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MADISON AVENUE AT PUNCH BOWL ROAD
- MORRISTOWN, N.J. 07960
- 201-s39-6111 g.Z., Public uhhteet Corporahon cen.rai March 22, 1978 Mr. Boyce H. Grier, Director Of fice of Inspection and Enforcement United States Nuclear Regulatory Cornission Region 1 631 Park Avenue King of Prussia, Pennsylvania 19406
Dear Mr. Grier:
Subject:
Oyster Creek Nuclear Generating Station Docket No. 50-219 IE BulIetin No. 78-03 The purpose of this letter is to respond to the directives set forth in IE Bulletin No. 78-03 which is concerned with potential detonable gas mixture accumulations associated with BWR offgas system operations.
The Oyster Creek Offgas System is comprised of piping and components which are part of the plant's original construction along with piping and components installed recently as part of the Augmented Offgas (A0G) System.
Originally, offgas release via the plant stack was delayed by the use of a holdup pipe which provides for a holdup time of approximately one hour.
With the installation of the A0G System, the delay time can be substantially increased prior to release through the plant stack.
The A0G System is an air dilution, recombination, charcoal absorption system designed to provide a minimum heldup time of 20 days for Xenon and 26 hours3.009259e-4 days <br />0.00722 hours <br />4.298942e-5 weeks <br />9.893e-6 months <br /> for Krypton.
The Oyster Creek entire offgas system was reviewed in accordance L
with the action items listed in Bulletin No. 78-03 Our response to each action item is given below:
Item No. 1 - Operations and maintenance procedures have been a: reviewed and are considered to be satisfactory with regard to the operation Q @ of the system in accordance with the design parameters used in the construction N
of the system.
Inadvertent actions (such as arc strikes) which might cause n
ignition of the mixture of gases contained in the offgas piping are prevented l
- n by the prerequisites, precautions and limitations, and specific operator i
% $cctions incorporated into the present procedures.
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New procedures being draf ted for use associated with the oficas sys t e..
will centinue to contain the necessary steps to orevent any in-
@%ccvertent actions which would icnite the cases contained in the offgas piping.
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'UQ[. VERMONT YAN KEE NUCLEAR POWER CORPO
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V SEVENTY sEVEN GROVE STREET RUTLAND. VERMONT 05701 REPLY TO:
ENGINEERING OFFICE April 14, 1978 TuRNeixE RC Ao WEsTBORO, M ass ACHUsETTs 015E TELEPMoNE 697*346-1098 WY 78-37 United States Nuclear Regulatory Co-4ssion Region I 631 Park Avenue King of Prussia, Pennsylvania 19406 Attention: Office of Inspection and Enforcement Boyce H. Grier, Director Ecference:
(a)
License No. DPR-28 (Docket No.30-271)
(b)
Letter USNRC to VTEPC dated February 10, 1978.
(c)
IE Eulletin 78-03 dated February 8,1978.
(d-) Letter VYNPC to USAEC dated June 16, 1972; Proposed C*,ange No. 1.
(e) Letter VYNPC to USAEC dated July 25, 1972; Proposed Change No. 1, Supple =ent 1.
(f) Letter VYNPC to USAEC dated July 26, 1973; Proposed Change No. 1, Supple =ent 2.
(g) Letter USAEC to VYHPC dated August 29, 1973.
(h) Telephone conversation R. H. Groce, YAEC, to E. McCabe, USNRC, March 27, 197S.
Dear Sir:
Subj ect:
Potential Explosive Gas Mixture Accu =ulations Associated with B W Off-gas System Operations References (b) and (c) required that Ver=cnt Yankee Nuclear Power Corporation verify the adequacy cf the current operational and procedural practices asscciated with the Advanced Off-Gas (ACG) System and, if necessary, describe acticns that can be taken 'to mitigate the potential for future hydrogen detonations.
Our responses to the ite=s listed in Reference.(c)'are as follows:
Item 1.
Review the operations and maintenance procedures related to the off-gas system to assure proper operation in acccrdance with all desien paraceters.
Include in this reviev =easures you have taken or vill take to prevent inadvertent actions (such as are strikes) which
=1;;ht cause ignition of the mixture of gases centained in the off-
- as piping.
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. t United States' Nuclear Regulatory Co= mission April 14, 1978 ittn: Office of Inspection and Enforce =ent Page Two Response 1.
-A review of Vermont Yankee procedures has been conducted and it has been determined that operation is within the parameters of
- the Advanced Off-Gas (AOG) System design. -
The original off-gas system at Vermont Yankee is no longer in use.-
In 1972, a modification to the originally
-installed off-gas system was made operable. References (d), (e), (f), and (g) provide infor=ation related to -
that modification.
To prevent inadvertent actions which might cause ignition of mixtures of gases, Plant Fire Protection Procedure A.P. 0042 restricts Hot Work (welding, grinding, heating of base metals, etc.) in permanent or ta=porary Fire Control Areas, including the entire AOG building. Ver=ont Yankee will revise this procedure to include the Stea= Jet Air Ejector (SJAE) Room, and any other area where te=porary openings in the off-gas piping are =ade, as temporary Fire Control Areas.
It should be pointed out that the possibility of a hydrogen explosien in the A0G system'is considered extre=ely unlikely because the steam dilution provided in-the second stage air ejectors and the control and instru=entation syste=s are designed to prevent an explosive mixture of hydrogen from propagating beyond the air ejector stages, i.e., an explosive mixture of hydrogen should never exist in the reco=biner sub-system, "30 minute" delay pipe, condenser / dryer, or charcoal adsorber beds. During shutdown, st'eam or air purge is utilized to force the residual off-gas steam mixture containing hydrogen through the preheater and reco=biner to prevent an explosive mixture frc= occurring in the reco=biner/preheaters and. upstream 2
i lines.
Start-up procedures insure that sufficient steam is introduced upstream of the preheater to dilute any hydrogen entering the A0G system as the air. ejector line is made ready for operation.
Also, the reco=biner subsyste= is purged with steam to remove any remote explosive mixture before it 14 i
brought on the line.
Instru=entation would detect an explosive mixture and prevent inadvertent unsafe operation.
Hydrogen control is acce=plished by providing fully redundant hydrogen analyzers on the cutlet frc= the reco=hiner syste=.
These analyzers initiate reco=biner syste= shutdown and l
switchover if the hydrogen concentration at the systa= cutlet
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exceeds 2% by volu=c.
Two hydrogen analy:crs are used on each j
train to assure sufficient redundancy and the systems are periodically tested to confirm continued operability. During l
an autecatic shutdcen, two nain process valves close to tseletc the rece=biner syste=.
Alar =s associated with icv reco=biner cass flow varn of impending lack of dilution stea= in the fluid i
strea= frc= the SJAE's to the reconbiners. Additionally, the i
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.. a United States Nuclear Regulatory Co--ission April 14, 1978 Attn:
Office of Inspection and Enforcement
.Page Three recombiner bed temperatures provide insight into recombiner performance to insure that flar able hydrogen mixtures do not get beyond the recochiner. Typically, the recombiners reduce the amount of hydrogen present to less than 0.1% by volume in any downstream ga.s flow.
Item 2.
Review the adequacy of the ventilation of spaces and areas through which off-gas system piping containing. explosive mixtures of gases pass. The review should consider ventilation losses and off-nor=al off-gas system operation, such as lack of dilution steam, lest loop seals, blown rupture disks, bypassing reco=biners, and leakage of off-gas into isolated portions of syste=s.
Response 2.
Redundant ventilation systems are provided for the A0G building and Turbine building essentially eliminating the possibility of a loss of. ventilation in these areas.
System operation following off-nor=al conditions such as lack of dilution steam, or a bloun rupture disc is not allowed by procedural' control.
Bypassing the reco=biners is not possible with the current system configuration.
The only loop seal in the A0G System prior to the recombiners is the intercondenser loop seal which is self-filling and drains back to the main condenser.
The other loop seals on the A0G piping are not exposed to hydrogen gases in excess of 0.1% and pose no threat to explosions even if they were blown due to transient conditions.
Leakages into unused portions of the old off-gas system have occurred at Vercent Yankee and consequently became the subject of a comprehensive engineering evaluation. As a result,
, several system changes have been implemented to prevent possi-bility of future leaking.
Ite= 3.
4 For those spaces and areas identified, describe what action you have taken or plan to take to assure that explosive mixtures cannot accuculate, that cenitoring ecuip ent vill warn of such an accunulation, and that disposal of such mixtures'will be controlled without resulting in a damaging.
explosion.
Respense 3.
See our response to Item No. 1.
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United States Nuclear Regulatory Cc= mission-fpril 14, 1978.
Atta: Office.of Inspection and Enforcenent Page Four Item 4.
Loop seals are potential off-gas leakage paths following a pressure transient in the off-gas system piping. Describe your design features to ein4m4ze and detect the loss of liquid from loop seals and describe operating procedures which assure prompt detection and reseal of the blown loop seals.
r Response 4.
As discussed in our responses to Items 1 and 2, loop seals downstrern of ths recenbiners pose no problem due to possible explosive concentrations of gas. -The loop seals on the mechanical vacuum pump line to the stack also do not present a problem since the mechanical vactum pump is operated only in shutdown, refuel or startup modes when hydrogen generation is not a problem. Present operating procedures require that loop seals be refilled on a monthly basis.
Should loop seals in the Radwaste Building deteriorate in spite of the monthly inspections, a Radiation Monitor woulc alars to alert Control Room personnel of off-nor=al conditions.
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The remaining loop seals at the stack base will be isolated.
4 following~a modification planned for the-1978 refueling outage.
r Item 5.
Review operating and energency procedures te assure that your operating staff has adequate guidance to respond properly to off-gas system explosions.
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Response 5.
We have reviewed our operating and e=ergency procedures and conclude that our operating staff has adequate guidance to respond properly to off-gas system explosions.
l We trust you will find this infor=ation satisfactory; however, should you desire additional infor=ation, please contact us.
i Very truly yours, t
VERMONT YANKEE NUCLEAR POWER COR?O?ATION I2_ } c.,(qh 4.b. I CT.at
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.i D. E. Moody Manager of Operations
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