ML19332F812

LER 89-028-00:on 891115,penetration Room Developed Noble Gas High Airborne Radioactivity Excursion of 86.1 Max Permissible Concentration Fraction.Caused by Inadequate Latching Mechanism on Access door.W/891212 Ltr

ML19332F812
Person / Time
Site:	Sequoyah
Issue date:	12/12/1989
From:	Bynum J, Spencer S TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-028, NUDOCS 8912190088
Download: ML19332F812 (5)

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. 1 TENNESSEE VALLEY AUTHORITY ,

6N 38A Lookout Place  !

December 12, 1989 l

U.S. Nuclear Regulatory Commission .

ATTN! Document Control Desk Washington, D.C. 20555 ,

Gentlemen:

TENNESSEE VALLEY AUTHORITY - SEQUOYAH NUCLEAR PLANT UNIT 1 - DOCY.ET NO.

50-327 - FACILITY OPERATING LICENSE DPR LICENSEE EVENT REPORT (LER) 50-327/89028 The enclosed LER provides details concerning a missing auxiliary building gas treatment system duct access cover that resulted in an auxiliary building secondary containment enclosure breech that exceeded the plant design basis.

This event is reported in accordance with 10 CFR 50.73, paragraph a.2.ii.B.

Very truly yours, TENNESSEE VALLEY AUTHORITY .

. R. Bynum, ice President Nuclear Power Production i 4

Enclosure cc (Enclosure):

Regional Administration U.S. Nuclear Regulatory Commission Office of Inspection and Enforcement '

Region II L 101 Marietta Ptreet, Suite 2900 l Atlanta, Georgia 30323 INPO Records Center l' Institute of Nuclear Power Operations 1100 Circle 75 Parkway, Suite 1500 Atlanta, Georgia 30339

1. NRC Resident Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy. Tennessee 37379 I

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At approximately 0430 on November 15, 1989, the Unit 1, Elevation 714 penetration room developed a noble gas high airborne radioactivic excursion cf 86.1 maxirum permissible etncentration fraction. The source of the airborne racioactivity was a missing access l csver in the auxiliary building gas treatment system (ABGTS) ductwork that allowed beckflow from the Unit I containment purge system. The plant was in a condition outside the design basis because the 144 square-inch breech caused by the missing access cover w;s greater than the maximum allowable breech of 30.4 square inches. As a consequence, the ABCTS may not have developed and maintained the required negative pressure under l cccident conditions. As immediate corrective action, the breech was located, the ABGTS l Fcn A-A was started to prevent backflow and to clean up the auxiliary building air and

( the access cover was reinstalled. The root cause of this incident has been determined I to be an inadequate latching mechanism for the application. The other access covers i l casociated with the auxiliary building secondary containment enclosure and control room l isolation system will be evaluated for adequate latching mechanisms and upgraded as rsquired.

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O jo 0l2 or 0 l4 no m . < M ,w Nec w ,asum Description of Event At approximately 0430 Eastern daylight time (EDT) on November 15, 1989, with Unit 1 in Mode 1 (100 percent power, 2,235 pounds per square inch gauge (psig), 578 degrees F hrenheit [F)) and Unit 2 in Mode 1 (12 percent power, 2,235 psig. 550 degrees F) the p;netration room on Elevation 714 in Unit I had an airborne radiation excursion during a ctntainment purge (EIIS Code VA). The source of this airborne radioactivity was identified as a missing access cover in the auxiliary building gas treatment system (ABG7S) (EIIS Code VF) duct that allowed backflow from the Unit I containment purge system (shield building exhaust stack) into the penetration room.

Prior to the initiation of the containment purge. Operations personnel requested R:diological Control to sample the air in the penetration room when the Unit I centainment purge was started. An airborne radioactivity problem was anticipatad b:cause the penetration room had experienced airborne radioactivity during a containment purge conducted on October 29, 1989. The October airborne radioactivity incident was investigated; however, the source of the airborne radioactivity could not be identified.

After the high airborne incident on October 29, 1989, Radiological Control recommended that the system engineer walk down the containment purge system to determine if leaks cxisted. The system engineer verified proper operation of the system during the containment purge. The system engineer walked down portions of the purge supply and cxhaust but found no leaks.

Th3 system engineer developed a test to recreate the conditions existing on October 29, 1989, during the next containment purge. On November 15, 1989, in cecordance with the system engineering test plan, Operations notified Radiological Control prior to initiating the purge. When the purge was started, Radiological Control tcok samples when high airborne radioactivity was again detected. They identified the highest concentration of 86.1 maximum permissible concentration fraction in the vicinity l

of the ductwork near the ceiling of the ABGTS room, which is located adjacent to the Elcvation 714 penetration room.

Radiological Control personnel felt airflow coming from the ductwo d at the point of the

highest radioactivity concentration. An auxiliary unit operator (AUO) investigated the l possible air leak on the ABGTS A-A fan ductwork and discovered an open 12- by 12-inch cecess door located in the downstream ABGTS ductwork. The duct access cover was found inside the ductwork on a horizontal ledge approximately three feet from the opening.

Th3 open ABGTS exhaust provided a flow path from the Unit 1 shield building exhaust stick to the ABGTS room and the Elevation 714 penetration room of the auxiilary building.

Ths missing access cover provided a breech of approximately 144 square inches in the auxiliary building secondary containment enclosure (ABSCE) boundary. The design basis ellows a maximum breech of 30.4 square inches, while still ensuring that ABGTS can p2rform its design function of reducing and maintaining the auxiliary building to a nigative 1/4-inch of water pressure. With the access cover not installed, the ABGTS l would not exhaust sufficient airflow to maintain the required negative pressure level.

I Th2 missing access cover would act as a register and recirculate air through the ABSCE.

l As a result, the ABSCE was in a condition outside the design basis of the plant.

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The root cause of the event.has been determined to be that the latching mechanir.m for j the access door was inader.uate for the application and vibrations loosened the latch ,

mechanism and allowed the access cover to fall off.  ;

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Analysis of Event This event is being reported in accordance with 10 CFR 50.73. paragraph a.2.ii.B. as a etndition that resulted in Units 1 and 2 being in a condition that was outside the d; sign basis of the plant.

The ABGTS serves to reduce radioactive releases from the ABSCE during accidents. The ABSCE includes the principle components that collectively function together to form a s2condary containment barrier, including the auxiliary building structure, and the isolation valves and dampers in the penetrations. ,

The missing ABGTS access cover, loc 2.ted on the downstream ABGTS ductwork provided a breech of approximately 144 square inches in the ABSCE boundary. The current design b: sis for the plant allows a maximum ABSCE breech of 30.4 square inches while still cnsuring ABGTS can perform its intended design function. Had an accident occurred, the <

ABGTS may not have been able to meet its design function of reducing the auxiliary building to a negative 1/4-inch of water pressure and the postaccident offsite dose guidelines of 10 CFR 100 could have been exceeded. The consequences of this scenarios would be the same for Modes 1-4. c Corrective Action As immediate corrective action, the ABGTS Fan A-A was started to clean up the auxiliary building air and to pressurize the downstream ABGTS duct, thus preventing shield building stack backflow into the auxiliary building. A work request (WR) was issued and ccmpleted to replace the access cover. .Because no other airborne areas were identified during this event, SQN considers the missing access cover to be an isolated case.

As long-teta corrective action, the system engineer has issuad WRs to upgrada the access cover latching mechanism frem two latches to four latches on both trains of ABGTS. The system engineer, with assistance from Nuclear Engineering, will evaluate other access covers for the ABSCE boundary and control room isolation system to determine if latching

.cchanisms are adequate.

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A review of LERs, potential reportable occurrences, workplam . .ud WRs identifled no previous similar events with the exception of the high airborne radioactivity event of I October 29, 1989, previously described.

Commitment The system engineer, with assistance from Nuclear Engineering will evaluate other access envers for the ABSCE boundary and control room isolation system to determine if latching mechanisms are adequate. This evaluation will be completed by February 16, 1990.

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