United States Nuclear Regulatory Commission - Protecting People and the Environment

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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