Part 21 Report - 1996-350
ACCESSION #: 9605070110
ABB
April 15, 1996
LD-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 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 occur 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 limits, Based on the result of
these evaluations ABB-CE has concluded that because of the discretionary
conservatism built into the input parameters 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 vendor designs utilizing a
reactor protection system trip for subcritical events based upon absolute
power if nuclear instrumentation decalibration factors are not accounted
for appropriately. The Enclosure provided herewith summarizes the
information available to ABB-CE regarding the subject condition.
ABB Combustion Engineering Nuclear Power
Combustion Engineering. P.O. Box 500 Telephone (860) 688-1911
Inc. 2000 Day Hill Rd. Fax (860) 285-5202
Windsor, CT 06095-0500
Document Control Desk LD-96-009
April 15, 1996 Page 2
[f 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 ENGINEERING, INC
J. C. Rickard, Director
Operations Licensing
Enclosure: As stated
cc: R. S. Siudek (ABB-CE)
ENCLOSURE
ABB Combustion Engineering Nuclear Operations
Report of a Deviating Condition in a Reactor Trip Function
April 15, 1996
ABB Combustion Engineering Nuclear Operations
Report of a Deviating Condition in a Reactor Trip Function
(i) Name and address of the individuals informing the Commission:
I. C. Rickard, Director
Operations Licensing
Combustion Engineering, Inc.
2000 Day Hill Road
Windsor, CT 06095-0500
(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 Electric 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
(iv) Nature of the defect or failure to comply and the safety hazard
which is created or could be created by such defect or failure
to comply.,
Although the High Logarithmic Power trip function is only
credited at low power levels (from </=10**-4 % to 2% rated
power), the log channel instrument calibration is routinely
performed at 1 00% power. Calibrating the log channels at near
full power conditions, however, does not account for the
effects of lower primary coolant temperatures, higher boron
concentrations, changes in control rod position (all rods in),
etc., when a plant is operating in Mode 2 subcritical 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 potentially be higher than the analytical limit
utilized in the safety analysis when a High Logarithmic Power
trip or 3 CPC Zero Power Bypass removal occurs.
An Investigation of the decalibration effects identified cycle
specific core design differences that were not fully
compensated for in the original evaluation of the log power
trip setpoint.
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 instruments 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 effect
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 change
in signal (decades per minute) for any given rate of approach
to critical would not be adversely impacted. As such, the
protective function provided by the Startup Rate trip would not
be adversely impacted.
(vii) The corrective action which has been, is being, or will be
taken; the name of the individual or organization responsible
for the action; and the length of time that has been or will be
taken to complete the action:
Affected utilities should ensure that log power channels are
properly calibrated and 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 the 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 calibration and the actual use point of less than 10**-4%
power without performing plant specific evaluations. The log
power trip is typically bypassed above the 10**-4% power level.
Further, affects on instrumentation calibration should be
specifically considered if fuel management schemes are changed.
As an Interim corrective action, ABB-CE determined that
lowering the existing trip setpoint by a factor of 10 would
conservatively compensate for the decalibrating effects
discussed above. Based on the result of plant specific
evaluations, ABB-CE has concluded that
because of the discretionary conservatism built into the input
parameters 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
Infobulletin (attached) to advise owners of ABB-CE designed
NSSSs.
ABB-CE Infobulletin No. 96-01
Page 1 of 1 (Rev 04, 2/20/96
ABB
Combustion Engineering Infobulletin No. 96-01
Instrumentation 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 at one
plant identified effects that can contribute 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 10**-4% power. An investigation of the decalibration effects
identified core design and power level differences that were not fully
compensated in the original evaluation of the log power trip setpoint.
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. These effects could
result in a lower flux 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 or a CPC Zero
Power Bypass removal occurs.
Periodic recalibration of this trip setpoint would help identify
potential instrument drift problems, but would not eliminate the
decalibrating factors between 100% power and zero power. Further, an
"electrical-only" calibration of the instrument (which reads in volts)
without cross-correlating against flux means that the voltage calibration
may not accurately correlate with core power (via the flux measurement of
the excores).
Recommendation: Affected utilities should ensure that log power channels
are properly calibrated and cross-correlated to the linear power channels
at 100% reactor power. Since the actual plant conditions and core design
enter into the determination of flux levels needed for instrument
calibration, ABB is unable to generically specify the data needed to
account for differences between 100% power calibration and the actual
conditions of requiring 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
interim corrective action, CE believes that lowering the existing log
power trip setpoint by a factor of 10 will compensate conservatively for
the decalibrating effects discussed above.
Applicability: The potential for low power decalibration affects all CE
NSSS units. However, only those operating plants with digital reactor
projection system, including ANO-2, SONGS-2 & 3, Waterford-3, Palo Verde-
1, 2 & 3, and Yonggwanq-3 & 4, incorporate a trip that could be adversely
affected by a change in the absolute magnitude of the log power signal.
Earlier plants with analog protection systems are not affected by this
decalibration effect since they rely upon the 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 the absolute magnitude of the
signal (percent power).
Technical Contact: Kelly McQuoid, 860-285-2326 or Steve O'Hearn, phone:
860-285-2770
The information contained in this Infobulletin is provided by ABB-CE as a
service to your organization. Since operation of your plant is completely
within your control and responsibility, and involves many factors not
within ABB-CEs knowledge, this information may be utilized only with the
understanding that ABB-CE makes no warranties or representations, express
or implied , including warranties of fitness for a particular purpose of
[Illegible] with respect to the accuracy, completeness or usefulness of
the information contained. ABB-CE disclaims and you assumed, all
liability in negligence or otherwise, as a result of your use of this
information.
ABB COMBUSTION ENGINEERING NUCLEAR OPERATIONS
2000 DAY HILL ROAD
WINDSOR, CONNECTICUT 06096-0500
FROM: CHUCK MOLNAR TELEPHONE: (203) 285-5205
FAX NO: (203) 285-2337
TO: NRC Operations Center COMPANY: NRC
SUBJECT: Report of a Potential Deviating Condition in a Reactor Trip
Channel
Attached is a copy of a letter documenting a potential deviating
condition in a reactor trip channel. The evaluation of this condition
for ABB-CE plants has determined that there is no resultant safety
concern.
DATE: April 15,1996 S I G N E D
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