ML20086D939

Updated AO 73-27:on 730909,apparent Detonation in Offgas Sys Caused Sufficient Back Pressure to Fracture Air Ejector Rupture Disc,Thereby Degrading Boundary Designed to Contain Radioactive Matl

ML20086D939
Person / Time
Site:	Vermont Yankee
Issue date:	09/17/1973
From:	Riley B VERMONT YANKEE NUCLEAR POWER CORP.
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086D943	List: ML20086D939 ML20086D945
References
AO-73-27, VYV-3025, NUDOCS 8312050687
Download: ML20086D939 (3)

Text

__ _ _. _

. O O ,

0 w

F VERMONT YANKME NUCLEAR POW Mu UORPOR ATION J S E V E t4 T '. SCVCN OttoVE STREET

. Rirri.Axo, ViamoNT 03701 H LPL.Y TO:

VYV-3025 e. o. nox m7 VERNON. VEftMOr4T 05354 September 17, 1973 Di re c to r Directorate of Licensing United States Atomic Energy Commission .-

Washington, D.C. 20545 REFEl'ENCE : Operating License DPR-28 Docket I:o. 50-271 Abnormal Occurrence No. A0-73-27 Gentlemen: ,

~

' \ 'l As defined by Technical Spccifications for the Vermont Yankee' v . . *; ., , n.,.,, c ., w a n., c .eten L *1 0 1 ..n en ve n v e 4 ,, v%n Gn11r.,4 n HDuornal UCcurrence US AU-/J-4/.

At 0210 on September 9,1973, while operating at approximately 65% power, an apparent detonation in the Off-Gas System caused sufficient back pressure to fracture the air ejector rupture disc, thereby degrading a boundary designed to contain radioactive materials as defined by Technical Specificat. ions, Section 1.A.S.

There was no evidence of Jightning or electrical storm at the time of this incident. The station operators became aware of the incident by the following events:

1. Off-gas flow recorder went downstale

2. tilgh AP stack gas filter alarm

3. iiigh steam pressure on SJAE

4. Immediate visual inupection of the SJAE rupture disc verified it had fractured 0312050687 730917 PDR ADOCK 0D000271 S PDR I

m r

VERMONT YLKEE NUCLEAR POWER CORPORATION 4

Directorate of Licensing September 17, 1973 Page 2 Turbine roof ventilation was immediately secured and an orderly shutdown of the plant was initinted as well as effecting a temporary repair of the fractured disc to minimize turbine building gaseous activity. Turbine Building samples taken at 0347 indicated a maximum airborne activity of <1.18x10-6 pCi/cc. The Interim Off-cas System was not in service as it had been previously isolated on August 31, 1973. The reactor was completely shutdewn at 0723 on September 9,1973.

During the shutdown period, the following areas were addressed:

1. The air ejector rupture disc was replaced.

2. The " north" stack particulate filter was replaced and tested.

I

3. All instrumentation associated with the of f-gas piping system was tested and found to be in proper calibration. This instrumentation was again rechecked to assure its incapability of being a spark producing agent.

" , , . L . . . ... .. '. . ... :... :: .... .. . . . :. :.. : u. . . :.. .

a proper ground existed between the lines and the of f-gas piping.

5. The stack particulate filter cartridge and inner cover plate were positively grounded to the filter case to prevent the possibility of ungrounded internal components building up a static charge and ultimately sparking over internally as an ignition source.

?, 6. The stack sump pumps were inspected for possible electrical

• ^

problems and none were found. Pump starts are now being monitored

in an atteept to associate their starting with any future off-gas problem, f 7. The of f-gas line low point drain at the stack was extended five (5) inches into the suinp to provide a greater length of water seal.

8. The of f-pas holdup line was opened at two (2) locations along -

its length and inspected. Nothing of an abnormal nature was observed.

l

, I i

l l

9 I

O VCRMONT Yg(CC NUCLCAR POWCR CORPORATION Directorate of Licensing September 17, 1973

,Page ,3

9. The of f-gas system bypass valve (ICV-1) , downstream of the tilter, was removed and inspected in an atterr.pt to identify an upstream or downstream direction of detonation source.

This proved inconclusive. No other evidence of abnormality was apparent.

10. A thorough ground continuity check was conducted on bolted pipe flangen, bolted valve bonnets, tubing, and ground straps associated with the off-gas piping and no indication of f aulty grounding uns evident.

11. The off-gas flow meter was inspected and found to be in operating condition. Based upon stainless steel and aluminum metals of the flow meter, it is not considered capable of being an ignition source.

12. All routine and non-routine operating evolutions on or about the timen of the last two detonations were identified'and exnlorml to deretnino their nousth1r. cnnerihurton en c o..e n nr J. ....J.,.. . n i .. a v i c i, s vsn.i ow u uu u: Aut t uiso.up ue wecis cause and effect.

Prior to returning the plant to operation on September 13, 1973, a uritten emergency operating procedure was established addressing detonation in the off-gas syntom. Additionally, a continuous air purge has been introduced to the off-gas line in an attempt to reduce the hydrogen concentration below the explosive level of approximately 15%.

The Plant Operations Review Committee (PORC) and the Nuclear Safety and Audit Review Committee (NSARC) met to review the incident and agreed that the steps taken in an attempt to preclude another incident of this nature were extensive and appropriate.

Very truly yours, VERMONT YANKEE NUCLPAR POWER CORPORATION

% /

M ^

Brndford U. Riley Plant Superintendent WFC/kbd L _ e- . --m