Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
November 22, 1995
NRC INFORMATION NOTICE 95-13, SUPPLEMENT 1: POTENTIAL FOR DATA COLLECTION
EQUIPMENT TO AFFECT PROTECTION
SYSTEM PERFORMANCE
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 to potential problems that may occur when using
data acquisition system (DAS) devices to collect data on protection system
performance. These devices have the potential to affect the signals being
monitored. 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.
Background
The NRC issued Information Notice (IN) 95-13 to alert addressees to the
potential for DAS devices in protection system circuits to affect parameters
being monitored. Fermi 2 experienced problems with a decrease in reactor
vessel level indication with the DAS unit connected but deenergized. The
decrease in indication was a result of the low internal impedance of the
(deenergized) DAS unit, whereby the signal was in parallel with both the DAS
and the indicator, resulting in a smaller signal to the indicator. NRC has
since learned that Quad Cities Station Unit 2 had a very similar experience
shortly before the incident at Fermi. A more recent event at Quad Cities
Unit 2 demonstrates potential for unplanned interactions of an energized DAS
and the circuits being monitored.
Description of Circumstances
Quad Cities 2
During a routine surveillance on December 13, 1994, testing personnel found
that the reactor low water level analog trip associated with the reactor
protection system (RPS) scram setpoint was apparently reading out-of-
tolerance. The testing personnel found that an installed DAS unit was
affecting the trip setpoint. The DAS had been installed under a temporary
modification and a 10 CFR 50.59 evaluation was performed.
9511160384. IN 95-13, Supp. 1
November 22, 1995
Page 2 of 4
However, no post-installation testing was performed because the licensee
believed that the DAS unit was nonintrusive and would not affect the trip
function. After the change in reactor water level setpoint was identified,
the licensee performed additional troubleshooting and found that with the DAS
turned off, the DAS unit became a low internal impedance path. Therefore, the
transmitter output signal would divide in parallel with the DAS and the analog
trip unit, causing the process parameter to decrease. The DAS was reinstalled
to finish the collection of data, with instructions and warnings to prevent
the DAS from being turned off. The effects of a potential loss of 120-volt
(V) ac power to the DAS unit were not addressed.
On July 19, 1995, the DAS unit was reinstalled to monitor and establish
baseline data for redundant channels of reactor vessel level and pressure,
core flow, steam flow, and average power range monitor (APRM) flux. The next
day, during a startup (at 2 percent power), the nuclear shift operator noted
that the B recirculation pump speed had increased from about 32 percent to
about 60 percent (causing a slight increase in reactor power), even though the
A and B recirculation pumps were in manual mode, and no change in pump speed
demand had been made by the operators. The operator was able to return the
pump speed to 32 percent. Six hours later, the operator observed an indicated
decrease of 20 centimeters(cm) [8 inches] in one reactor water level channel
and indicated changes in core flow. It should be noted that neither water
level nor core flow actually changed during this last event. The licensee
made immediate plans to disconnect the DAS.
The cables that connect to the parameters being monitored were completely
shielded except at the terminal block before entering the DAS unit. These
cables did not use any type of isolation device to protect the instrumentation
from influences of the DAS unit. A 100 conductor, unshielded ribbon cable was
connected at one end to the unshielded terminal block external to the DAS, and
on the other end to a circuit board within the DAS unit. The licensee had
performed some preliminary testing and found that voltages from 0.4 V to 1.7 V
ac were present on the dc process parameter signals. The ac voltage is
present because the voltage supply to the instruments and control circuits is
120V ac. During normal operation (without the DAS installed), this stray ac
voltage did not affect the dc signals. These dc signals are inputs into the
GE-MAC analog control and to control room instrumentation. The signals are
normally 10-50 milliamp (ma) dc but are converted to millivolt (mV) dc before
entering the DAS.
The licensee explained that because the ribbon cable is not shielded, the
process parameters were subjected to electromagnetic interference (EMI). This
EMI is caused by the ac voltages riding on the ribbon cable, which may cross
over to other circuits along the length of the ribbon cable (a length of about
20.5 cm [8 inches]). Testing personnel demonstrated that noise on one circuit
can feed back into the original circuit, or to the other circuits. The
licensee report of the event stated that the stray ac voltage was carried into
the DAS and this ac voltage induced noise on the circuits being monitored,
causing the signal problems previously noted. The vendor of the DAS was
consulted and verified that such a scenario was possible. The vendor also
stated that because of the high ac noise voltage compared to the signal being . IN 95-13, Supp. 1
November 22, 1995
Page 3 of 4
monitored (10-50 mV dc), the DAS unit was not capable of completely filtering
out the noise. The vendor stated that the DAS unit was capable of filtering
out noise that did not exceed 50 percent of the parameter being monitored
(about 20 mV for a 10-50 mV signal). The vendor also stated that high ac
noise signals can damage the DAS multiplexer.
Discussion
The event on December 13, 1994, at Quad Cities (low internal impedance of a
deenergized DAS) was similar to that experienced at Fermi as described in
IN 95-13. The second event at Quad Cities involved a problem with the DAS
unit turned on. Before putting the DAS unit in service, the licensee had
observed ac noise riding on the dc signal. The licensee shielded all the
output signals. However, the licensee did not consider induced voltages at
the ribbon cable that would affect the parameters being monitored. The
induced ac voltage superimposed on the dc signal, combined with the inability
of the DAS unit to filter out the large ac noise signals, caused unplanned
changes to certain control and indication signals.
Neither the Fermi nor the Quad Cities licensee performed testing adequate to
indicate the potential for affecting the instrument signals for indication and
control. Testing could have detected the changes to the input impedance to
the DAS unit. In addition, possible loss of voltage to the DAS units,
combined with the possible effects on redundant channels, was not considered.
Furthermore, although the licensee for Quad Cities was aware of the ac noise
present on the dc lines, the licensee performed no further investigation or
testing to determine whether any adverse effects on the dc signal would occur.
As part of its corrective actions, the licensee for Quad Cities was
formulating a policy that would allow use of the DAS with recorders with
isolators. The isolation device would prevent the signal from feeding back or
interfering with the process parameters. The licensee stated that the LaSalle
nuclear station had used this approach with success.
The DAS units used at Fermi 2 are manufactured by Intelligent Instrumentation
and utilize a Model No. PCI-20098C multifunction carrier board. The DAS units
at Quad Cities are manufactured by Keithly Metrabyte. This equipment may be
widely used in the nuclear power industry to collect and monitor plant data
for analysis of various input parameters.
The Institute of Electrical and Electronics Engineers (IEEE) Standard
279-1971, "Criteria for Protection Systems for Nuclear Power Generating
Stations," specifies that a single failure in the protection system not cause
a loss of function, and redundant channels be independent and physically
separated. IEEE Standard 338-1975, "Criteria for the Periodic Testing of
Nuclear Power Generating Station Safety Systems," specifies that test
equipment not cause loss of independence between redundant channels. The
licensing basis of each plant contains the specific applicable commitments for
channel independence. . IN 95-13, Supp. 1
November 22, 1995
Page 4 of 4
Licensees are required [10 CFR 50.59(b)(1)] to maintain a written safety
evaluation which provides the bases for the determination that a change, test
or experiment does not involve an unreviewed safety question.
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.
/s/'d by DMCrutchfield
Dennis M. Crutchfield, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Roger Mendez, RIII
(708) 829-9745
Paul Loeser, NRR
(301) 415-2825
Attachment:
List of Recently Issued NRC Information Notices
