05000328/LER-2003-003

LER-2003-003, 1 OF 6

Sequoyah Nuclear Plant (Sqn) Unit 2
Event date:	02-28-2003
Report date:	04-23-2003
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function
3282003003R00 - NRC Website

I. PLANT CONDITION(S)

Unit 2 was in power operation at approximately 100 percent power.

II. DESCRIPTION OF EVENT

A. � Event:

On � February � 28, � 2003, � at � 1851 � Eastern � standard � time � (EST), � Technical Specification (TS) Limiting Condition for Operation (LCO) 3.6.1.9, Action b, was exceeded because of excessive leakage through a containment purge exhaust [EllS Code VA] containment isolation valve [EllS Code V] and the inability to repair the valve within the TS allowed outage time (AOT) of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. On February 27, 2003, during performance of a surveillance instruction it was determined that containment penetration X-6 leakage was above TS acceptance criteria.

Penetration X-6 contains two 24-inch air-operated butterfly valves. � Before expiration of the TS AOT, NRC staff enforcement discretion was obtained for an additional 144 hours0.00167 days <br />0.04 hours <br />2.380952e-4 weeks <br />5.4792e-5 months <br /> to identify the source of leakage, repair or replace the components, and perform verification testing. Troubleshooting found the inboard containment isolation valve had failed to fully close resulting in leakage through the valve seat.

B. Inoperable Structures, Components, or Systems that Contributed to the Event:

None.

C. Dates and Approximate Times of Major Occurrences:

February 27, 2003, at Began local leak rate testing (LLRT) of containment 1520 EST � purge valves at penetration X-6.

February 27, 2003, at Entered TS LCO 3.6.1.9 Action b (containment ventilation 1851 EST � system) because penetration X-6 valves failed LLRT.

The leak rate criterion was less than or equal to 11.25 standard cubic feet per hour (scfh) or .054 February 28, 2003, at NRC staff provided verbal approval for requested notice 1806 EST � of discretionary enforcement of LCO 3.6.1.9.b for extension of the TS action time by an additional 144 hours0.00167 days <br />0.04 hours <br />2.380952e-4 weeks <br />5.4792e-5 months <br />.

February 28, 2003, at TS LCO 3.6.1.9.b allowed action time of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 1851 EST � expires.

March 4, 2003, at Valve repair completed, LLRT performed and found 1626 EST acceptable, and TS LCO 3.6.1.9, Action b, was exited.

D. Other Systems or Secondary Functions Affected:

None.

E. Method of Discovery:

The containment purge valve leakage was identified during routine surveillance instruction performance.

F. Operator Actions:

Control room operators evaluated the condition and took action to maintain the plant in a safe condition. The appropriate TS LCO was entered, plant management was notified, and work documents were initiated to troubleshoot and restore the valves to an operable condition.

G. Safety System Responses:

No safety system responses were required.

III. C CAUSE OF THE EVENT

A. Immediate Cause:

The immediate cause of the event was the failure of a 24-inch air operated butterfly valve to close. Limit switch operated position indication lights indicated the valve moved to the closed position; however, penetration LLRT found leakage through one of the valves in excess of TS acceptance criteria for this penetration.

B. Root Cause:

The root cause of the event was inadequate engagement of an actuator yoke key, between the actuator yoke and valve stem, resulting in failure of the actuator yoke key. Actuator yoke key engagement was found to be approximately 1/2 to 3/4 inch.

The actuator yoke key sheared resulting in the valve disk not responding to the valve operator. The valve is air operated, with the valve operator mounted below the valve centerline.

Before completion of Unit 2 construction, a lack of key engagement was discovered on Unit 1, in a valve. That condition led to installation of actuator yoke key position retention bushing for valves with operators configured below the valve centerline.

This configuration could result in the key shifting and becoming disengaged from the operator. Review of the document used to install key retention bushings found that the amount of key engagement was not verified during installation of the bushing. The failed containment purge valve did have a key position retention bushing in place, however; it could not be determined if the observed key engagement occurred during valve assembly by the factory or at some point before installation of the position retention bushing.

C. � Contributing Factor:

None

IV. ANALYSIS OF THE EVENT

The containment ventilation system (reactor building purge system) provides mechanical ventilation of the primary containment (upper and lower), the instrument room (located inside containment), and the annulus area located between the steel containment vessel and the concrete shield building. The system is designed to supply fresh air for breathing and contamination control to allow personnel access for maintenance and refueling activities. The system consists of two purge air supply fans, two purge air exhaust fans for the containment and annulus areas, dampers, piping, and containment purge isolation valves at the containment penetrations. Each purge penetration is designed to isolate upon a containment isolation signal or upon detection of high radiation in the purge exhaust.

Leakage for 10 CFR 50, Appendix J, Type B and C penetrations combined, including the as found purge exhaust penetration leakage, results in a worst-case minimum path leakage of 33.5 scfh. This is much less than the TS limit for an allowable leakage of 135 scfh. Additionally, the leakage was much less than the overall containment leakage limit of 225 scfh. Therefore, the assumptions used for the plant accident analysis are not affected by the fail-to-seal condition of the purge valves and the results of the accident analyses remain bounding.

Additionally, compensatory measures were put in place to ensure that containment penetration X-6 was administratively controlled such that the penetration flow path would not contribute to additional containment leakage beyond the allowable limits.

V. � ASSESSMENT OF SAFETY CONSEQUENCES Based on the above "Analysis of the Event" and the compensatory actions taken, this event did not adversely affect the health and safety of plant personnel or the general public.

VI. � CORRECTIVE ACTIONS

A. Immediate Corrective Actions:

Measures were put in place to ensure that containment Penetration X-6 was administratively controlled such that the penetration flow path would not contribute to additional containment leakage beyond the allowable limits.

Troubleshooting identified a failed valve. The failed valve was removed, repaired, reinstalled, and successfully tested.

B. Corrective Actions to Prevent Recurrence:

To determine the extent of condition and prevent recurrence, air operated valves with operators configured below the valve centerline, that could impact TS LCOs of less than or equal to seven days, will be inspected to determine whether actuator key engagement is acceptable. This action is contained in the corrective action program.

A. Failed Components:

An actuator yoke key (Part Number 21084) of a 24-inch Pratt valve (Model Number N-SL-2FII) with a Bettis Actuator (Model Number 732C-SR80 ) sheared.

B. Previous LERs on Similar Events:

A review of previous reportable events for the past three years did not identify any similar events.

C. Additional Information:

None D. � Safety System Functional Failure:

This event did not result in a safety system functional failure in accordance with 10 CFR 50.73(a)(2)(v).

VIII. COMMITMENTS

None.