ML20028C431

Forwards Response to IE Bulletin 78-03, Potential Explosive Gas Mixture Accumulations Associated W/Bwr Offgas Sys Operations, Per NRC 780210 Ltr.Plant Design Changes Unnecessary

ML20028C431
Person / Time
Site:	Browns Ferry, 05000000
Issue date:	03/29/1978
From:	Gilleland J TENNESSEE VALLEY AUTHORITY
To:	James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML20027A594	List: ML20028C411 ML20028C420 ML20028C424 ML20028C428 ML20028C431 ML20028C433 ML20028C435 ML20028C437 ML20028C443 ML20028C446 ML20028C449 ML20028C451
References
FOIA-82-545, REF-SSINS-6820 IEB-78-03, IEB-78-3, NUDOCS 8301100127
Download: ML20028C431 (8)

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' TENNESSEE VALLEY AUTHORITY CH ATTANOOG A. TENNESSEE 374ot 830 Power Building MAR 291978 Mr. James P. O'Reilly, Director Office of Inspection and Enforcement U.S. Nuclear Regula'tcry Cc= mission Region II - Suite 1217 230 Peachtree Street, KW.

Atlanta, Georgia 30303

Dear Mr. O'Reilly:

OFFICE OF I urwn0N AND ENFCRCSSNT BULLETD 78 RII:JPO 50-259, -260, -296 - BROWNS m!RY EUCLEAR FIANT UNITS 1,

2. AND 3 In response to your February 10, 1978, letter which transmitted IE Bulletin 78-03, we are enclosing the results of our investi-gations at Browns Ferry.

Very truly yours, J. E. G111 eland Assistant Manager of Power Enclosure cc (Enclosure):

Office of Inspection and Enforcement L/

Division of Reactor Operations Inspectics U.S. Nuclear Regulatory Commission Washington, DC 20555 9

e 8301100127 821207 PDR FOIA .

HIATTB2-545 PDR i . . ._

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.- ENCLOSURE RESPONSE TO IE BULLETIN 78-03 POTENTIAL EXPLOSIVE GAS MIXTURE ACCUMULATIONS ASSOCIATED WITH BWP OFFGAS SYSTD4 OPERATIONS This is in response to your letter to Godwin Williams, Jr. , dated r

February 10, 1978, on the subject bulletin. Our response corresponds l to the related item numbers in the subject ti.11etin.

Item 1

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System Operating Instruction No. 66 ensures that operations are performed in accordance with design parameters. ,

Adninistrative procedures BFS27 and BFM8 describe maintenance procedures '

to prevent fire or explosion in the offgas system.

3tyms 2 and 3 A. Influent to Recombiner

1. Condenser to Steam Jet Air Ejector.(SJAE)

Offgas piping originates from the back of each condenser and enters a large pipe chase before branching into reor.s housing an individual i SJAE. During operation of the unit, this piping as well as the condenser is under negative pressure. Small breaches in system integrity would cause building air to leak into the offgas system increasing the dilution of detonable mixtures. ,

2. SJAE ,

Each SJAE is located in a room connected to the turbine building i

ventilation system. Room air is pulled from an outside corridor

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through the room and into a branch ventilation system exhaust duct.

leak on the discharge side of the' SJAE would release noncondensibles plus dilution steam. The noncondensibles would be diluted further by room air and then by other exhaust streams entering the ventilation system.

Signifi-cant itaks would produce and be detected by increased activity in the .turbin building exhaust duct. A sample from this duct is monitored continuously by a radiation monitor. There are no rupture disks associated with the Browns Ferry Nuclear Plant SJAE's. .

B. Recombiners Both of a unit's recombiners and a common offgas condenser are located in a single room in tho turbine building adjacent to the SJAE's.

Ventilation and leak detection are identical to that of the SJAE's described above. .

C. Effluent from the Recombiner The offgas reconbiners catalytically recombine the explosive mixture of hydrogen and o).7 gen in the offgas stieam. The construction of the offgas system precludes bypassing of the recombiners. The recombiner effluent is thus comprised principally of fission gases and condenser air inicakage. This effluent is monitored to ensure that detenabic mixtures

• are not present. Additionally, recombiner performance is monitored by periodic observation of the recombiner AT. If dilution steam is lost to the SJAE's, a high temperature alarm on each recombiner vill provide control room awareness. Therefore, it is highly unlikely that detonable mixtures will exist downstream of the. recombiners. In all cases, if high air-borne activity is detected in an area, no danger of detonation exists if checks indicate proper recembiner operation and no downstream hydrogen concentration. The following evaluation of the system has been performed.

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1. Recombiner Effluent to Underground Holdup Volume The effluent from the recombiner is routed through a pipe chase and vault room before exiting the turbine building to connect with an underground holdup pipe. Significant leakage in either location v'o uld by diffusion be transported to the building ventilation system.

Once in the ventilation system it would be diluted by other exhaust streams. An increase in activity on the turbine building continuous air monitor would alert operating employees to the presence of a significant leak. Depending on the actual location and size of the l leak,carlier detection might be provided by the area radiation monitors s

located throughout the turbine building.

2. Pretreatment Samnline and Monitoring System This system consists of piping and valves necessary to route a reprocentative sample of the offgas stream to a process radiation monitor and grab sample station. The process radiation monitor is located in a room on the ground floor of the turbine building. The two pretreatment process radiation detectors are located inside the room external to a cylindrical metal sample chamber. Significant leakage inside the room would therefore be readily sensed when the detectors eenitored the presence of unshielded offgas activity.

Additional sample piping and a sacple station are located in a Concrally open area of the turbine building. Leakage originating in this area would be transported to and diluted by the turbine building ventilation system. An area radiation monitor is located near the sample station and could lead to earlier detection of a significant l'eak. Each unit has two offline process hydrogen analy:cra to continuously monitor the concentration of hydrogen in the recombiner

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-h-effluent. A separate building ventilation exhaust duct is provided for each monitor in the event of leakage associated with the instrument!

The monitor 9 sample stream is returned to the condenser; therefore,

, offgas could only escape on the suctico side of the sample pumps.

Significant leakage from this line would disrupt normal sample flow to the monitors and would initiate a control room annunciation.

D. Turbine Building Concrrl Most of the drains asscelated with the offgas system in the turbine building -

discharge to the condenser. The exception is the offgas dehumidification coil  ;

which discharges into the turbine building equipment drain sump. In the unlike"l event the automatic isolation logic fails and the loop seal is lost, offgas activity would escape the sump and be diluted by the turbine building's once- i through ventilation syctem. Significant leakage would be detected by. I increased activity in the turbine building ventilation system or by i annunciation of en area radiation monitor. '

E. Underground Holdup Volt me .

The approximately 8,00C cubic feet of holdup volume is located entirely underground. It was designed for and used with the original offgas system.

The loop seals associated with the' drains frem the hol' dup volumes are routed to a co= mon sump located in the basement of the radwaste building. In the unlikely event the automatic isolation logic fails and a loop seal is lost, activity would exit the sump and be diluted by the radwaste building's once-through ventilation system. Increased airborne activity in the venti-lation system and/or ar nunciation of an area radiation monitor would alert j operations' employees cf the problem. l l l i -

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l- F. Off gas Treatment Building This building houses the offgas pretreatment' HEPA filters and the charcoal (

,adsorber beds associated with all three units. For shielding purposes, individual ventilated vaults house major system components. With the exception of the charcoal adsorber beds, air outside the vault in exhausted  !

by a common exhaust system and discharged into the main stack. A significant leak in any one of these vaults would be diluted by the once-through venti- l

., lation system and could be detected by an unexplained charge in the post-treatment and/or stack gas activity. All six of a unit's charcoal adsorber beds are located in a large vault with a ventilation system indapendent of the of fgas building. Vault air is recirculated through an external heating and cooling system. Each vault 'contains an area radiation monitor to detect any significant leakage. Small sample lines transport offgas from vault rooms to a common grab sample station and unitized process radiation monitors.'

Leakage from the sample station or radiation monitors would be diluted and exhausted by the building's once-through ventilation system. A continuous ,

air monitor is located in the same room with the sa=ple station and could provide early detection of system leakage. Experience has demonstrated that small offgas leaks originating near the process radiation monitors can be detected by significant increases above the monitor's normal readings." Loop seals on drains in the offgas treatment building empty into a common building sunp. The sump is connected to the suction side of the building ventilation exhaust fans and kept under constant negative pressure. Should a loop seal i be lost, offgas leakage would be diluted and removed by the ventilation system.i, Lost loop seals would increase the radiction levels associated with the venti-lation system exhaust ducts. This would be detected during the performance of periodic health paysics radiation surveys.

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C. ,ftack Underground piping transports the offgas from the treatment build'ing to the base of the stack. There the offgas is routed through the post-treatment HEPA filtera. These redundant HEPA filters are located in individual vaults A filter cubicle ex'haust or. cubicles located in the basement of the stack.

system is used to pull air through these vaults and discharges it to mix with the stacks other inputs. A leak in one of these would be diluted by the exhaust inputs from other vault's. A large leak might be detected by a change in the stack gas activity. Once detected the spare filter could be placed i

in service to eliminate the source of the leak. The effluent from the filters is routed by way of a valve room and pipe chase to the third floor of the stack. i Each unit's valve rooms open onto the stack's well-ventilated ground floor. i i

The'offgas system ends with the effluent from the post-treatment filter being

' mixed into the discharge of redundant dilution air fans. At least one of  !

these fans per unit is operated to provide dilucion and upward momentum to i

the offgas. The dilution fans were designed for and used with the crigindl  ;

offgas system. During operation of the system before the addition of the l

recocibiners, these fans demonstrated their ability to maintain hydrogen in the stack effluent below detonabic concentrations. Ofigas loop seals in the stack discharge to a common sump located in the basement of the stack. In f I  !

the unlikely event that automatic isolation logic associated with these drains .

fails and a loop seal is lost, offgas activity would be pulled from the sump and diluted by the same ventilation duct exhausting the HEPA filter cubicles. i The offgas building and each turbine building have redundant ventilation system exhaust fans to minimize the potential for losing airflow through these buildings.

l Based on the revieu performed under item 2 above, no area was discovered with a high probability of accumulating a large undiluted and/or undetected volume

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of detonable gases originating fro m the Browns Terry offgas system. Therefore no changes in plant desil;n are beitg considered.

Item 4 operating Instruction No. 66 ensures that all loop seals are established before the system is placed in operation. If a loop seal is lost while the system is

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operating, it will be detected by an it. crease in radiation or activity level on the nearest area radiation monitor or continuous air monitor. The area radintion ,

monitors and continuous air monitors, located inside the plant, are recorded in the unit control room. An alarm is provided for.high activity levels, Existing plant procedures require the operators and health physics technicians to locate the scuren of the activity and correct or isolate the problem.

Item 5 Operating Instruction No. 16 gives adequate guidelines to ensure that <4% H2 cixtures are maintained in :he offgac piping downstream of the recombiners or the unit sdll be shut down. If an explosien should occur that resulted in high pressure and/or temperature in the offgts system, the operators are instructed to manually scram the reactor. The plant emergency plans manual covets situs-tions for fires or c=plosioas external.to the system piping and outlines operator

. reuponce to fires and explosions. Fires in the charcoal adsorbers are also covered in the energency plans m.tnual.

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