiring Unit shutdown if not restored
within 8 hours.
Cannot contact Duty Supervisor
Sr. Mgr, Chemistry notified, heading to the lab

6. Chemistry Breakroom

7. CDAS Monitors

8. Data Acquisition Alarm/Response

9. Timeline 3/24/15
• 16:05 - 16:30 TB 685’ Lab walkdown performed:
 Validated local inline instrument readings
 Instrument panel (front) pressure/flow parameters appear normal
 Grab sample by Waltron DO Meters confirms high DO
 Raised hydrazine injection flow rate to Unit 1 to a maximum
• 16:30 Chemistry Management Team and Secondary
Chemist arrive. Additional technicians called in.
Duty Team directed to staff the OCC.

10. TB 685’ Secondary Plant Lab

11. Timeline 3/24/15
• 16:45 Operations is requested to restore nitrogen to
the Turbine Building and locate/isolate (alleged)
demin water header leak.
• 17:00 A similar condition now starts on Unit 2, with
Action Level 2 and 3 limits being exceeded.
 Overlay of Condensate DO spikes and CST makeup correlate
 Same actions previously taken on Unit 1 now performed on Unit 2
 Carbo/Hydrazine is added both the ‘A’ and ‘B’ CSTs
• 18:00 Unit 2 DO is restored to within AL1 limits.
• 18:45 Grab samples from CSTs are <50 ppb DO.

12. Timeline 3/24/15
• 19:00 Secondary Chemist recognizes that the
hotwell/condensate process flow is actually DI water.
 Elevated sample line pressure at the sample panel.
 DI pressure regulators were set to max pressure.
 The regulators were adjusted from 40 psig back to the
normal setting of 19 psig.
• 19:05 Grab samples from Condensate Headers
(locally) are <20 ppb DO.
• 19:15 Normal DO values are restored to both Units
as inline instrument process flow is switched back
from DI water to Secondary Plant points.

13. TB 685’ Panels (rear)

14. TB 685’ Panels (rear)

15. Sample Panel Design

16. Why did it Happen?
1. Off normal sample system configuration was not
fully understood or immediately recognized.
2. Weak status control and procedure guidance for
adjusting the DI water pressure regulators.
3. System design with DI water used as a backup.
4. Communication gap between Operations and
Chemistry for DI water restoration. Clearance did
not have Chemistry Lab restoration guidance.
5. Off normal DO procedure did not provide detailed
instructions (tribal knowledge not captured).

17. Lessons Learned
1. Maintain clear communications, turnover and
system monitoring when performing alignment
changes to ensure proper restoration to normal
conditions.
2. Capture tribal knowledge actions and OE into
procedures.

18. Questions?
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