Sequoyah Nuclear Plant

Dual Unit Secondary Plant High Dissolved Oxygen (DO) Levels

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

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.

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.

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 SupervisorSr. Mgr, Chemistry notified, heading to the lab

Chemistry Breakroom

CDAS Monitors

Data Acquisition Alarm/Response

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.

TB 685’ Secondary Plant Lab

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.

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.

TB 685’ Panels (rear)

TB 685’ Panels (rear)

Sample Panel Design

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).

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.

Questions?

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