05000410/LER-2014-008

LER-2014-008, Secondary Containment Inoperable Due to Reactor Building Exhaust Fan Trip

Nine Mile Point Unit 2
Event date:	06-10-2014
Report date:	08-08-2014
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
4102014008R00 - NRC Website

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A. PRE-EVENT PLANT CONDITIONS:

Prior to the event, Nine Mile Point Unit 2 (NMP2) was in Mode 1 operating at rated thermal power.

B. EVENT:

On June 10 at 2032, Nine Mile Point Unit 2 (NMP2) declared secondary containment inoperable due to a Reactor Building Exhaust fan trip. While restoring Reactor Building ventilation to normal lineup following planned maintenance, Fan 2HVR-FN5A (Exhaust Fan "A"), tripped which caused secondary containment differential pressure (dP) to exceed Technical Specification (TS) limits. Exhaust Fan "A" was being started after maintenance to replace the fan power supply breaker when the event occurred. The exhaust fan trip was attributed to a fan flow switch malfunction. At the start of the event, the station entered the action statement for TS 3.6.4.1 Condition A.1 to restore dP within TS limits within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.

Procedure N2-EOP-SC, "Secondary Containment — Flowchart" was also entered. Standby Exhaust Fan, 2HVR-FN5B, was started per procedure and Reactor Building dP was subsequently restored within TS limits. At 2036, the secondary containment was declared operable. The TS 3.6.4.1 action statement and N2-EOP-SC were subsequently exited.

Nine Mile Point Unit 1 (NMP1) was unaffected by the occurrence of secondary containment inoperability at NMP2.

This event has been documented in the plant's corrective action program as CR-2014-005610.

TO THE EVENT:

No other systems, structures, or components contributed to this event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

The dates, times and major occurrences for this event are as follows:

June 10 2032 Reactor Building Exhaust Fan 2HVR-FN5A Tripped 2032 Entered TS Action Statement 3.6.4.1, Condition A.1 2035 Entered N2-EOP-SC 2036 Standby Exhaust Fan 2HVR-FN5B started 2036 Declared Secondary Containment Operable, Exited TS Action Statement 3.6.4.1, Condition A.1. Exited N2-EOP-SC

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

No other systems or secondary functions were affected.

F. METHOD OF DISCOVERY:

This event was discovered when Local Alarm 101107, RB VENT FN5A-5B Auto Trip/Fail to Start, annunciator was received at the time of the 2HVR-FN5A trip.

G. MAJOR OPERATOR ACTION:

The station entered TS action statement 3.6.4.1, Condition A.1 and N2-EOP-SC for the Reactor Building dP being greater than TS limits. These measures were exited after Standby Fan 2HVR-FN5B was started and dP returned to greater than the TS minimum.

H. SAFETY SYSTEM RESPONSES:

The duration of the reportable condition was approximately 4 minutes. Fan 2HVR-FN5B was started at 2036 and secondary containment dP returned to within TS limits. Division I Standby Gas Treatment System (GTS) was operable and in operation.

II. CAUSE OF EVENT:

This event was caused by the tripping of 2HVR-FN5A due to a faulty flow switch.

III. ANALYSIS OF THE EVENT:

The event described in this LER is reportable under 10 CFR 50.72 (b)(3)(v)(C) and 10 CFR 50.73(a)(2)(v)(C) as "any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material.

Reactor Building Exhaust Fan "A" tripped subsequent to planned maintenance while Reactor Building ventilation was being restored to normal lineup. The causal analysis for this event identified that associated Flow Switch 2HVR-FS12A did not change state following a 35 second delay. The exhaust fan tripped as a result of this malfunction then secondary containment dP exceeded TS limits.

Operator actions taken on June 10, 2014 to address this condition were consistent with plant procedures. When Standby Exhaust Fan "B" was activated, secondary containment dP returned within TS limits.

The required TS Limiting Condition for Operation (LCO) action statement during this event was entered and subsequently exited once the secondary containment dP was restored to above the required TS minimum value. In all cases, the TS required completion time was not exceeded. The secondary containment structural integrity, the ability to automatically isolate the non-safety related Reactor Building Ventilation System, and the GTS availability were not impacted. Secondary containment would have continued to perform its intended safety function.

Based on the above discussion, it is concluded that the safety significance of this event is low and the event did not pose a threat to the health and safety of the public or plant personnel.

This event does not affect the NRC Regulatory Oversight Process Indicators.

IV. CORRECTIVE ACTIONS:

A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL

STATUS:

Compensatory measures taken to restore secondary containment to pre-event status included the following:

Entering and exiting TS Action Statement 3.6.4.1 and N2-EOP-SC Starting Standby Exhaust Fan 2HVR-FN5B

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

The following are actions planned or taken to prevent recurrence:

Replaced Flow Switch 2HVR-FS12A.

Revise the Preventative Maintenance (PM) strategy for Reactor Building ventilation supply and exhaust fan flow switches.

Revise N2-OP-52, "Reactor Building Ventilation", to ensure exhaust and supply fan discharge dampers are 100% open prior to a fan start.

A. FAILED COMPONENTS:

There were no other failed components that contributed to this event.

B. PREVIOUS LERs ON SIMILAR EVENTS:

There were no previously submitted similar LERs identified.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) COMPONENT FUNCTION

COMPONENT

IEEE 803 FUNCTION IEEE 805 SYSTEM

IDENTIFIER IDENTIFICATION

Reactor Building Ventilation System N/A VA Reactor Building Exhaust Fan FAN VA Standby Gas Treatment System N/A BH

D. SPECIAL COMMENTS:

None