Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 10, 1992
NRC INFORMATION NOTICE 92-12: EFFECTS OF CABLE LEAKAGE CURRENTS ON
INSTRUMENT SETTINGS AND INDICATIONS
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 a safety problem that could result from
inaccuracies introduced into safety-related instrument loops because of
increased leakage currents from instrument cables when subjected to a harsh
environment. 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 May 15, 1989, while reviewing instrument loop accuracies, test
technicians of the Virginia Electric and Power Company (VEPCO), the licensee
for the Surry Power Station, determined that during a harsh environmental
condition, the leakage currents in cables could prevent performance of
safety functions (Licensee Event Report 50-280/89-20). These potential
failures would be caused by reduction in the cables' insulation resistance
(IR) in the harsh environment. Such a harsh environment could be produced
inside containment by a loss of coolant accident (LOCA) or by a high energy
line break (HELB) event. The instrument cables installed at the Surry plant
were environmentally qualified, but the previous safety system calculations
for the accuracy of the instrument loops and for trip setpoints did not
account for the additional uncertainties that could be introduced by the
reduced IR values.
In particular, the licensee noted that this phenomenon could potentially
mask the reactor trip signal for steam generator low level and the safety
injection signal for pressurizer low pressure and, thus, prevent the
required protective actions. Furthermore, the licensee also noted that the
pressurizer level and reactor coolant system wide range pressure instrument
systems could be adversely affected by leakage currents.
The licensee reviewed all safety-related instrument loops and replaced,
where appropriate, affected cables in both units with new cables having a
higher IR value. The licensee reviewed setpoint calculations and verified
the margins
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IN 92-12
February 10, 1992
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taking into consideration errors caused by cable leakage currents. The
licensee also reviewed the loop accuracy calculations for indication loops
and revised affected emergency operating procedures to address errors in
indications caused by leakage currents.
The NRC is aware that many licensees are revising instrument setpoints using
the latest industry standards and are assessing the effects of leakage
currents. However, since most licensees for operating plants may not have
addressed these effects in their original design calculations, the problem
described above for Surry may be generic.
Discussion
Under conditions of high humidity and temperature associated with either a
LOCA or a HELB, the IR may decrease in components of the instrument loop
such as cables, splices, connectors, terminal blocks, and containment
penetrations. Consequently, leakage currents increase and measurement of
process variables becomes more uncertain. In a normal environment, however,
leakage currents are small enough to be essentially calibrated out of
consideration.
The instruments of a safety-related system provide monitoring and control to
ensure the system will perform its intended safety function. The decreased
IR of the instrument loop components may disable such monitoring and
control.
In June 1984, the NRC issued Information Notice (IN) 84-47, "Environmental
Qualification Tests of Electrical Termination Blocks." In this information
notice, the staff identified the potential for errors caused by leakage cur-
rents at terminal blocks when these blocks are subjected to a harsh
environment.
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.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: S. V. Athavale, NRR
(301) 504-2974
Jerry L. Mauck, NRR
(301) 504-3248
Attachment: List of Recently Issued NRC Information Notices
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