Information Notice No. 89-68: Evaluation of Instrument Setpoints During Modifications
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 25, 1989
Information Notice No. 89-68: EVALUATION OF INSTRUMENT SETPOINTS
DURING MODIFICATIONS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being issued to alert addressees to a potential
safety problem resulting from inadequate evaluation of operating and design
characteristics when modifying instrumentation and control (I&C) systems. It
is expected that recipients will review the information for applicability to
their facilities and consider actions, if applicable, to avoid similar
problems. Suggestions contained in this information notice do not constitute
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
Several design inspections of plant modifications in the I&C area by the
Nuclear Regulatory Commission have revealed that calculations relating to the
setpoints of the modified instrument loops were not performed properly to
verify that the original design objectives of the safety systems were still
satisfied. Modifications to the instrument system(s) may introduce
undesirable operating characteristics because of a change in the margin
between the nominal setpoint and the technical specification (TS) limit or a
change in the system's response time. The changed attributes of the I&C
components may degrade the safety system's ability to meet its design
requirements. Summarized below are inspection findings from three recent and
two earlier inspections that illustrate these concerns.
Oyster Creek Nuclear Power Plant (November - December 1988):
The reactor instrumentation system at the Oyster Creek station was modified to
convert certain reactor protection system pressure switches to an analog trip
system. In determining the setpoint values for the modified I&C loops, the
licensee arbitrarily established margin values and was not thorough in
accounting for all potential error contributors to the total uncertainty for
I&C measurement loops. Because of this, I&C technicians could leave
calibrated instrument channels at the upper limits of their calibration bands
and create conditions that permit setpoints of plant process parameters to
deviate beyond their TS limits without the condition being detected. In this
regard, the licensee identified several instrument loops with a history of
exceeding TS limits.
8909200010
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IN 89-68
September 25, 1989
Page 2 of 3
Zion Nuclear Power Station (March - April 1988):
For modification packages in the I&C area at the Zion station, setpoint calcu-
lations for modified instrument loops did not consider head correction changes
caused by changes in instrument tap location. Also, calculations did not ad-
dress compensations for temperature and/or density changes where applicable.
The value of allowance for instrument drift was not related to the interval
between surveillances. The inspection team found that some calculations used
a value equivalent to 12 months drift for instrument loops that had a surveil-
lance interval of 18 months. In addition, calculations used assumed values
for uncertainty in measuring and test equipment (M&TE), assumed values of
calibration tolerances, and assumed values of instrument dead bands. All
these errors resulted in a nonconservative value of setpoint margin. The
failure to account correctly for drift, head correction, temperature and
density correction, accuracies of M&TE, and values of calibration tolerances
could create an unanalyzed situation in which the instrument may not be able
to initiate the required safety function even if the process variable is in
the non-conservative direction with respect to the allowable value.
Indian Point, Unit 2 (January - February 1988):
An inspection of the preventive maintenance program at Indian Point Unit 2
revealed that the licensee was not trending for directional changes (positive
or negative) in instrument accuracy occurring between successive calibrations.
The inspection team found that in several loops, accuracy and drift values of
the instruments were changing only in one direction between successive
surveillance intervals. Uncertainty calculations for setpoint margins of
these loops were performed by combining uncertainty attributes using the
square root of the sum of the squares (SRSS) method. This method is
acceptable provided uncertainty attributes of loop components have random
directions. In a situation in which uncertainty attributes are known to be
changing only in one direction, use of the SRSS method for computing the
setpoint margin will result in a non-conservative value. This value, when
applied to the setpoint, may compromise the ability of an instrument to
initiate a safety function before the process variable exceeds its process
safety limit. Furthermore, instrument drift values occurring in only one
direction over several surveillance tests warrant review by the licensee to
determine their cause.
Dresden Nuclear Power Station, Unit 3 (November - December 1985):
The reactor instrumentation system at Dresden Unit 3 was modified by replacing
the old mechanical instruments with new solid-state instruments. The new in-
strument loop had increased response time and drift. Also, the accuracy
values of the new instruments were different. The inspection team found that
an engineering evaluation of the impact of characteristics of new
instrument(s) on the system operation was not performed. Also, an analysis of
the impact of the increased drift on the existing surveillance frequency was
not performed. After installation, the new instruments were adjusted to the
setpoints established for the original instruments. In this situation, the
effectiveness of the modified system to meet the original design objectives
could not be ascertained, creating a potentially unanalyzed situation.
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IN 89-68
September 25, 1989
Page 3 of 3
Edwin I. Hatch Nuclear Plant, Units 1 and 2; Joseph M. Farley Nuclear Plant,
Units 1 and 2 (May 1986):
Design changes at the Hatch and Farley units included replacement of the
existing mechanical-type instruments with either a single instrument or a
string of instruments consisting of a primary sensor, a signal conditioner,
and bistables. An analysis of the setpoint margin using characteristics of
the new instruments was not performed, and the new instruments were set to
either a setpoint established for the original instruments or to a new
setpoint chosen by the instrument vendor. In the absence of any engineering
analysis, the ability of the modified system to meet the original design
objectives could not be ascertained, thus creating a potentially unanalyzed
situation.
Discussion:
It is important that an engineering analysis be performed to verify that the
static and dynamic characteristics of a system, when modified by the
installation of new instrumentation, continue to meet the design objectives.
In modifying I&C systems, it is important that careful consideration be given
to the necessity of recalculating setpoints, setpoint margins, and values of
the TS limits to ensure that improper operating characteristics have not been
introduced by the modification. Useful guidance is provided in ISA
67.04-1982, "Setpoints for Nuclear Safety-Related Instrumentation used in
Nuclear Power Plants" which has been endorsed by Regulatory Guide 1.105
Revision 2-1986,"Instrument Setpoints for Safety-Related Systems".
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: S. V. Athavale, NRR
(301)492-0974
J. Mauck, NRR
(301)492-3264
S. C. Guthrie, NRR
(301)492-0991
Attachment: List of Recently Issued NRC Information Notices
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