United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-29: Problems with the Use of Unshielded Test Leads in Reactor Protection System Circuitry

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                                April 12, 1993


NRC INFORMATION NOTICE 93-29:  PROBLEMS WITH THE USE OF UNSHIELDED TEST 
                               LEADS IN REACTOR PROTECTION SYSTEM CIRCUITRY


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees that induced current from unshielded test leads may
cause problems with safety-related circuitry.  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

On June 22, 1992, Duke Power Company engineers were performing an
incore/excore calibration on the power range neutron monitoring system at the
William B. McGuire Nuclear Station.  The fuses in the control power circuit
opened causing the actuation of the alarms for overpower rod stop, high flux
rate, and high setpoint alert.  The Duke Power Company engineers did not think
the test probe connection caused the fuse to fail, because the fuse was in a
different drawer and was completely isolated from the meter and test points. 
The engineers replaced the fuse and continued the test.  Later in the test,
the test probe was connected to a different channel.  The control power fuse
opened in that channel with the same results as above.  The engineers
responded by suspending the test. 

On May 15, 1992, Southern Nuclear Operating Company technicians were
performing an incore cross-calibration on the power range neutron monitoring
system at the Joseph M. Farley Nuclear Plant while the plant was operating at
35 percent power.  A reactor trip occurred because of a concurrent high flux
trip and high flux rate trip on one channel, and a high flux trip and high
flux rate trip on a separate channel.  After the reactor trip, the technicians
discovered that the high leg control power fuse for one channel and the
neutral leg control power fuse for the other channel had opened.

Discussion

Each utility investigated the cause of the fuse failures at its plant. 
Southern Nuclear Operating Company testing revealed that a 0.4 volt

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                                                            April 12, 1993
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peak-to-peak (60 Hz) signal was induced on meter leads from unshielded test
leads.  This induced signal was verified to be present at the summing
amplifier output.  When the same digital volt meters were connected with
shielded test leads, no induced signals were present.

Duke Power Company found that the particular digital multimeter used in the
test could retain a charge for approximately five minutes after the leads were
disconnected from the test points.  If one of the leads inadvertently made
contact with the drawer chassis during the test connection, the instrument
could discharge and create a voltage spike.

Subsequent discussions with Westinghouse revealed that similar incidents had
occurred at other sites.  Westinghouse determined that the use of long,
unshielded test leads may induce radio frequency (RF) noise into the leads of
the digital multimeter.  The induced RF is amplified by the power range drawer
summing and level amplifier, then channeled into the Nuclear Instrumentation
Systems bistable circuitry.  The induced signal in the bistable circuitry may
cause the silicon control rectifier to oscillate rapidly and produce coupling
within the output transformer.  The coupling within the transformer may have
produced an excessive current, causing the associated control power fuses to
open.  This type of event may occur when the combination of induced RF and
normal current/voltage signals pass through the bistables but are of
sufficient magnitude to trip the channel or in some cases, blow the fuse.  A
blown fuse could itself result in a spurious channel or reactor trip.  The
blown fuse also would prevent resetting the channel until it is replaced.  In
other applications, where there are no obvious indications, blown fuses could
remain undetected until the next surveillance or a valid demand.

Duke Power Company and Southern Nuclear Operating Company have revised
procedures to require that digital multimeters be connected with shielded test
leads and to limit test connections to one nuclear instrumentation channel at
a time.  Duke Power Company and Southern Nuclear Operating Company also
revised procedures to require specific digital multimeters and test point
connection methods when performing nuclear instrumentation testing.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact 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 contact:  Todd A. Cooper, RII
                    (704) 875-1681

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