United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-68: Evaluation of Instrument Setpoints During Modifications

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C. 20555

                               September 25, 1989

                                   DURING MODIFICATIONS


All holders of operating licenses or construction permits for nuclear power 


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 

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. 


                                                            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.  

                                                            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 


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 

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  S. V. Athavale, NRR

                     J. Mauck, NRR

                     S. C. Guthrie, NRR

Attachment: List of Recently Issued NRC Information Notices 
Page Last Reviewed/Updated Tuesday, November 12, 2013