United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-57: Potential Loss of Safe Shutdown Equipment Due to Premature Silicon Controlled Rectifier Failure

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                 August 8, 1988

                                   DUE TO PREMATURE SILICON CONTROLLED 
                                   RECTIFIER FAILURE 


All holders of operating licenses or construction permits for nuclear power 


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 

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 


                                                            IN 88-57
                                                            August 8, 1988
                                                            Page 2 of 3

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. 


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."


                                                            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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