ML20086D950

Updated AO 73-26:on 730903,apparent Detonation in Offgas Sys Caused Sufficient Back Pressure to Fracture Air Ejector Rupture Disc,Thereby Degrading Boundary Designed to Contain Radioactive Matls

ML20086D950
Person / Time
Site:	Vermont Yankee
Issue date:	09/11/1973
From:	Riley B VERMONT YANKEE NUCLEAR POWER CORP.
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086D956	List: ML20086D950 ML20086D958
References
AO-73-26, VYV-3020, NUDOCS 8312050697
Download: ML20086D950 (3)

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j D VERMONT YANKEE NUCLEAR POW MR CORPOR ATION i sevenry ocven onovc oracer I Run.,um, Vmmo rr onvoi VYV-3020 "f['gy ,

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j VERNON, VERMONT 05354 i

l September 11, 1973 I .

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Director l 4 &

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\/ h Directorhte of Licensing U.S. Atomic Energy "ormission 0F - -

q Washington, D.C. 23545 SEP1407#

g - ge mc E ".Y REFERE!'CE: Operating License DPR-28 "O g'Com s

Dochet o. 50-271 , fy c,j;n 3 Abnormal Occurrence !!o. A0-73-26 o, ,

4 cP Genticuen:

A9  !! efined by Technical Specifications for the Yem' ntoYankee Nuclear l'cuet* Starten. Sectnen 616.I, wo are reportug the g:ettosame Abnormal, Occurrence es A0-73-26.

l At 68% pou(lj0250 r, an apparent on detonation September 3,1973, in the while Off-gas System operating caused at approximately sufficient back pressure to fracture the air ejector rupture disc, thereby degrading a boundary designed to ccntain radioactive materials as defir ad by Technical Specifications Section 1. A.5. '

The re was n'o evidence of lightning or electrical storm at the time of this incident. The operatore became aware of this incident by the .

followit .g events: s.

1. Off-gas flow recorder vent dounscale

2. liigh AP stach gas filter alarm

3. li.igh steam pressure on SJAE alarm _

4. Off-gas radiation and stack gas radiation recorders indicated upscale spil<es '

5. limr.cdiate visual inspection of SJAE rupture disc verified it was fractured l

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8312050697 730911 PDR S ADOCK 05000271 PDR t .

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f} . O VCRMONT A. KCC NUCt. CAR POWCR CORR'ORATIO!!

Directorate of Licensing September 11, 1973 Page 2 Turbine roof ventilatica was immediately secured and an immediate orderly shutdo.:n of the plant was initiated as well as effecting a temporary repair of the f ractured disc to minimize turbine building gaseous activity. Air ejector room air particulate samples taken at 0430 indicat ed a 1,:aximum particulate activity of 4.5x10 9 pC1/ce. 'Ihe Interim Off-nas System wan not in service as it had been previously isolated on August 31, 1973, and was verified isolated on September 3, 1973. ihe reactor was completely shuta.r.:n at 0950 on September 3,1973.

During the chutdwn period, the air ejector rupture disc uns replaced, the " north" stack particulate filter uns replaced, the of f-gas pipin;; at the stack and in the steam jet air ejector room was inspected cad no da age uns evident, the stack loop seals were checked, and all instruncatation associated with the of f-gas piping systera was tested and found to be in proper calibration. This instrurentation uns also cheched to assure its incapability of being a spark producing agent.

To extinguish a possible fore *gn cor.humtible smoldering material uithin the off-can ninine the sv s h e un n n v el for am - t el" W- bi- b"

......~..o . . . ~ m m.

-f .n. o.. .u.u goint in one o w.ua j e t an ejectui roon. Final scqle results t aken at the stack indicated a quantity of 96% CO2 in the pipe line.

Prior to the detonation, the stach release rate of radicactive gases was at a steady state 1cvel of 14,000 uCi/sec. The detonation produced a brief surge in the off-gan flow rate to the stack, increasing the activity release rate, as recorded by the stach radiation monitoring system, to a maxir um value of 50,000 pCi/sec. The duration of this release rate was less than one minute. Follwing this namentary increase, the stack release rate decreased steadily in renponse to the orderly plant. shutdown that was i n i t. Ia t ed. The reduction rate vas such that the average stack release rate during the hour subsequent to the detonation was 5,000 pCi/sec.

The instantaneous stack release rate limit, as defined in Technical Specifications, Section 3.fl. A.2, is 220,000 pCi/sec. Thus, the release rate during the period preceding the detonation was 6.4% of that permitted, the peak was 22.7% of the limit, and the level for the hour after the detonation was 2.3% of the limit. For the t.uo hour period centered at the time of the peak (one hour before, one hour af ter) the average stack release rato, disregarding the peak, was 9500 nCI/sec. Including the detonation-induced peak, this raises the IcVel for the two hour period to 9300 pCi/sec.

'lhe detonation caused a 3% increase in stack release rate for the two hour period.

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• O O VERMONT YANKEC NUCLF An POWCR CORPORA *Tio

. Directorate of Licensing -

Ecptember 11, 1973 ,

Page 3 Total calculated garcia exposure to personnel in the plant as documented by "hermo Luminescent Dosinater was <25 mr. A torso count, perforned on four uorkers involved with tenporary repair of the ruptiire dicc, indicated a figure of <1% of the whole body burden.

The plant returned to operation on Septetaber 6,1973, without the Interim Of f-p.as System In service per previous Plant Operations '

Review Committee (PORC) recommendation.

PORC taet to review the incident and made the following recommendations:

1. Install a scal-in feature in the logic circuitry of the steam jet air ejector isolation valve (FCV-12) requiring plant operator action to reopen the valve af ter the initiating signal has cleared.

2. Install a means of monitoring the integrity of the air ejector i rupture disc 'so that control room indication is available in the event th,e the ator f r,c ri.vaa .

1 In response to PORC reconmendations, the Nuclear Services Division will perform an engineering evaluation to determine the feasibility of irrplenenting the above two iters.

The indicated evaluations are currently in progress and will be completed as soon as possible.

l Very truly yours, I

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} VERP.ONT YANKEE NUCLEAR PO'.7ER CORPORATION

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'c Bradford W. Riley Plant Superintendent WFC/kbd .

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