ML17311A461

LER 94-006-00:on 941029,reactor Tripped on Low DNBR Trip Signal Due to Erroneous Penalty Factor.Ceac #1 Processor Board Replaced & Equipment Root Cause Failure Analysis of Processor Board Being performed.W/941123 Ltr

ML17311A461
Person / Time
Site:	Palo Verde
Issue date:	11/23/1994
From:	Grabo B, James M. Levine ARIZONA PUBLIC SERVICE CO. (FORMERLY ARIZONA NUCLEAR
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
192-00914-JML-B, 192-914-JML-B, LER-94-006-02, LER-94-6-2, NUDOCS 9412060114
Download: ML17311A461 (15)

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(ACCELERATED RIDS Pl<OCI'.SSliG I I REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)

ACCESSION NBR:9412060114 DOC.DATE: 94/11/23 NOTARIZED: NO DOCKET FACIL:STN-50-529 Palo Verde Nuclear Station, Unit 2, Arizona Publi 05000529 AUTH. NAME AUTHOR AFFILIATION GRABO,B.A. Arizona Public Service Co. (formerly Arizona Nuclear Power LEVINEiJ.M. Arizona Public Service Co. (formerly Arizona Nuclear Power RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 94-006-00:on 941029,reactor tripped on low DNBR trip signal due to erroneous penalty factor.CEAC gl processor board replaced 6 equipment root cause failure analysis of processor board being performed.W/941123 ltr.

DISTRIBUTION CODE: IE22T COPIES RECEIVED:LTR TITLE: 50.73/50.9 Licensee Event Report (LER), QIncident ENCL 1 SIZE:

Rpt, etc.

NOTES:Standardized plant. 05000529 RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL PD4-2 PD 1 1 HOLIAN, B 1 1 TRAN,L 1 1 INTERNAL: ACRS 1 1 AEOD/iROAB/DSP 2 2 AEOD/SPD/RRAB 1 1 CEILE CENTERS 02 1 1 NRR/DE/EELB 1 1 NRR/DE/EMEB 1 1 NRR/DORS/OEAB 1 1 NRR/DRCH/HHFB 1 1 NRR/DRCH/HICB 1 1 NRR/DRCH/HOLB 1 1 NRR/DRSS/PRPB 2 2 NRR/DSSA/SPLB 1 1 NRR/DSSA/SRXB 1 1 NRR/PMAS/IRCB-E RES/DSIR/EIB 1,, 1 RGN4 FILE 01 1

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1 EXTERNAL: L ST LOBBY WARD 1 1 LITCO BRYCE,J H 2 2 NOAC MURPHY,G.A 1 1 NOAC POORE,W. 1 1 NRC PDR 1 1 NUDOCS FULL TXT 1 1 YOTE TO ALL"RIDS" RECIPIENTS:

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FULL TEXT CONVERSION REQUIRED TOTAL NUMBER OF COPIES REQUIRED: LTTR 28 ENCL 28

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Arizona Public Service Company PALO VERDE NUCLEAR GENERATING STATION P,O. BOX 52034 ~ PHOENIX, ARIZONA85072-2034 192-0091 4-JMQBAG/DLK JAMES M. LEVINE November 23, 1994 VICE PRESIDENT NUCLEAR PADDUCTIDN U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station P1-37 Washington, DC 20555

Subject:

Palo Verde Nuclear Generating Station (PVNGS)

Unit 2 Docket No. STN 50-529 (Ucense No. NPF-51)

Licensee Event Report 9M66~

File: 942th404 Attached please find Licensee Event Report (LER) 94-006-00 prepared and submitted pursuant to 10 CFR 50.73. This LER reports a reactor trip on low Departure from Nucleate Boiling Ratio (DNBR) which was attributed to an apparent malfunction in a Control Element Assembly Calculator (CEAC) processor board. In accordance with 10 CFR 50.73(d), a copy of this LER is being forwarded to the Regional Administrator, USNRC Region IV.

If you have any questions, please contact Burton A. Grabo, Section Leader, Compliance, at (602) 393-6492.

Sincerely, JMUBAG/DLK/pv Attachment cc: L. J. Callan K. E. Perkins K. E. Johnston f cl 07$

0 9412060114 941123 PDR ADOCK 05000529 B PDR

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION AQUTYNAME DOCKET NUMBER LER NUMBER PAGE gEAR SEOUEMINL RENQO NUMBER NUMBER Paio Verde Unit 2 0 5 0 0 0 5 2 9 9 4 <<0 0 6 - 0 0 0 2 OF 0 6 DESCRIPTION OF WHAT OCCUEG&D:

A. Initial Conditions:

On October 29, 1994, at 2044 MST, Palo Verde Unit 2 was in MODE 1 (POWER OPERATION) operating at approximately 100 percent power.

B. Reportable Event Description (Including Dates and Approximate Times of Major Occurrences):

Event Classification: An event that resulted in the automatic actuation of the Reactor Protection System (RPS) (JC).

On October 29, 1994, at approximately 2044 MST, Palo Verde Unit 2 was in MODE 1 (POWER OPERATION), operating at approximately 100 percent power when the reactor tripped on a low Departure from Nucleate Boiling Ratio (DNBR)~ trip signal. The trip signal was initiated from the Core Protection Calculators (CPC) (JC) after processing an erroneous DNBR penalty factor generated by Control Element Assembly Calculator (CEAC) g1 (JC). Required plant equipment and safety systems responded to the event as designed. No Engineered Safety Features (JE) actuations occurred and none were required. The Shift Supervisor diagnosed the event as an uncomplicated reactor trip. By approximately 2115 MST on October 29, 1994, the plant was stabilized in MODE 3 (HOT STANDBY).

Just prior to the reactor trip, several Control Element Assembly Calculator (CEAC) gl Fail and CPC Sensor Failure alarms (IB) were received in the Control Room. One function of the CEACs is to generate' DNBR penalty factor for the CPCs when Control Assembly (CEA) (AA) position deviations are detected. 'lement An apparent malfunction in a processor board located in CEAC gl generated an erroneous DNBR penalty factor causing all 4 CPC channels'o alarm and initiate a reactor trip signal. The reactor tripped on low DNBR. Required plant equipment and safety systems performed as designed. No Engineered Safety Features Actuation System (JE) functions were activated. The

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION AQUlYNAME DOCKET NUMBER LER NUMBER PAGE fEAR SEQUENTIAL NUMBER REIASIO NUMBER Palo Verde Unit 2 0 5 0 0 0 5 2 9 9 4 - 0 0 6 - 0 0 0 3oFO 6 Shift Supervisor diagnosed the event as an uncomplicated reactor trip. By approximately 2115 MST on October 29, 1994, the plant was stabilized in MODE 3 (HOT STANDBY).

C. Status of structures, systems, or components that were inoperable at the start of the event that contributed to the event:

Although CEAC g1 generated an erroneous DNBR penalty factor, there were no indications that any structures, systems, or components were inoperable at the start of the event which contributed to the event.

D. Cause of each component or system failure, if known:

Initial troubleshooting was not successful in isolating a specific failed subcomponent on the CEAC 41 processor board. By evaluating available data (obtained from the auto-restart log Point IDs [PID], sensor status word PIDs, failed sensor stack PIDs, CPC and CEAC trip buffers,, and the plant computer alarm printer logs), Control Room Operator observations, and past CEAC failures, APS Engineering personnel determined that an apparent malfunction in a processor board caused CEAC g1 to generate an erroneous DNBR penalty factor. The CPCs correctly processed the

,erroneous DNBR penalty factor and initiated a reactor trip signal as designed. The processor" board was cpxarantined. An independent investigation of the processor board was initiated to determine the root cause of failure.

E. Failure mode, mechanism, and effect of each failed component, if known:

The apparent malfunction in the processor board resulted in CEAC g1 sending an erroneous penalty factor to the CPCs which, in turn, generated a low DNBR trip signal and initiated the reactor trip.

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION DOCKETNUMBER LER NUMBER PAGE

~R SEQVENAAL NUMBER REVlSO NUMBER Palo Verde Unit 2 0 5 0 0 0 5 2 9 9 4 - 0 0 6 - 0 0 0 4 oF0 6 F. For failures of components with multiple functions, list of systems or secondary functions that were also affected:

Not applicable no failures of components with multiple functions were involved.

G. For a failure that rendered a train of a safety system inoperable, estimated time elapsed from the discovery of the failure until the train was returned to service:

Not applicable no failures that rendered a train of a safety system inoperable were involved.

H. Method of discovery of each component or system failure or procedural error:

Engineering concluded that the processor board located in CEAC g1 had apparently malfunctioned based on an evaluation of available data, Control Room Operator observations, and past equipment failures that exhibited similar failure symptoms.

There were no procedural errors which contributed to this event.

I. Cause of Event:

The cause of the reactor trip was attributed to an apparent malfunction in the processor board located in CEAC g1 which sent an erroneous penalty factor to the CPCs. All 4 CPC channels processed the erroneous penalty factor and generated a low DNBR trip signal (SALP Cause Code X: Other).

J. Safety System Response:

Not applicable there were no safety system responses and none were necessary.

K. Failed Component Information:

The processor board on CEAC $ 1 is a Concurrent Computer Corporation model number 3117150-01, serial number 195.

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION DOCKET NUMBER LER NUMBER PAGE SEOUENllAL PAQO NUMBER NUMBER Palo Verde Unit 2 0 5 0 0 0 5 2 9 9 4 - 0 0 6 - 0 0 0 5 oF0 6 EXT Assessment of the Safety Consequences and Implications of this Event:

The low DNBR trip is provided to prevent the DNBR in the limiting coolant channel in the core from exceeding the fuel design limit in the event of design bases anticipated operational occurrences.

The reactor trip occurred when all 4 channels of CPCs calculated a DNBR value that exceeded the low DNBR trip setpoint. The CPC calculated DNBR resulted from an erroneous penalty factor generated in CEAC g1 due to an apparent malfunction in the processor .boar'd. The actual DNBR safety limit was not exceeded.

Primary and secondary pressure boundary limits were not approached due to the reactor tripping from a steady state condition, followed by a "quick open" of the Steam Bypass Control System (JI). The transient did not cause any violation of the Specified Acceptable Fuel Design Limits. Therefore, there were no safety consequences or implications as a result of this event. This event did not adversely affect the safe operation of the plant or health and safety of the public.

CORRECTIVE ACTION:

A. Immediate:

Control Room Operator action was taken to place the reactor plant in a stable condition in accordance with the appropriate operating procedures.

Initial troubleshooting was conducted but was not successful in isolating a specific failed subcomponent. APS Engineering evaluated available data, Control Room Operator observations, and past CEAC failures, and determined that an apparent malfunction in a processor board caused CEAC g1 to generate an erroneous DNBR penalty factor. The CPCs correctly processed the erroneous DNBR penalty factor and initiated a reactor trip signal. The processor board was quarantined. An independent investigation of the event and a separate independent

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION AQUlYNAME DOCKET NUMBER LER NUMBER PAGE

'fEAR SEOUENllAL NUMBER NUMBER Palo Verde Unit 2 0 5 0 0 0 5 2 9 9 4 - 0 0 6 - 0 0 0 6 OF 0 6 investigation for the processor board are being conducted in accordance with the APS Incident Investigation Program.

B. Action to Prevent Recurrence:

The CEAC gl processor board was replaced.

As part of the investigation, an ERCFA of the processor board is being performed by APS Engineering. The preliminary evaluation has not identified any specific subcomponent failure(s) which would cause the CEAC processor board to malfunction. If information is developed that would significantly affect the readers'nderstanding or perception of this event, a supplement will be submitted.

V. PREVIOUS SIMILAR EVENTS:

A reactor trip resulting from a CEAC generating an erroneous DNBR penalty factor was previously reported in LER 528/89-004. The root cause of the 1989 reactor trip was identified, through troubleshooting, as a failure of a CEAC processor board's arithmetic logic unit integrated circuit.

The ERCFA being performed on the processor board from CEAC 41 for this event (LER 529/94-006), has not isolated the cause(s) of the malfunction. Both events were similar in that the reactor tripped on low DNBR due to erroneous penalty factors; however, a similar cause for the CEAC processor boards to generate an erroneous penalty factor has not been established.