ML20195G809

Forwards Response to RAI Made During 990511 Telcon Re LARs 184 & 170 for SONGS Units 2 & 3.Amend Applications Proposed Restriction on Operation with Channel of RAS or EFAS in Tripped Condition

ML20195G809
Person / Time
Site:	San Onofre
Issue date:	06/14/1999
From:	Nunn D SOUTHERN CALIFORNIA EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9906160170
Download: ML20195G809 (5)

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/ Ey SOUTHERN CALIFORNIA L

EDISON

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' Ancmsougmwouu-cmip=y June 14, 1999 U. S. Nuclear Regulatory Commission i

Attention:' Document Control Desk L

. Washington D.C. 20555 Gentlemen:

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Subject:

Docket Nos. 50-361 and 50-362' l

Response to Request for Additional Information Regarding Amendment Applications 184 and 170 RAS /EFA.S Channels in Tripped Condition San Onofre Nuclear Generating Station, Units 2 and 3 -

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Reference:

Letter dated December 31,1998, from D. E. Nunn (SCE) to Document i

Control Desk (NRC),

Subject:

Docket Nos. 50-361 and 50-362, Amendment L

' Application Nos.184 and 170, RAS /EFAS Channels in Tripped Condition, l

San Onofre Nuclear Generating Station Units 2 and 3 l

l Provided as an Enclosure is a response to an NRC request for additional information l

regarding Southern California Edison's (SCE's) license Amendment Applications 184 and l

170 (Referenced) for the San Onofre Nuclear Generating Station, Units 2 and 3. These amendment applications proposed a restriction on operation with a channel of

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Recirculation Actuation Signal (RAS) or Emergency Feedwater Actuation Signal (EFAS) in//

the tripped condition. The request for additional information was made during a telephone call on May 11,1999, if you have any additional questions on this subject, please call me or Jack Rainsberry at (949) 368-7420.

l Sincerely,

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t Enclosure cc:

E. W. Merschoff, Regional Administrator, NRC Region IV '

J. A. Sloan, NRC Senior Resident inspector, San Onofre Units 2 & 3 L. Raghavan, NRC Project Manager, San Onofre Units 2 and 3

- S. Y. Hsu, Department of Health Services, Radiologic Health Branch 9906160170 990614 PDR ADOCK O'JOOO361 P. O. Box 128

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PDR-San Clemente, CA 92674-0128

.sT4d 949-368-1480

• g,,i-Fax 949-368-1490

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ENCLOSURE' RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING AMENDMENT APPLICATIONS 184 AND 170 I

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,r* L f.n the discussion of the risk impact of placing one channel of RAS in trip, why were LOCA events the only initiating events evaluated?

l Southern California Edison (SCE) Response: The risk assessment performed to evaluate the impact of placing a Recirculation Actuation Signal (RAS) channel in trip included those initiating events which met the following criteria:

1) the initiating event must require safety injection for accident mitigation, AND i

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2) the post-accident containment pressure must be great enough to force the High Pressure l

Safety Injection (HPSI) pumps to take suction from the dry containment emergency sump

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instead of the Refueling Water Storage Tank (RWST). This would cause the HPSI pumps to fail due to gas binding.

Of all the accident initiating events evaluated in the San Onofre Nuclear Generating Station (SONGS)

Units 2 and 3 Probabilistic Risk Assessmen+ (PRA), only Steam Generator Tube. Ruptures (SGTRs) and Loss of Coolant Accidents (LOCAs) (including plant transients with induced pressurizer safety valve failures to close) require safety injection (Criteria 1). However, Criteria 2 is not met in the case l

of a SGTR event since the post-SGTR containment pressure will not e: ceed that of the RWST.

Therefore, only LOCA initiating events were evaluated for the risk impact of placing a channel of l

RAS in trip.

2. Explain how the CDF of 5.4E-6 was obtained.

SCE Response: The SONGS 2/3 PRA model contains a detailed Plant Protection System (PPS) model based on a Combustion Engineering generic PPS fault tree logic (CEN-327) which was later I

customized for San Onofre Units 2 and 3. Converting the PPS fault tree logic to address spurious signal actuations, a fault tree was developed to model postulated LOCA initiating event scenarios

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coincident with the dominant failures of the remaining 3 channels leading to an inadvertent RAS.

j The component failure rates used in the fault tree were taken from the SONGS 2/3 Living PRA and the Nuclear Computerized Library for Assessing Reactor Reliability (NUCLARR) Generic Component Failure Rate Database for Light Water and Liquid Sodium Reactor PRAs, Report l

EGG-SSRE-9975, Febmary 1990, prepared by EG&G Idaho.

1 Solving this fault tree, the conditional core damage frequency (CDF) due to an inadvertent RAS i

following a LOCA event when a RAS channel is in trip was estimated to be 1.6E-5/yr. This value is a conditional CDF based en an assumed exposure time of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> where any one of the other three channels could inadvertently trip and cause an inadvertent RAS. However, an inadvertent RAS actuation has unacceptable consequences following a LOCA event only before a valid automatic RAS actuation occurs. The exposure time for the other three RAS channels is the time interval between initiation of the LOCA event and the switch over from injection to recirculation. Based on Modular Accident Analysis Program (MAAP) calculations, the longest time interval between LOCA initiation and valid RAS actuation for a LOCA is conservatively estimated to be 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. LOCAs where the switch over to recirculation would be greater than 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> would either utilig shutdown cooling instead of post-LOCA recirculation aRer 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> or result in a containment sump inventory sufficient

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., after 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> to prevent containment gas entry into the HPSI suction lines. Therefore, the conditional CDF of 1.6E-5/yr is multiplied by the ratio of 8/24 to reflect the more realistic 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> exposure time for RAS channel random failures, resulting in a conditional CDF of 5.4E-6/yr when a RAS channel is placed in trip.

The 5.4E-6/yr conditional CDF represents the incremental increase in core damage risk if one channel of RAS were to remain in trip for the entire year. During normal power operations, if a PPS channel fails or requires testing or maintenance, it is San Onofre's practice to place the channel in bypass (not in trip). The only time it is anticipated that a channel would be placed in trip is when one channel has failed and one of the three remaining channels requires testing. In this case, the failed channel would be placed in trip, while the other channel is placed in bypass for testing purposes. The amount of time the plant would be placed in this configuration (one channel in trip /one channel in bypass) is substantially less than a year. Therefore, the annual increase in risk would in actuality be a fraction of the 5.4E-6/yr reported.

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Subscribed on this Kl day of kz a -

, 1999.

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0 Respectfully submitted, SOUTHERN CALIFORNIA EDIS0N COMPANY f

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By: (

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Dwi[E.Nunn Vice President State of California County of San Diego On b.L 14 1999 before me,

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-Mb, personally appea ed Don (hb E. ManrL

,pe[donallyknowntometobethepersonwhose name is subscribed to the within instrument and acknowledged to me that he executed the same in his authorized capacity, and that by his signature on the instrument the person, or the entity upon behalf of which the person acted, executed the instrument.

WITNESS my hand and official seal.

Eefuses sessynase-omuns My CE Eg*w R$31 MWk Signature 0

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