ML17305B462

Forwards Info Provided to INPO Re Two Major Contributors to Core Damage Frequency in PRA Discovered During Individual Plant Exams Per Generic Ltr 88-20.Plant Change Requests Initiated to Address Reduction in Transient Effects

ML17305B462
Person / Time
Site:	Palo Verde
Issue date:	04/02/1991
From:	Conway W ARIZONA PUBLIC SERVICE CO. (FORMERLY ARIZONA NUCLEAR
To:	Martin J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V)
References
GL-88-20, NUDOCS 9104160376
Download: ML17305B462 (8)

Text

ACCELERATED DISTRIBUTION DEMONSTRATIONS SYSTEM

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REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)

. ACCESSION NBR:9104160376 DOC.DATE: 91/04/02 NOTARIZED: NO DOCKET FACIL:STN-50-528 Pal'o Verde Nuclear Station, Unit 1, Arizona Publi 05000528 STN-50-529 Palo Verde Nuclear Station, Unit 2, Arizona Publi 05000529 STN-50-530 Palo Verde Nuclear Station, Unit 3, Arizona Publi 0500'0530 AUTH. NAME'UTHOR Arizona Public Service Co. (formerly Arizona Nuclear Power AFFILIATION'ONWAY,W.F.

R RECIP.NAME RECIPIENT AFFILIATION MARTIN,J.B. Region 5 (Post 820201) .I

SUBJECT:

Forwards info provided to INPO re two major contributors to D core damage frequency in PRA discovered during .individual plant exams per Generic Ltr 88-20.Plant change requests S' initiated to address reduction i.'n transient effects.

DISTRIBUTION CODE: IEOID TITLE: General (50 COPIES RECEIVED:LTR Dkt)-Insp Rept/Notice of Violation J ENCL g SIZE:

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NOTES:STANDARDIZED PLANT 05000528 Standardized plant. 05000529 Standardized plant. 05000530 D

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RECEfVED NRC REGloI! V Arizona Public Service Company P.O.SOX53999 ~ PHOENIX,ARIZONA85072-kY9f AFi> -0 A>f f f: 07 WILLIAMF. CONWAY

.EXECUTIVE VICE PRESIDENT NUCLEAR April 2, 1991 Mr. John B. Martin Region V Regional Administrator U. S. Nuclear Regulatory Commission 1450 Maria Lane, Suite 210, Walnut Creek, California 94596-5368,

Dear Mr. Martin:

Enclosed is a copy of the information we provided INPO'o that it could be transmitted via their information network.

I indicated we would do so during: the meeting with you and members of your staff on March 15, 1991.

Sincerely,.

WFCnbs Enclosure 9104160376 910402 F'DR F'.

ADGCK 0.=00052:=:

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'SUBJECT PROBABLISTIC RISK ASSESSMENT IDENTIFIED TWO MAJOR CONTRIBUTORS TO CORE DAMAGE FRE UENCY PL'ANT PALO VERDE 1 g 2 g & 3 ('PWR/CES ) 'EVENT'ATE ARiZONA PUBLIC SERVICE CO. ~LOG-" : OE N/A'OURCF.:

4476 EVENT:

During the performance of our Individual Plant Examination (IPE),

per Generic Letter 88-20, Palo Verde Nuclear Generating Station's Probablistic Risk Assessment (PRA) identified, two major contributors to core damage frequency,(CDF)., The combined CDF for these two events without taking credit for operator recovery is 8x10 E-4/Rx year.. The two initiating events resulting in the CDF contributions are: Loss of Class 1E'C Control Power Channel 'A',

and Loss of Ventilation (cooling) to the Class 1E DC equipment rooms. The high CDF results from both the potential transient impact on the plant and the loss of the plant"s capability to mitigate the transient effects.

The design of the Main Steam and Feedwater Isolation valves at Palo Verde is configured to actuate closed'hen a single channel of DC control power is lost. This results in a simultaneous load rejection and loss of feedwater.

The plant's ability to mitigate this potential transient is impacted by the initiator. One entire train of safety equipment (ECCS pumps, Auxiliary Feedwater and support systems including the field flash for the Train 'A'DG) would be disabled. In the case of Channel 'A'C power loss, two of the three .Auxiliary Feedwater pumps are impacted. Auxiliary Feedwater is required to remove decay heat. A random failure in-the remaining train of Auxiliary Feedwater would lead to complete loss of,a saf'ety function.

Operator action, both inside and outside the Control Room, is

-required to mitigate postulated consequences.

Plant Change Requests (PCRs) have been initiated to address both the reduction in the transient effects of these initiators and the plant's ability to mitigate them.

Interim compensatory measures are being taken before permanent plant changes are implemented. These measures include staging temporary ventilating equipment to be used in the event of loss of ventilation to the DC equipment rooms; 'perator training to sensitize the operators to the importance and complexity of addressing these events; and changes to the emergency operating procedures, including the Function Recovery Procedure, to ensure local, manual action can occur early enough to avoid postulated core uncovery. This included a new instruction for local manual operation of the steam-driven Auxiliary Feedwater pump w'thout DC control power.

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As stated in GL 88-20, licensees are, expected to move expeditiously to correct any identified vulnerabilit'ies that. warrant correction.

The scenarios identified by the PRA team were determined by APS to be significant and immediate actions are being taken to ensure reliable and safe operation of the Palo Verde Units.

4'ith the compensatory measures in place and an aggressive -plant change implementation schedule, APS is confident that pl'ant risk has been reduced. These findings further demonstrate that the IPE process is an effective tool in identifying .dependent failures and subtle event scenarios that may contribute significantly to calculated core damage risk.

APS has discussed. the measures being taken with NRC. 'This Operating Experience (OE) report alerts all other li;censees to similar event scenarios. at the'ir plants and the need to take expeditious measures to reduce calculated core damage risk significant scenario is identified through the IPE process.

if any If you would like more detailed information on this issue, please contact the individual listed below.

INFORNATION CONTACT.: Chuck Stevens, (602') 340-4081

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