United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-49: Improper Integration of Software Into Operating Practices

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                                July 8, 1993

                               INTO OPERATING PRACTICES


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to recent events involving improper integration of
software-based digital systems into operating practices.  It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems.  However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.  

Description of Circumstances

AMSAC Time Delay Error

On December 31, 1992, the New York Power Authority (the licensee for 
Indian Point, Unit 3) performed a routine semiannual logic test for the
anticipated transient without scram (ATWS) mitigation system actuation
circuitry (AMSAC).  The AMSAC system failed the test when a required 40-second
time delay was not observed.  The absence of the time delay would have
prevented the automatic initiation of the motor-driven auxiliary feedwater
pumps in response to an AMSAC initiation signal under certain conditions.

After initial review, the licensee concluded that the deficiency had existed
since July 8, 1992, when a Foxboro (vendor) field technician reinstalled the
hard drive and manipulated software in the AMSAC logic.  When the hard drive
was reinstalled, the vendor technician loaded AMSAC software from an
uncontrolled version of the software in his possession.  The controlled,
plant-specific version of the software had not been retained by the licensee
nor had the licensee made arrangements for the vendor to maintain
configuration management.  The vendor technician attempted to modify the
uncontrolled version of the software to customize it for plant-specific use. 
Use of the improper version of the software caused the system to reboot
incorrectly.  The system failed the surveillance test, and the vendor
technician modified the software to allow proper system reboot.  During this 


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software manipulation, the 40-second time delay was incorrectly implemented in
the software logic.  This activity was not documented, and after the changes
were made, the AMSAC system was not adequately retested.  Because the actual
system logic was not retested, the vendor technician and the licensee were
unaware of the fact that the location of the 40-second time delay of the AMSAC
signal had been mistakenly altered during the software manipulations,
rendering the AMSAC inoperable under certain conditions.

Annunciator Driver Failure

On December 13, 1992, with the Salem Nuclear Generating Station, Unit 2, at
100-percent power, the overhead annunciator (OHA) system in the control room
was inadvertently placed in a configuration in which it did not update the
OHAs to indicate true alarm status.  The inoperable status of the OHAs went
unrecognized by the operators for 90 minutes until an alarm typewriter printed
a change in alarm status while the corresponding OHA failed to respond.  The
OHAs remained inoperable until the OHA sequence event recorder computer was

The OHA system is a real-time, multi-tasking, distributed processing computer
system with 35 microprocessors and the associated software.  The OHA system
design permitted an operator to place the sequence event recorder in the data
transfer mode versus the operating mode and enter the password-protected
software without warning to the operator, which allowed unauthorized system
manipulation.  The event occurred because the operator at a remote
configuration workstation failed to follow procedure while attempting to
obtain system status data by having the "black box" switch placed in the
incorrect position.  The incorrect position routed commands entered on the
remote configuration workstation to a high priority link on the sequence event
recorder.  The operator miskeyed the command characters, but the miskeyed
command characters happened to be valid commands on the high priority data
link which required additional data input.  The sequence event recorder
processed the command and suspended communications to other data links
(including the OHAs), while it waited for additional data input over the high
priority link, until the condition was recognized after 90 minutes and the
system was rebooted.

Diverse Scram System Failure

On March 13, 1993, at the Maine Yankee Nuclear Power Plant, flashing trouble
indications appeared on the intelligent non-nuclear safety digital automation
control system (INNSDACS).  An instrumentation and controls (I&C) technician
attempted to clear the alarms by rebooting the control processor.  On
March 14, the plant engineer determined that the diverse scram system had been
inoperable since the reboot.  The diverse scram system was restored by 
March 16.  The I&C technician did not have sufficient training on INNSDACS to
respond to system malfunctions without rendering the diverse scram system
inoperable.  Licensee implementation of the diverse scram system did not
ensure comprehensive training and administrative controls for maintenance

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Inoperable Torus Temperature Monitoring System

On November 14, 1991, at the James A. FitzPatrick Nuclear Power Plant, the
licensee found that 3 of 12 circuit cards in the torus temperature monitoring
system "A" train had defective solder joints.  The torus temperature
monitoring system consists of 15 resistance temperature detectors (RTDs)
positioned at various locations throughout the torus that feed two redundant
instrumentation channels and provide a bulk temperature output via an
averaging circuit.  The defective cards in the "A" channel were replaced, and
the channel was declared operable.  Checkout testing of the system on 
November 15, 1991, showed that the programming of a module in the "A" channel
was loaded with an incorrect software algorithm.  The algorithm is designed to
discard RTD input signals that deviate more than 100 percent from the average
signal.  The as-found setting for the module (which controls four of the RTDs)
would have discarded any RTD readings deviating more than 10 percent from the
average.  This could have affected bulk temperature readings in a
nonconservative direction in the event of localized torus heating.  The
correct software was immediately loaded into the module.


The events described above are examples of how inadequate integration of
software-based digital systems into operating practices and how inadequate
knowledge of the intricacies of software-based digital systems on the part of
technicians and operators caused systems to become inoperable.  The above
events indicate the susceptibility of software-based digital systems to
failure modes different from those of analog or hardware-based digital

Related Information Notices

                               PLANT TECHNICAL SPECIFICATIONS


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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 Office of
Nuclear Reactor Regulation (NRR) project manager.

                                       ORIGINAL SIGNED BY

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Jerry L. Mauck
                     (301) 504-3248

                     Eric J. Benner
                     (301) 504-1171

List of Recently Issued NRC Information Notices.
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