Information Notice No. 87-24: Operational Experience Involving Losses of Electrical Inverters

                                                   SSINS No.: 6835    
                                                      IN 87-24

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

                                 June 4, 1987

Information Notice No. 87-24: OPERATIONAL EXPERIENCE INVOLVING LOSSES
                                  OF  ELECTRICAL INVERTERS

Addressees:

All nuclear power reactor facilities holding an operating license or a 
construction permit.

Purpose:

This notice is provided to alert recipients of potential problems
involving  electrical inverter losses that have led to unplanned plant
transients and/or  inoperability or improper functioning of safetyrelated
and other important  plant equipment. It is expected that recipients will
review this information  for applicability to their facilities and
consider actions, as appropriate, to  preclude similar problems from
occurring at their facilities. However,  suggestions contained in this
notice do not constitute NRC requirements;  therefore, no specific action
or written response is required.

Past Related Correspondence:

Information Notice No. 84-80, Plant Transients Induced By Failure of 
  NonNuclear  Instrumentation Power," November 8, 1984

IE Bulletin 79-27, "Loss of Non-Class IE Insrumentation and Control Power 
  system BUS During Operation," November 30, 1979

Information Notice No. 79-29, "Loss of Nonsafety-Related Reactor Coolant
  System  Instrumentation During Operation," November 16, 1979

IE Circular 79-02, "Failure of 120 Volt Vital AC Power Supplies," 
  January 11,  1979

Background:

Inverters in nuclear power plants provide "uninterruptible" vital ac 
electrical power to safety- and non-safety-related instrumentation and
control  systems. Generally, loss of this function results in some type of
undesirable  system condition and/or plant transient, including
unnecessary actuation of  safety systems such as reactor protection and
engineered safeguards systems;  loss of indicators that provide plant
status information; system disturbances,


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                                                            IN 87-24  
                                                            June 4, 1987 
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including reactor coolant system transients; improper response of the 
feedwater and steam generator water level control systems; loss of 
safetyrelated electrical equipment functions; damage to mechanical
equipment;  and challenges to operators and the remaining functional
equipment. Such  conditions and/or transients clearly have significant
safety implications  since they result in challenges to safety equipment
and plant operations  and/or a degradation of plant equipment.

The NRC case study report, AEOD/C605 dated December 1986, "Operational 
Experience Involving Losses of Electrical Inverters," includes the review
of  94 licensee event reports (LERs), totaling 107 events involving
inverter  losses that occurred during 1982 through 1984. The study
includes 35  additional events from the Nuclear Plant Reliability Data
System (NPRDS) that  occurred in the same timeframe. These 142 events
occurred at 51 distinct plants: 26  designed by Westinghouse, 11 by
General Electric, 9 by Combustion Engineering,  4 by Babcock & Wilcox, and
1 by General Atomic. The total number of events  included in the study for
each of the 3 years along with the number of reactor  units which were
operating during each of those years is summarized below.

Parameter                    1982      1983       1984

Operating Reactor Units       72        74          82 
Inverter Loss Events          34        51          57 
Losses/Operating Reactor     .47       .69         .70

As indicated above, the NRC has issued information on inverter losses
since  1979; and industry groups have issued approximately 14 reports
related to this  issue.

Description of Circumstances:

The NRC case study report identified three potential failure mechanisms
for  inverters. One of these involves relatively high ambient temperature
and/or  humidity within inverter enclosures. This condition appears to
result in  accelerated aging of components that form a part of the
inverter circuitry  causing a significant reduction in component life
expectancy and inverter  loss.

Another mechanism for inverter failure involves the electrical
interconnecting  and physical arrangements for the inverter circuitry
components. In some  installations, these arrangements are such that when
certain components fail,  other components also may fail or degrade.

The third failure mechanism involves voltage spikes and perturbations.
Many of  the electrical loads in a plant have inductive characteristics.
During plant  operations that involve energizing and deenergizing these
loads, voltage  spikes and perturbations are generated. The solid-state
devices in the  inverter circuitry are sensitive to these voltage spikes,
and this has  resulted in component failure, blown fuses, and inverter
losses. Additionally,  secondary voltage perturbations caused by lightning
strikes or switching  surges can have an adverse effect on inverter
operation.


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                                                            IN 87-24 
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Discussion:

The NRC case study report indicates that the failure mechanisms involving 
service condition parameters (e.g., ambient temperature and/or humidity
and  voltage spikes and perturbations) have common-cause implications.
However,  none of the events reviewed and evaluated in the report involved
the  simultaneous loss of redundant inverter-powered buses.

The dominant cause of inverter losses was attributed to component
failures.  Such components include diodes, fuses, silicon controlled
rectifiers,  capacitors, transistors, resistors, printed circuit boards,
transformers and  inductors. It also appears that major contributing
factors for the occurrence  of component failure events are high ambient
temperature and/or humidity  within inverter enclosures and electrical
disturbances at the inverter  input/output terminals.

In addition, incorrectly operating circuit breakers, improperly setting up 
test equipment, removing the wrong Inverter unit from service, and
improperly  transferring power sources for a bus are some personnel
actions that make them  the second largest contributor to inverter losses.

It is suggested that licensees consider monitoring of temperature and/or 
humidity internal to inverter enclosures and evaluating input and output
voltages of the inverter unit during steady-state and transient conditions
to  assure that manufacturer's recommendations are being considered.
Additionally,  to minimize the number of inverter loss events resulting
from personnel  actions, licensees might consider reviewing related
maintenance and testing  procedures and practices for inverters, Further,
specialized training and  practice sessions with involved plant personnel
and verification of  appropriate sequence of steps to achieve desired
related maintenance and  testing activities also may be considered.

No specific action or written response is required by this information
notice.  If you have any questions about this matter, please contact the
Regional  Administrator of the appropriate regional office or this office.

Charles E. Rossi, Director
Division of Operational Events Assessment Office of Nuclear Reactor
Regulation

Technical Contact: Vincent D. Thomas, NRR
(301)4924414

Attachment: List of Recently Issued NRC Information Notices

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