ML17311A301

LER 94-005-01:on 940619,CPC Channel D Was Declared Inoperable.Caused by Hot Leg Temperature Anomaly.Cpc Channel D Was Calibrated & Declared operable.W/940907 Ltr

ML17311A301
Person / Time
Site:	Palo Verde
Issue date:	09/07/1994
From:	Grabo B, James M. Levine ARIZONA PUBLIC SERVICE CO. (FORMERLY ARIZONA NUCLEAR
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
192-00905-JML-B, 192-905-JML-B, LER-94-005, LER-94-5, NUDOCS 9409280243
Download: ML17311A301 (16)

Text

(ACCELER:KTED RIDS PROCI;SSIi' REGULATORY INFORMATION 'DISTRIBUTION SYSTEM (RXDS)

ACCESSION NBR:9409280243 DOC.DATE: 94/09/07. NOTARIZED: NO DOCKET FACIL:STN-50-528 Palo Verde Nuclear Station, Unit 1, Arizona Publi 05000528 AUTH. NAME AUTHOR AFFILIATION GRABO, B.A. Arizona Public Service Co. (formerly Arizona Nuclear Power LEVINEgJeMo Arizona Public Service Co. (formerly Arizona Nuclear Power RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 94-005-01:on 940619,CPC channel D was declared inoperable. Caused by hot leg temperature anomaly.CPC channel D was calibrated & declared operable.W/940907 ltr.

DISTRIBUTION CODE: IEZZT COPIES RECEIVED:LTR ENCL Z SIZE: 2 LIncident TXTLE: 50.73/50.9 Licensee Event Report (LER), Rpt; etc.

NOTES:STANDARDIZED PLANT 05000528 RECIPIENT COPIES'TTR RECIPIENT COPIES ID CODE/NAME ENCL ID CODE/NAME LTTR ENCL PD4-2 PD 1 1 HOLIAN, B 1 1 TRAN,L 1 1 INTERNAL: ACRS 1 1 AEOD/-ROA'B/DSP 2 .2 AEOD/SPD/RRAB 1 1 ZZZ CENTER=OO~.i 1 1 NRR/DE/EELS 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 RES/DS IR/EIB 1

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FULL TEXT CONVERSXON REQUIRED TOTAL NUMBER OF COPIES REQUIRED: LTTR 28 ENCL... 2'8.

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Arizona Public Service Company PALO.VERDE NUCLEAR GENERATING STATION P.O, 8OX 52034 ~ PHOENIX. ARIZONA 85072-2034 JAMES M. LEVINE 192-00905-JM4'BAG/R JR VICE PREEIOENT NUCLEAR PRODUCTION September 7, 1994 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Mail Station P1-37 Washington, DC 20555

Subject:

Palo Verde Nuclear Generating Station (PVNGS)

Unit 1 Docket No. STN 50-528 (License No. NPF-41)

Licensee Event Report 94-005-01 File:. 94-020-404 Attached please find supplement 1 to Licensee Event Report (LER)94-005 prepared and submitted pursuant to 10CFR50.73. This supplement provides additional information on the cause, corrective actions, and safety significance of the fluctuations in the Core Protection Calculator Delta-T Power reported in the original submittal. The August 19, 1994, supplement submittal date was extended to. September 16, 1994, by Howard Wong, Branch Chief, Reactor Projects (Telecon August 16, 1994). In accordance with 10CFR50.73(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, Supervisor, Nuclear Regulatory Affairs, at (602) 393-6492.

Very truly yours, JM4'BAG/RJR/rv Attachment cc: W. L. Stewart (all with attachmen

~p L. J. Callan K. E. Perkins K. E. Johnston INPO Records Center I

5'40 j2B024~ 940907 PDR ADOCK 05000528 F'DFi gpss

0 LICENSEE EVENT REPORT (LER)

I ACILITYHAMI((I) DOCKET NUMBER (2) PAGE (3)

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LICENSEE CONTACT FOR THIS LER (12)

I IAMB TELEPHONE NUMBER Burton A. Grabo, Supervisor, Yuclear Regulatory Affairs A AC 60'7 c

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At approximately 2258 MST on June 19, 1994, Control Room personnel determined that the Core Protection Calculator (CPC) Channel D, Delta-T Power signal could no longer be adjusted to within +/- 2 percent of actual power as deeermined by secondary plant calorimetric and required by Technical Specification (TS) 3.3.1, Table 4.3-1, Notaeion (2). CPC Channel D was declared inoperable. This condition was caused by loop 2 Hot leg temperaeure (T-h) Resiseance Temperature Detector (RTD) fluctuations. Fluctuations in T-h RTDs have been identified ae other planes and attributed to temperature stratification. On June 29, 1994, after reviewing plant logs, the condition was determined to be reportable. The loop 2 (T-h) Resistance Temperature Detector (RTD) creating the Delea-T power fluctuations was electronically switched with the T-h RTD used in the Core Operating Limits Supervisory System. At approximately 1812 MST on July 2, 1994, CPC Channel D was calibrated and declared OPERABLE. The event did not adversely affect safe operation of the plant.

No previous similar evenes have been reported pursuant to 10CFR50.73.

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION PACll.ltY t<A<<r. nOC<lEV RV<<EEn LER NUMBER PACE

~ SEOVEVT<AL REVIS<OR

/EAR <JVMEER RVMEER PRIO VerCIR Unit

'l P v 'f osooo QF0 6 DESCRIPTION OF WHAT OCCURRED:

Initial Conditions:

At approximately 2258 HST on June 19, 1994, Palo Verde Unit 1 was in Hode 1 (POWER OPERATION) at normal operating temperature and pressure.

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

Event Classification: Operation prohibited by the plant's Technical Specifications (TS)

At approximately 2258 HST on June 19, 1994, Unit 1 Control Room personnel (utility, licensed) determined that,, due to frequent fluctuations of approximately 5 percent in Core Protection Computer (CPC) Channel D, Delta-T Power signal, Delta-T power could no longer be adjusted to within +/- 2 percent of actual power as determined by secondary plant calorimetric and required by TS 3.3.1 Table 4.3-1 Notation (2). CPC Channel D was declared inoperable and placed in by-pass.

TS Limiting Condition for Operation (LCO) 3.3.1 Action 2 states that STARTUP and/or POWER OPERATION may continue with the number of channels OPERABLE 1 less than the total number of channels, provided the inoperable channel is placed in the bypassed or tripped condition within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. This TS LCO Action also requires that the desirability of maintaining the channel in bypass be reviewed in accordance with TS 6.5.1.6.g and returned to OPERABLE status no later than during the next COLD SHUTDOWN.

During the performance of the calibration of CPC Channel D Delta-T Power on June 19, 1994, questions were raised as to the validity of the calibration by a Control Room operator (utility, licensed).

Prior to June 19, 1994, the CPC Delta-T Power signals had been averaged by Control Room personnel for determining agreement with the calorimetric. The averaging of this fluctuating signal was not proceduralized or a subject of formal operator training. The initial investigation identified that the method of determining an average for this signal varied between operators.

Il II LICENSEE EVENT REPORT (LER) TEXT CONTINUATION rnC1Ln Y11AMr. OOCt:ET RUMOEll LER NUMBER PAGE v EAR 'EOUEUT1AL REV<S<OU UUL1eER UUMEER Pain VerCle Unit 0 G 0 0 0 g"89 O O> O>>~OFO 6 TEXT The validity of the calibration was questioned because the Control Room operator felt the fluctuations had..become larger than before; and even after adjusting the average signal to within +/- 2 percent of actual power, the fluctuations would Delta-T Power signal on CPC Channel D to periodically swing still cause the outside its allowable band of +/- 2 percent power. Because of this and the lack of written guidance on how to obtain an average signal, it was determined by the Control Room staff that the fluctuations should not be averaged. Thus, CPC Channel D Delta-T Power could not be calibrated as required and the channel was declared inoperable. During the investigation a review of past data on CPC Channel D Delta-T power showed that fluctuations of the same magnitude have existed for several fuel cycles.

To calculate Delta-T power, the CPCs monitor several primary plant parameters. One of those parameters is Hot Leg temperature (T-h).

T-h data is received from 8 Resistance Temperature Detectors (RTD). The RTDs are arranged with 4 in each hot leg. Each hot leg is a 42 inch diameter pipe. The RTDs are inside of thermal wells which protrude approximately 3 inches into the process stream. The thermal wells are located radially around the hot leg in approximately the same plane. An additional thermal well and RTD exists 10 inches closer to the steam generator inlet and is used as an 'input to the Core Operating Limits Supervisory System (COLSS).

Each CPC channel receives one T-h signal from each Hot Leg. For example, CPC Channel D receives T-h inputs from 1 Loop 1 RTD and 1 Loop 2 RTD. These signals are combined with signals from cold leg temperature, mass flow rate, and Reactor Coolant System (RCS) pressure to produce the Delta-T Power calculation. The Delta-T Power calculation is one input to the Maximum Power Calculation auctioneering algorithm.

The Maximum Power Calculation auctioneering algorithm also receives two other power signals (Neutron Flux and a 20 percent minimum signal) and auctioneers the highest of the 3 powers. The output of the Maximum Power Calculation is used in calculating Departure from Nucleate Boiling Ratios (DNBR) and Local Power Density (LPD). The same T-h signals are also used as an input to the Auxiliary Trip logic for the generation of an auxiliary Hot Leg Saturation Trip. The auxiliary trip is provided when the highest T-h, including uncertainties, reaches or exceeds the saturation temperature.

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACIETTy tlhAIF ~

DOCKET NUMBER VER IIVMBER PACE yt'AR ' SEOVENTIAL REVISICIII, tIUMBEIT c.".NUMBER Polo Verde Unit I TEXT osooo 0,0 5 0~>oFO 6 Piior to declaring CPC"Channel D inoperable, Unit 1 was operating a 86.08 percent actual power as determined by secondary plant calorimetric. The CPC Channel D thermal power indications were observed to be between approximately 81.6 and 86,.6 percent power even though Unit power was not varying. This was attributed to fluctuati:ons in one of the T-h RTDs (loop 2) which supply a signal to CPC Channel D Delta-T Power, a condition that has existed for several fuel cycles.

Prior to this event, APS Engineering had been investigating these fluctuations since January, 1991, when ABB Combustion Engineering (ABB-CE) responded to a request by APS for an explanation of a T-h anomaly which had been observed in Unit 1,. Loop-1. The response described the normal coolant temperature stratification effects which occur in reactor hot legs and the effects and postulated cause for the phenomenon known as the Hot Leg Temperature Anomaly.

Hot Leg Temperature Anomaly is a phenomenon observed in the RCS hot legs where temperatures .measured at the same distance from the core exit, but at different radial locations, may differ by several degrees. Although flow at the core exit is highly turbulent and mixing is expected to be complete, hotter water from the center of the core and colder water from the periphery do not always mix completely. This shows as varying temperature readings.

ABB-CE's response concluded that the T-h trends in 'PVNGS Unit 1 were consistent with trends observed in other ABB-CE reactors.

This caused APS Engineering to follow the changes in T-h RTD fluctuations and conduct several reviews to verify that the RTDs were accurately responding to temperature changes. At no .time during these investigations did the results identify problems specific to the instrumentation.

On February 18, 1994, the investigation team concluded that:

~ The RTDs were. functioning correctly,,

~ There was no safety concern since the fluctuations did not cause the CPC system to perform its safety function in a non-conservative manner,, and based on the available industry information and plant observations, the temperature variations seen in the hot legs .were being caused by thermal-hydraulic effects of the phenomenon known as "Hot Leg Anomaly" or "Hot Leg Stratification."

LICENSEE EVENT REPORT(LER) TEXT CONTINUATION rAr;rr.rrY rrn>rr; nocrrrr<vMpER EEA NUMBER PAGE AFAR EEOrrsrr7rAE REQIPIQ'I rrrrlrpER rIUMPER P;llO Ver(je Unjt IEXT 0IG 0 OIO 0 105 OF 0 6 When CPC Channel D was declared inoperable on June 19, 1994, a temporary modification, which substituted the COLSS Loop 2 T-h RTD with the CPC Channel D T-h RTD, was initiated. The loop 2 T-h RTD creating the Delta-7 power fluctuations was electronically switched with the COLSS T-h RTD. CPC Channel D was successfully calibrated and was declared OPERABLE at approximately 1812 MST on July 2, 1994.

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

Not applicable - no structures, systems, or components were inoperable at the start of the event which contributed to this event.

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

Based on extensive testing and analysis of the T-h RTDs, the RTDs are functioning correctly. The variance seen in T-h temperature by CPC Channel D is due to thermal stratification anomalies occurring at the location of this T-h RTD.

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

There were no component failures.

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.

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 - there were no failures that rendered a train of a safety system inoperabl'e were involved.

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

Not applicable - there have been no component or system failures or procedural errors identified. There were no procedural errors which contributed to this event.

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LICENSEE EVENT, REPORT (LER) TEXT CONTINUATION FAI II.ITT nOGIIF T IIVMaFn LER NUMBER PAGE IIAMI'alo

'YEAR, EECVEIITIAL AEVISIOII RVMEER IIVMEER Verde Unit TEXT O 5 OlOIOl =

' 0 0I' O.: n6 OF 0 6 I. Cause of Event:

When the condition of fluctuating T-h RTD'ignals was first identified in 1991, APS determined that fluctuations were not safety significant in that the trips provided by the CPCs were not negatively effected. APS continued to investigate and'eview the condition and based on the best available industry and'lant information, the cause of the T-h fluctuations is attributed to a phenomenon known as Hot Leg Temperature Anomaly (SALP Cause Code X: Other). An investigation of this event is continuing. If information is developed which would affect the reader' understanding or perception of this event, a supplement will be submitted.

Safety System Response:

iNot applicable - there were no safety system responses and none were necessary.

Failed Component Information:

Not applicable - no component failures were involved.

II. ASSESSMENT OF THE SAFETY CONSEQUENCES AND IMPLICATIONS OF THIS EVENT:

The CPC uses T-h indications for calculating the Hot -Leg Saturation Trip, Thermal Power, and RCS Flow.

The Hot leg Saturation Trip includes a 13 degree uncertainty which is sufficient to offset the observed abnormal behavior and prevent hot leg saturation even with a large temperature difference among RTDs.

The T-h signals used by the CPCs to calculate thermal power are first averaged. The calculated thermal power is then compared to a calculated, reactor power based on neutron flux density (when above 20 percent power). The higher of the two calculated signals is used in the calculation of DNBR and LPD. Because of this, the likelihood that the averaging techniques used by the Control Room operators would have had an adverse affect on DNBR and LPD is significantly reduced.

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