Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 8, 1988
Information Notice No. 88-57: POTENTIAL LOSS OF SAFE SHUTDOWN EQUIPMENT
DUE TO PREMATURE SILICON CONTROLLED
RECTIFIER FAILURE
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to recurring
problems experienced with silicon controlled rectifiers (SCRs) that
potentially can cause the loss of safety-related circuits. 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 do not constitute
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
The NRC Vendor Inspection Branch conducted an inspection at the Elgar Corpora-
tion on January 25-28, 1988. This inspection was conducted because of
recurring problems experienced with Elgar 25-kVA electrical inverters by the
Palo Verde Nuclear Generating Station (PVNGS), Units 2 and 3, and is
documented in NRC Inspection Report 99900871/88-01. The inspection revealed
that the Elgar maintenance instruction for its 25-kVA inverters did not
contain a critical torque requirement for the installation of replacement
SCRs. Elgar has omitted the torque requirement value in its instructions.
The PVNGS licensee reported to the NRC in Licensee Event Report (LER)
87-013-00 that a fuse was found blown and subsequently replaced on May 9, 1987
and again on May 10, 1987, on the "C" train inverter that powers a 400-amp bus
that supplies power to portions of the plant protection system, the engineered
safety features actuation system, and various plant instrumentation. The
licensee found nine SCRs shorted and replaced them. Attempts to energize the
inverter caused the fuse to blow again. The licensee retested the SCRs and
inspected all ac output circuitry components. The ac output circuitry
components were
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IN 88-57
August 8, 1988
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found to be in satisfactory condition. Further investigation found that the
SCR connections and mountings were loose. The licensee attributed this loose-
ness to improper torquing.
Discussion:
An SCR, also known as a thyristor, provides rectification and regulation in
power conversion equipment. A review of the operating experiences of
inverters and battery chargers used in nuclear power plants indicates that
numerous SCR failures have occurred due to fabrication and installation
errors. These SCRs conduct high electrical currents that generate heat. The
heat is dissipated through heat sinks to which the SCRs are mounted.
Typically, SCRs are either stud mounted or are arranged in a circular
configuration, known as disc-type. In either case, the proper mounting of the
SCR to its heat sink is extremely important.
Disc-type SCRs, such as the Elgar type, use a bracket to uniformly apply a
force over the surface area of the SCR. This bracket is important not only
for dissipating the heat from the SCR, but also for properly conducting the
electrical current through the SCR. Loose brackets result in the current
being carried through only a small area of the SCR. Therefore, without the
proper torque applied to the SCR, the device will heat up beyond its normal
operating temperature and prematurely fail or malfunction which may lead to
the loss of safe shutdown equipment. This potential for premature failure is
neither limited to one manufacturer or supplier, nor is it limited to only
electrical inverters.
If a replacement SCR is installed without the appropriate torque, premature
failure is likely to occur, resulting in the loss of a safety-related circuit.
For a normally energized inverter or battery charger (INV/BC) application, the
consequences could be a loss of power to important instrument and control
functions, causing an electrical transient that could include a reactor trip.
Conversely, if an SCR is replaced on an INV/BC used for standby equipment, a
premature SCR failure could cause a loss of that equipment during an actual
transient when the INV/BC is energized for emergency operations.
Additional detailed discussions on the operating experiences of battery charg-
ers, inverters, and SCRs are available in NUREG/CR-4564, "Operating Experience
and Aging-Seismic Assessments of Battery Chargers and Inverters," and
NUREG/CR-5051, "Detecting and Mitigating Battery Charger and Inverter Aging."
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IN 88-57
August 8, 1988
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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the techni-
cal contacts listed below or the Regional Administrator of the appropriate
regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Joseph J. Petrosino, NRR
(301) 492-0979
Jaime Guillen, NRR
(301) 492-1170
Attachment: List of Recently Issued NRC Information Notices
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