05000390/LER-2017-008

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LER-2017-008, Shield Building Inoperability and Potential Loss of Safety Function Resulting from Spurious Equipment Operation
Watts Bar Nuclear Plant, Unit 1
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material

10 CFR 50.73(a)(2)(v), Loss of Safety Function
3902017008R00 - NRC Website
LER 17-008-00 for Watts Bar, Unit 1, Regarding Shield Building Inoperability and Potential Loss of Safety Function Resulting from Spurious Equipment Operation
ML17226A297
Person / Time
Site: Watts Bar Tennessee Valley Authority icon.png
Issue date: 08/14/2017
From: Simmons P
Tennessee Valley Authority
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML17226A297 (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 - 00 008

I. PLANT OPERATING CONDITIONS BEFORE THE EVENT

Watts Bar Nuclear Plant (WBN) Unit 1 was at 100 percent rated thermal power (RTP) . WBN Unit 2 was in Mode 5 and was not impacted by the event.

II. DESCRIPTION OF EVENT

A. Event Summary On June 15, 2017, at 1219 Eastern Daylight Time (EDT), Technical Specification (TS) Limiting Condition for Operation (LCO) 3.6.15 Condition B was entered for the WBN Unit 1 annulus pressure not within limits, resulting in Shield Building {EIIS:NH} inoperability. At 1221 EDT, the WBN Unit 1 annulus pressure returned to normal and LCO 3.6.15 Condition B was exited.

Because the shield building is a non-redundant safety system, operation outside of TS allowable limits represents an event that could have prevented fulfillment of a safety function.

The temporary loss of the Shield Building resulted from a loss of pressure control in the Auxiliary Building caused by a loss of Auxiliary Building General Ventilation due to a spurious cross zone fire alarm. The Auxiliary Building Gas Treatment System (ABGTS) {EIIS:VF} was started to maintain Auxiliary Building pressure within limits and the non-safety related Annulus Vacuum system automatically restored annulus pressure. The detectors that resulted in the loss of the WBN Unit 2 Auxiliary Building ventilation supply fans were replaced.

This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(v)(C) and (D) as an event or condition that could have prevented fulfillment of a safety function needed to control the release of radioactive material and mitigate the consequences of an accident.

B. Inoperable Structures, Components, or Systems that Contributed to the Event No inoperable equipment contributed to this event.

C. Dates and Approximate Times of Occurrences Date Time Event (EDT) 6/15/17 1215 Cross zone fire alarm results in fire pump start and trip of Auxiliary Building General Supply fans 2A and 2B 6/15/17 1217 Operations starts B train of ABGTS in accordance with system operating instruction 0-S01-30.06, Auxiliary Building Gas Treatment System.

6/15/17 1219 TS LCO 3.6.15 Condition B entered for annulus pressure not within limits.

6/15/17 1221 TS LCO 3.6.15 Condition B exited when annulus pressure is within limits.

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 NEOB-10202, (3150-0104). Office of Management and Budget, Washington, DC 20503. If a means 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.

D. Manufacturer and Model Number of Components that Failed During the Event The fire detectors that failed are Model DI-2 provided by Pyrotronics {EIIS:DET}.

E. Other Systems or Secondary Functions Affected

No other systems or secondary functions were affected .

F. Method of discovery of each Component or System Failure or Procedural Error Plant alarms were received for the start of the high pressure fire pump and the trip of the WBN Unit 2 auxiliary building general supply fans.

G. Failure Mode and Effect of Each Failed Component Not applicable.

H. Operator Actions

Upon receipt of the alarms, operations personnel responded in accordance with operating procedures.

I. Automatically and Manually Initiated Safety System Responses Not applicable.

III. CAUSE OF THE EVENT

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

A spurious cross zone fire alarm resulted in the start of two electric fire pumps and the stopping of the auxiliary building supply fans 2A and 2B.

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

No human performance causes have been attributed to this event.

IV. ANALYSIS OF THE EVENT

The WBN containment design includes a free standing steel pressure vessel surrounded by a reinforced concrete shield building. The shield building is maintained at a negative pressure of -5 inches water gauge (WG) during normal operation by the annulus vacuum control system, which is not safety-related. In the event of an accident, the safety-related Emergency Gas Treatment System (EGTS) {EIIS:VC} would filter the exhaust from the shield building, reducing the offsite dose to members of the public and to control room operators from postulated leakage of the containment pressure vessel.

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 NEOB-10202. (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means 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.

The design of the EGTS is that it is capable of achieving an acceptable negative pressure in the shield building annulus assuming the annulus is not at a negative pressure. While the annulus is normally expected to be at a negative pressure relative to atmospheric, the dose analysis conservatively assumes the annulus is at atmospheric pressure at event initiation. After blowdown, the annulus pressure will increase rapidly due to expansion of the containment vessel as a result of primary containment atmosphere temperature and pressure increases. The annulus pressure will continue to rise due to heating of the annulus atmosphere by conduction through the containment vessel. After a delay, the EGTS operates to maintain the annulus pressure below atmospheric pressure. Review of data traces for this event indicate that the annulus pressure went as low as approximately -4 inches WG, which bounds the analysis limit of atmospheric pressure.

Accordingly, while the shield building was outside its normal pressure limits, the safety function of the shield building to limit dose to the public and to control room operations personnel was not lost.

V. ASSESSMENT OF SAFETY CONSEQUENCES

As described in the previous section, the safety function of the shield building to mitigate a design basis accident was not lost. Therefore the consequences of this event are low.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event Both trains of EGTS remained operable 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 The shield building was outside of its normal allowed negative pressure for approximately 2 minutes.

VI. CORRECTIVE ACTIONS

This event was entered into the Tennessee Valley Authority (TVA) Corrective Action Program and is being tracked under Condition Report (CR) 1307486.

A. Immediate Corrective Actions

Operations personnel took prompt action to start the Train B ABGTS to support proper auxiliary building ventilation operation.

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 Infocollects.Resource@nrc.gov. and to the Desk Officer, Office of Information and Regulatory Affairs, 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.

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CONTINUATION SHEET

3. LER NUMBER

2017 - 00 008 B. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future The detectors that resulted in the spurious alarms were found outside of the required acceptance criteria and were replaced.

VII. PREVIOUS SIMILAR EVENTS AT THE SAME SITE

power ascension testing or were the result of equipment failures. As described above, no actual loss of safety function capability occurred because the safety function of the EGTS to reduce radiological dose is performed even if the shield building is not at a vacuum.

VIII. ADDITIONAL INFORMATION

None.

IX. COMMITMENTS

None.