LD-96-009, Part 21 Rept Re Potential Instrumentation Decalibr at Low Power.Affected Utils Should Ensure That Log Power Channels Properly Calibr & cross-correlated to Linear Power Channels at 100% Reactor Power

Part 21 Rept Re Potential Instrumentation Decalibr at Low Power.Affected Utils Should Ensure That Log Power Channels Properly Calibr & cross-correlated to Linear Power Channels at 100% Reactor Power

ML20108C709
Person / Time
Site:	Palo Verde, Arkansas Nuclear, Waterford, San Onofre
Issue date:	04/15/1996
From:	Rickard I ABB COMBUSTION ENGINEERING NUCLEAR FUEL (FORMERLY, ASEA BROWN BOVERI, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
REF-PT21-96 LD-96-009, LD-96-9, NUDOCS 9605070110
Download: ML20108C709 (8)

Text

JP,R IN'96 17: 48 FR CE OWNERS GROUP 203 285 2337 TO 13018165151 P.01/01 P% EN ED April 15,1996 LO-96-009 Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555

Subject:

Report of a Deviating Condition in a Reactor Trip Function

Dear Sir:

Combustion Engineering. Inc. (ABB-CE) hereby notifies the Nuclear Regulatory I Commission of a condition or circumstance involving a reactor protection system trip function which has the potential to contribute to the exceeding of a safety limit. The deviating condition evaluated involves nuclear instrumentation decalibration factors, which may not have been adequately compensated for fully when originally setting the  ;

trip setpoint, and which, therefore, have the potential to adversely affect the High '

Logarithmic Power channel trip function. The uncompensated decalibration factors are potentially nonconservative in nature and could, therefore, cause a trip, if needed, to )

cccur at a power level higher than that accounted for in safety analyses.

Evaluations were performed by ABB-CE to determine if this situation would in fact result in exceeding any safety limita. Based on the result of these evaluations ABB-CE has ,

coccluded that because of the discretionary conservatism built into the input parameters l i

for the one affected safety analysis, that the original High Logarithmic Power trip setpoint remains acceptable without creating the potential to exceed a safety limit. ABB-CE has chosen to report this condition because the situation could conceivably be )

applicable to other NSSS vender designs utilizing a reactor protection system trip for )

suberitical events based upon absolute power if nuclear instrumentation decalibration factors are not accounted for appropriately. The Enclosure provided herewith summarizes the information available to ABS-CE regarding the subject condition.

9605070110 960415 t PDR ADOCK 05000361 S PDR ABB Combustion Engineering Nuclear Power C ran:ce ! n7meerrg. Inc. P.O Bamto Teigrena 90) 6%101 2000 Cay H l. Pd Faz (640) 2454203 v.,ndzr. CT CM954500

[1PR.15'96 17:31 FR CE OWNERS GROUP 203 285'2737 TO 13010165151 P.02/07 Doc'ument' Control Desk LD-96-009 -

l . April 15,1996 Page 2 l

. If you have any questions, please feel free to contact me or Mr. Chuck Molnar of my staff at (203) 285-5205.

Very truly yours, COMBUSTION ENGN ING, INC.

l 1. C. r Director j Operations Licensing l

l l

Enclosure:

As stated l'

cc: R. S. Siudek (ABS-CE) l l

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,PR lS96 17:31 FR CE OWNERS GROUP 203 285 2337 TO 13018165151 P.03/07 ENCLOSURE 1

ABB Combustion Engineering Nuclear Operations Report of a Deviating Condition in a Reactor Trip Function April 15,1996 l

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! ,P R 15'96 17:31 FR CE OUNERS GROUP 203 285 2337 TO 10018165151 P.04/07 l

ABB Combustion Engineering Nuclear Operations Report of a Deviating Condition in a Reactor Trip FuncRon 1

(i) Name and addrcss of the individuals informing the Commission:

1. C. Rickard, Director i Operations Licensing I Combustion Engineering, Inc.

2000 Day Hill Road Windsor. CT 06095-0500 I

(ii) Identification of the facility, the activity, or the basic component supplied or such facility or such activity within the United States which fails to comply or contains a defect:

The condition being reported was initially identified at the Waterford Steam Electac Station Unit No. 3 (WSES-3). Specifically, the deviating condition involves nuclear instrumentation decalibration factors, which may not have been adequately compensated for fully when originally setting the trip setpoint. These decalibration factors are related to the reactor High Logarithmic Power channel trip function and are potentially nonconservative in nature.

(iii) Identification of the firm constructing the facility or supplying the basic component which fails to comply or contains a defect:

Combustion Engineering, Inc.

2000 Day Hill Road Windsor, CT 06095-0500 l

l (iv) Naturc of the defcct or failure to comply and the safety hazard which is created or could be created by such dcicct or failure to comply:

Although the High Logarithmic Power inp functron is only credited at low power levels (from 510" % to 2% rated power), the log channelinstrument calibration is routinely l performed at 100% power. Cahbrating the log channels at near full power cond:tions,

! however, does not account for the effects of lower primary coolant temperatures, higher boron concentrations, changes in control rod position (a!) rods in), etc., when a plant is operating in Mode 2 suberitical or Mode 3, the potential condition at the initiation of the subcritical transients.

The effects of the low power, low RCS temperature or more heavily rodded condition are to reduce the neutron flux leakage to the excore detectors. Therefore, the actual power in the core could potentia!!y be higher than the analyticallimit utiltred in the safety analysis when a High Logarithmic Power trip cr a CPC Zero Powcr Bypa is removal occurs.

An investigation of the decalibration effccts identified cycle specific core design differences that were not fully compensated for in inc ongnal evaluation of the log power inp setpoint.

P.R 15*9G 17:32 FR CE OWNERS GROUP 203 285 2337 TO 13018165151 P.05/07 1

These effects result in a lower flux reaching the excore detectors for a given power level.

The investigation showed that conditions exist at low power, where the High Log Power trip is required, that could introduce nonconservative factors relative to the full-power conditions where the instrurnents are calibrated.

Tripping the reactor at a power level higher than that analyzed could represent "A condition or circumstance...that could contribute to the exceeding of a safety limit . '

(v) The date on which the information of such defect or failure to comply was obtained:

ABB-CE concluded that NRC notification may be warranted on April 15,1996.

(vi) In the case of a basic component which contains a defect or fails to comply, the number and location of all such components in use at, supplied for, or being supplied for one or more facilities or activities subject to the regulations in this part:

The potential for low power decalibration may adversely affect ABB-CE designed NSSS units with Logarithmic Power Level trips (digital plants) as opposed to Start-up Rate trips (analog plants); the affected digital plants include ANO-2, SONGS-2 & 3. Waterford-3, Palo Verde-1,2 & 3 and Yonggwang-3 & 4.

ABB-CE plants with analog protection systems rely upon a Start-up Rate trip; where the trip is based upon how fast the signal changes in a given time period (decades per

minute) versus tripping on an absolute magnitude of the signal (percent power). While the input signal to this trip would still be subject to the same decalibration effects, the relative chango in signal (decades per minute) for any given rate of approach to critical i would not be adversely impacted. As such, the protective function provided by the Start-  !

up Rate trip would not be adversely impacted.

(vii) The corrective action which has been, is being, or willbe taken; the name of the individual or organization responsible for the action; and the Icngth of time that has been or will be taken to complcto the action:

Affected utilities should ensure that log power channels are properly calibrated anci cross-correlated to the linear power channels at 100% reactor power. This calibration procedure should include a term adequate to account for the potential differences in flux signal between the conditions at calibration and tne conditions where the protective function may be depended upon. Since the actual plant cycle specific conditions determine flux levels needed for instrument calibration, ABB-CE was unable to generically provide the specific data needed to account for differences between 100% power cahbration and the actual d

use point of less than 10 % power without performing plant specific evaluations. The log power trip is typically bypassed above the 10"% power level Further, affects on

instrumentation calibration should be specifically considered if fuel management schemes are changed.

2 As an interim corrective action, ABB-CE determined that lowenng the existing trip setpoint

[ by a factor of 10 would conservatively compensate for the decabbrating effects discussed above Based on the result of plant specific evaluaticns, ABB-CE has concluded that

t P,R 16'96 17:32 FR CE OWNERS GROUP 203 285 2337 TO 13018165151 P 06/0?

because of the discretionary conservatism built into the input paramotors for the one affected safety analysis, that the original High Logarithmic Power trip setpoint remains acceptable without creating the potential to exceed a safety limit. That is, the plant specific analyses show that the original trip setpoint (i.e., prior to the interim decrease by a factor of 10) remains acceptable. As a result, removal of the interim measure and continued operation using the original trip setpoint is justified.

(viii) Any advice related to the defect or failure to comply about the facility, activity, or basic component that has been, is being, or will be given to purchasers or licensees:

When the subject situation first developed ABB-CE issued an Infabulletin (attached) to advise owners of ABB-CE designed NSSSs.

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1 (P.R 25 9G 17:32 FR CE OWNERS GROUP 203 285 2337 TO 13018165151 P.07/07 i

ABB-CE Infobulletin No. 96-01 Page 1 of 1 (Rev 04, 2/20/96) l A BB Combustion Engineering Infobulletin No. 96-01 Instrurnentation Decalibration at Low Power introduction: The High Logarithmic Power trip protects against an inadvertent CEA withdrawal event at low power. An evaluation of readings from the log power excore channels and the linear power channels st one plant identified effects that can contnbute to an overall decalibration of the detector readings. The result of these effects can produce potential nonconservative instrumentation readings due to conditions at low power being different from the full power operating conditions at which the instrumentation is calibrated A conservative upper bound estimate for the magnitude of these decalibration effects and a compensating change in the High Log Power trip setpoint has been made to ensure that the results of certain design basis events remain within the bounds of results currently reported in the updated safety analysis report.

Discussion: Calibrating the log channels at near full power conditions does not account for the effects of lower primary coolant temperatures, higher boron concentrations, all rods in, etc., for a plant operating at less than 10d % power. An investigation of the decakbration effects identified core design and power level dif'crences that were not fully compensated in the original evaluation cf the log power trip setpoint. The f investigabon showed that conditions exist at low power, where the High Log Power trip is required, that

could introduce nonconservative factors relative to the full-power conditions. These effects could result in

[ a lower fiux reaching the excore detectors for a given power level. Therefore, the actual power in the core could potentially be higher than the analytical limit when a High Log Power trip cr a CPC Zero Power

! Bypass removal occurs.

l l Periodic recalibration of this inp setpoint wcu!d help identify potential instrument drift problems, but would not c!iminate the decalibrating factors between 100% power and zero power. Further, an "clectncal-only" cabbration of the instrument (which reads in volts) without cross-correlating against fiux means that the

voltage cahbration may not accurately correlate with core power (via the flux measurement of the l excores) 1 l Recommendation

Affected utilities should ensure that log power channels are properly calbrated and

{ cross-ccrrelated to the knear power channels at 100% reactor power. Since the actual plant conditions

! and core design enter into the determination of fiux levels needed for instrument cahbratien. ABB is unable I to generically specify the data needed to account for di'ferences between 100% power ca!!bration and the l actual conditions of requinng exact Log Power indication power without performing plant specific evaluations Also, this issue should be specifically considered when fuel management schemes are changed As an intenm corrcCtive action, CE beheves that lowenng the existing log power inp setpoint by a factor of 10 will comocnsate conservatively for the decalibrating effects discussed above Applicability: The potential for low power decalibration affects all CE NSSS units. However, only those l cperating plants with digital reactor protecticn system, inc!uding ANO-2, SONGS-2 & 3, Waterford-3, Pa:c l Verde-1,2 & 3, and Yongqwang-3 & 4, incorporate a trip that could be adversely affected by a change in j the absofute magnitude of the log power signal. Earlier plants with analog protection systems are not l affected by this decalibration effect since they rely upon the starbup rate trip, where the top is based upon i

hcw fast the signal changes in a given time penod (decades per minute) versus tnpping on the absolute l j magnitude of the signal (percent power)

Technical

Contact:

Kelly McCuoid,860-2c5-2326 cr Steve O'Heam, phone. 360-285-2770 1he ulx~taton coraseed m Ws 1-lhlen u pr:Wed by GB CE as a wr.a e v e egno,un. ku eperm .n <f y%r rl.mt a compie<ly d'" W r

cc:n:ol nd recon 41% .md m.nl.<> m.m im na wiiMn Arla.Crt. hio.ie4c. this mrermanon may te ett.:td crJy wra the uninunary tui ADO.CL mass no wansnrics or terresemmons. c. press er imphed, mdudm; unnannes or racss rot a tamcular pugw or unethmtiMay, nh re.peci se the xcuruv compicircris or usefulness or the crornacen onsned ABBG csdas. aM you asume, a:1 haNbev in nc;hr,cucc or other ne, a:. a r-.uh urwur u .e of w brormarion.

[RR.15'96 17:31 FR CE OtJNERS GROUP 203 285 2337 TO 13018165151 P.01/07 yW

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l })l2l 14350 ABB COMBUSTION ENGINEERING NUCLEAR OPERATIONS l

! 2000 DAY HILL ROAD l WINDSOR, CONNECTICUT 06095-0500 l 1 1

! FROM: CHUCK MOLNAR TELEPHONE: (203) 285-5205 FAX NO: (203)285-2337

. T0: NRC Operations Center CCMPANY: NRC

SUBJECT:

Report of a Potential Deviating Condition in a Reactor Trip i Channel i

! Attached is a copy of a letter documenting a potential deviating condition in a i reactor trip channel. The evaluation of this condition for ABB-CE plants has l determined that there is no resultant safety concern. l 4

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DATE: April 15,1996 SIGNED: 4/4I 7 b MIC This is Page 1, with 7 page(spo foHow.