05000391/LER-2017-003

LER-2017-003, Automatic Start of Auxiliary Feedwater System Due to Main Condenser Failure

Watts Bar Nuclear Plant, Unit 2
Event date:	03-23-2017
Report date:	05-22-2017
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3912017003R00 - NRC Website

LER 17-003-00 for Watts Bar, Unit 2, Regarding Automatic Start of Auxiliary Feedwater System Due to Main Condenser Failure

ML17142A107
Person / Time
Site:	Watts Bar
Issue date:	05/22/2017
From:	Simmons P Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 17-003-00
Download: ML17142A107 (7)

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. LER NUMBER

2017 - 00 003

I. PLANT OPERATING CONDITIONS BEFORE THE EVENT

Watts Bar Nuclear Plant (WBN) Unit 2 was at approximately 18 percent rated thermal power (RTP) .

II. DESCRIPTION OF EVENT

A. Event Summary On March 23, 2017, at 0014 Eastern Daylight Time (EDT), WBN Unit 2 experienced an unplanned trip condition of both Turbine Driven Main Feed Pumps (TDMFPs) following a loss of Main Condenser {EIIS:COND} Vacuum. The trip of both TDMFPs caused an automatic start of the Auxiliary Feedwater System (AFW) {EIIS:BA}, including both Motor Driven Auxiliary Feed Water Pumps (MDAFWPs) and the Turbine Driven Auxiliary Feed Water Pump (TDAFWP) as designed.

The plant was performing a normal startup, and had just synchronized the main generator to the grid. Subsequent to the event, the plant was transitioned to Mode 3 by inserting all control rods with a manual trip. All plant safety systems operated as expected.

This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(iv)(A) as a safety system actuation of the AFW system.

B. Inoperable Structures, Components, or Systems that Contributed to the Event The failure of the Unit 2 main condenser — B zone caused this event.

C. Dates and Approximate Times of Occurrences Date Time Event (EDT) 3/23/17 0014 Automatic turbine trip due to loss of both TDMFPs. 2A TDMFP tripped due to loss of condenser vacuum. 2B TDMFP was out of service due to the low power condition of the plant.

Automatic start of 2A and 2B MDAFWPs and Unit 2 TDAFWP.

3/23/17 0015 Unit 2 in MODE 2.

3/23/17 0023 Stabilized reactor power between 2 to 3 percent on AFW with control rods.

3/23/17 0128 Inserted manual reactor trip. Unit 2 in Mode 3.

D. Manufacturer and Model Number of Components that Failed During the Event The main condenser was designed by Ingersoll-Rand.

E. Other Systems or Secondary Functions Affected

The loss of the main condenser resulted in the trip of the TDMFPs. No safety systems were affected .

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

F. Method of discovery of each Component or System Failure or Procedural Error The investigation following the plant trip identified a large breach of the Unit 2 main condenser — B zone pressure boundary.

G. Failure Mode and Effect of Each Failed Component The main condenser neck support structural design was inadequate to maintain integrity within specification. This resulted in cracking and failure of gusset plates and the loss of condenser vacuum.

H. Operator Actions

With the loss of condenser vacuum, an automatic main turbine trip occurred with automatic AFW actuation. Operators promptly stabilized the plant at low power. Once the cause of the loss of condenser vacuum was identified, operations personnel performed a manual reactor trip.

I. Automatically and Manually Initiated Safety System Responses The loss of main condenser vacuum led to the trip of the TDMFPs, a main turbine trip, and the actuation of AFW. The plant was stabilized at approximately 2-3 percent RTP. Following investigation of the loss of condenser vacuum, a decision was made to shut the unit down and the plant was manually tripped.

III. CAUSE OF THE EVENT

A. The cause of each component or system failure or personnel error, if known.

The main condenser neck support structural design was inadequate to maintain integrity within specification due to a latent error in the original design. This resulted in cracking and failure of gusset plates and the loss of condenser vacuum.

B. The cause(s) and circumstances for each human performance related root cause.

No human performance causes have been identified as contributing to this event .

IV. ANALYSIS OF THE EVENT

A structural failure of the Unit 2 main condenser — B zone led to a loss of condenser vacuum and a plant main turbine trip and an automatic AFW actuation. An automatic reactor trip did not occur because of the low plant power level. Operations stabilized the plant at approximately 2-3 percent RTP. Upon identifying the condenser had suffered a structural failure, the decision was made to shut the unit down.

V. ASSESSMENT OF SAFETY CONSEQUENCES

The event is bounded by a loss of normal feedwater event, which is an anticipated operational occurrence.

Due to the reduced power level at the time of this event, operations personnel were able to quickly comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to. the information collection.

3. LER NUMBER

2017 - 00 003 stabilize the plant at low power. Based on this rapid recovery, the safety consequences of this event are considered minimal.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event All safety systems operated as designed during this event.

B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident Not applicable.

C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service All safety systems operated as designed during this event.

VI. CORRECTIVE ACTIONS

This event was entered into the Tennessee Valley Authority (TVA) Corrective Action Program and is being tracked under Condition Reports (CRs) 1275870.

A. Immediate Corrective Actions

The plant was stabilized in Mode 3. Unit 2 was subsequently transitioned to Mode 5 while condenser repairs are completed.

B. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future main condenser neck support structure to reduce loading on sub-components to within material specifications. Corrective actions to reinforce the condenser will be completed prior to returning WBN Unit 2 to operation.

The Unit 1 condenser is impacted by this design issue. Reinforcements to the Unit 1 condenser were implemented during the Spring 2017 refueling outage prior to its return to operation.

Additional modifications to the Unit 1 condenser are planned for the Fall 2018 outage.

VII. PREVIOUS SIMILAR EVENTS AT THE SAME SITE

On February 21, 2015, WBN Unit 1was manually tripped due to decreasing main turbine condenser vacuum as reported in LER 390/2015-001. The decreasing main condenser vacuum was due to a failure of an expansion joint boot seal at an inadequate joint splice. The failure described in LER 2015-001 is different because it was the result of an improperly performed maintenance activity.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

VIII. ADDITIONAL INFORMATION

None.

IX. COMMITMENTS

None.