05000461/LER-2016-002

LER-2016-002, Trip of Fuel Building Ventilation Exhaust Fan Due to Moisture Formation Resulting In the Loss of Secondary Containment Vacuum

Clinton Power Station, Unit 1
Event date:	02-13-2016
Report date:	04-13-2016
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
4612016002R00 - NRC Website

LER 16-002-00 for Clinton, Unit 1, Regarding Trip of Fuel Building Ventilation Exhaust Fan Due to Moisture Formation Resulting In the Loss of Secondary Containment Vacuum

ML16106A305
Person / Time
Site:	Clinton
Issue date:	04/13/2016
From:	Stoner T R Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
U-604272 LER 16-002-00
Download: ML16106A305 (6)

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PLANT AND SYSTEM IDENTIFICATION

General Electric—Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in .the text as [XX]

EVENT IDENTIFICATION

Trip of Fuel Building Ventilation Exhaust Fan Due to Moisture Formation Resulting In the Loss of Secondary Containment Vacuum A. Plant Operating Conditions before the Event Unit: 1 Event Date: 2/13/16 Mode: 1 Mode Name: Power Operation

B. DESCRIPTION OF EVENT

Event Time: 0206 Reactor Power: 99 percent On February 13, 2016 at 0206, during routine venting of the drywell, the main control room (MCR) received a fuel building ventilation (VF) system alarm. At 0208 Fuel Building exhaust fan 1VFO4CA tripped and 'B' exhaust fan 1VFO4CB auto started, however, secondary containment vacuum continued to degrade. Secondary Containment was declared inoperable when the vacuum degraded to less than 0.25 inch water gauge (WG). TS LCO 3.6.4.1 "Secondary Containment", Required Action A.1 was entered. When Secondary Containment differential pressure value became greater than 0 inch of vacuum WG with 1VFO4CB fan operating, procedure EOP-8, Secondary 'Containment Control, was entered.

At 0212, Secondary Containment vacuum was at 0.15 inch WG with 1VFO4CB running and 1VFO4CA in standby. With Secondary Containment vacuum still outside TS limits, the MCR secured the VF exhaust fans at 0255 per CPS 3404.01, Fuel Building HVAC (VF). At 0256 Standby Gas Treatment System (VG) Train A was started per CPS 3319.01, Standby Gas Treatment (VG).

Secondary Containment vacuum was restored to greater than 0.25 inch WG at 0257 and Secondary Containment was declared operable. TS LCO 3.6.4.1 Required Action A.1 and EOP-8 were exited.

A troubleshooting team determined that ice had formed resulting in a blockage in the low side sensing line to the environment as the most probable cause of the event. Other locations for sensing line clogging were also investigated.

After disassembling, thawing, and cleaning the intake tubing, technicians performed a blowdown of the instrumentation tubing. Following the blowdown, Operations successfully restarted VF exhaust fan 1VFO4CA. Secondary Containment vacuum was restored to normal.

Clinton Power Station, Unit 1 05000461 The event described in this LER was documented in Event Notification No. 51732 which was provided to the NRC on February 13, 2016 at 0909.

C. CAUSE OF EVENT

An investigation identified that ice formed in the low side of the VF 'A' sensing line causing an inaccurate Secondary Containment vacuum reading on both indication and control loop for the 1VFO4CA. This caused 1VFO4CA to trip which in turn led to a loss of Secondary Containment vacuum. Moisture had collected in the sensing line and froze due to externally cold weather conditions. At the time of the event, the outside air temperature was 8 degrees Fahrenheit. A subsequent causal evaluation established that prior instrument sensing line designs did not recognize the potential to trap water in the sensing line as the cause of this event.

D. SAFETY ANALYSIS

This event is reportable in accordance with 10CFR50 50.73(a)(2)(v)(C) as a condition that could have prevented fulfillment of a safety function to control the release of radioactive material because secondary containment was declared inoperable.

The VG system was fully operable at the time of the event and capable of performing the required safety function. Operations entered EOP-8 for Secondary Containment vacuum less than minus 0.25 inch water gauge and entered TS LCO 3.6.4.1, Required Action A.1.

Secondary containment vacuum was restored to within TS limits within the completion time requirement. The secondary containment vacuum is kept slightly negative relative to the atmospheric pressure to prevent leakage to the atmosphere. The VF system is anon-safety ventilation system which is normally in service to maintain secondary containment vacuum. The VG System is the safety-related system which is relied upon to perform this function following an accident. During the event the VG system was placed in service and restored Secondary Condition to operability consistent with its safety function. Therefore, the ability of the station to maintain secondary containment in an accident scenario was never jeopardized or challenged by the VF system fan trip.

E. CORRECTIVE ACTIONS

Technicians disassembled, thawed, and cleaned the intake tubing. Afterwards, they performed a blowdown of the instrumentation tubing. Following this activity, Operations successfully restarted the VF fan. Interim measures to prevent a recurrence of icing included the installation of heat tape inside the sensing line and a heat source (light bulb) inside the protective housing.

An Engineering Change (EC) will be completed to install an alternate VF sensing line design to prevent moisture accumulation in the instrument sensing line and ensure accurate indication and control of the Secondary Containment pressure. A subsequent review of VF performance will be conducted to ensure the modification effectively prevented moisture accumulation.

F. PREVIOUS SIMILAR OCCURENCES

A review for previous occurrences did not identify a similar licensee event report at CPS.

CPS Issue Report (IR) 2613542 identified a similar condition on January 17, 2016 in which VF Ventilation System exhaust fans repeatedly tripped on low Fuel Building vacuum. The VG System was started to maintain secondary containment vacuum. However, in this case, Secondary Containment vacuum was maintained greater than minus 0.25 inch WG, and therefore not reportable.

VF vacuum (Computer point VF-BA201) and Containment Building (VR) vacuum (Computer point VR-BA201) were reading outside their normal operating bands. The apparent cause of the VF exhaust fans tripping was due to blockage/moisture in the vacuum sensing line. This was confirmed when the sensing line was blown down to restore the indicated VF vacuum to normal operating band.

Corrective actions resulting from this event included scheduling a blowdown of the instrument sensing line during the upcoming refueling outage and evaluating long term plans to resolve this recurring condition.

This occurrence is directly applicable to the conditions noted in this LER and documents a prior occurrence of the same failure mode earlier in 2016.

G. COMPONENT FAILURE DATA

There were no component failures associated with this event.