05000458/LER-2016-001

LER-2016-001, Potential Loss of Secondary Containment Safety Function Due to Failure of Auxiliary Building Ventilation System

River Bend Station - Unit 1
Event date:	1-5-2016
Report date:	03-07-2016
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4582016001R00 - NRC Website

LER 16-001-00 for River Bend, Unit 1, Regarding Potential Loss of Secondary Containment Safety Function Due to Failure of Auxiliary Building Ventilation System

ML16084A144
Person / Time
Site:	River Bend
Issue date:	03/07/2016
From:	Vazquez S Entergy Operations
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RBG-47660 LER 16-001-00
Download: ML16084A144 (5)

Reported lessons learned are incorporated into the licensing prociss and fed back to industry.

Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, 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.

REPORTED CONDITION

On January 5, 2016, at 10:58 p.m. CST, with the plant operating at 100 percent power, the main control room alarm indicating high pressure in the auxiliary building [NF] actuated. Operators confirmed that the building pressure was out of specification. Secondary containment was declared inoperable, and the Division 2 standby gas treatment system [BI-1] was started. This action restored building pressure to the acceptable range, and secondary containment was declared operable at 12:27 a.m. on January 6. This condition is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that caused the secondary containment to be potentially incapable of performing its safety function.

BACKGROUND

The normal auxiliary building ventilation system [VF] is non-safety related, and comprises two 100 percent capacity supply fans, two 100 percent capacity exhaust fans, and the associated ductwork, dampers, and controls. A supply fan operates continuously, drawing air from outside and distributing it throughout the building. The supply fan discharge,dampers modulate automatically to maintain air flow at a nominal 10,000 standard cubic feet per minute. An exhaust fan continuously draws air from the building and expels it outside. The arrangement of the flow path through the building general equipment areas and pump rooms is balanced such that the system acts to maintain the building at a negative pressure.

During design-basis accident conditions, the auxiliary building is maintained at negative pressure by the standby gas treatment system.

That is a safety-related system that actuates automatically in response to certain plant parameters.

INVESTIGATION and IMMEDIATE ACTIONS The normal ventilation system was inspected, and it was determined that a combination of degraded components caused the condition:

• Seals on the exhaust fan (**FAN**) discharge damper (**BDMP**) vanes were degraded. This allowed air to flow backwards through the idle exhaust fan, causing the total exhaust outflow to decrease and building pressure to increase:

• The flow controller for the supply fan modulating dampers (**CDMP**) had failed, allowing excessive air flow that caused building pressure to increase.

In order to restore the normal ventilation system to service, the troubleshooting plan for this condition temporarily altered the operating configuration of the system to close the suction damper on the idle exhaust fan. This prevents backflow through the idle fan, allowing the system to control building pressure within the required operating range.

CORRECTIVE ACTION TO PREVENT RECURRENCE

Corrective maintenance is being planned to (1) replace the seals in the exhaust fan discharge dampers to eliminate the backflow path, and (2) calibrate the supply fan modulating damper control circuit.

PREVIOUS OCCURRENCE EVALUATION

No similar events have been reported by River Bend Station in the last three years.

SAFETY SIGNIFICANCE

The River Bend Updated Safety Analysis Report describes the sequence of events postulated to occur following a loss of coolant accident (LOCA). Part of that analysis is a projection of the maximum radiation dose received by a person at the site boundary. The LOCA dose calculation assumes that the standby gas treatment system is initiated 20 minutes into the event, and that secondary containment is at the required negative pressure within 30 minutes, such that filtration may be credited. As such, the safety function of secondary containment is maintained as long as the auxiliary building pressure is capable of being established within 30 minutes of the onset of the event. Since the ability of the standby gas treatment system to control building pressure within specifications under accident conditions was' demonstrated in the recovery from this condition, no loss of the safety function of secondary containment actually occurred. This event was, thus, of minimal significance to the health and safety of the public.

(NOTE: Energy Industry Identification System component function identifier and system name of each component or system referred to in the LER are annotated as (**XX**) and [XX], respectively.)