Information Notice No. 93-17: Safety Systems Response to Loss of Coolant and Loss of Offsite Power

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                                 March 8, 1993


NRC INFORMATION NOTICE 93-17:  SAFETY SYSTEMS RESPONSE TO LOSS OF COOLANT
                               AND LOSS OF OFFSITE POWER 


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 condition in which automatic safety systems
could have responded inappropriately to certain sequences of loss of coolant
and loss of offsite power.  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

In a report of May 1, 1989, the Virginia Electric Power Company, the licensee
for the Surry Power Station, reported a deficiency in the emergency diesel
generator (EDG) loading logic that could have resulted in overloading the EDGs
if a loss of offsite power had occurred after a loss of coolant accident
(LOCA) or other design basis event causing the automatic start of the safety
system electrical loads.  The loading logic was not designed properly to
respond to this sequence of events.  During recent work on the Improved
Standard Technical Specification program and through discussion with nuclear
steam supply system owners groups, the NRC has determined that licensees may
have similar design logic problems at other plants.  The logic at these plants
may have been designed to respond properly to a simultaneous LOCA and loss of
offsite power but may not have been designed to respond appropriately to other
sequences.

Discussion

General Design Criteria 17 (GDC 17) of Appendix A to Part 50 of Title 10 of
the Code of Federal Regulations requires that, following a loss of offsite
power, the onsite power system be sufficient to ensure that the core is cooled
and containment integrity and other vital functions are maintained in the 





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event of postulated accidents.  Licensees typically include an analysis in
Chapter 15 of the plant Safety Analysis Report for a simultaneous occurrence
of a loss of offsite power and a LOCA.  However, licensees may not always
analyze for other possible sequences.  Thus, the safety systems in some plants
may have been designed to respond properly if these events occurred alone or
if they occurred simultaneously, but not for other possible sequences. 

A LOCA with a delayed loss of offsite power may occur in various ways.  In one
scenario, the LOCA results in a turbine trip and a loss of power generation to
the grid causing grid instability and a loss of offsite power seconds later. 
In another scenario, the loss of power generation causes a degraded voltage at
the plant switchyard, which causes the degraded voltage relays to actuate,
resulting in a delayed loss of offsite power to the safety buses.  In plants
where the safety buses are normally fed from the unit auxiliary transformer
connected to the output of the main generator, the loss of offsite power could
result from a failure of the buses to transfer to the offsite source after the
LOCA, or a failure of the offsite transformer to carry the additional load. 
In such events, offsite power will be lost to the safety buses during the
sequencing of the LOCA loads (if LOCA loads are designed to be sequenced on
offsite power) or shortly thereafter.  The control logic and piping systems
(safety injection, service water, etc.) at some plants may not be designed to
meet the consequences of such events.  Possible adverse results include the
failure to reenergize loads required to respond to a LOCA, improper loading
and loss of the diesel generators, and water hammer in the piping systems. 

The opposite sequence is a loss of offsite power followed by a delayed LOCA. 
One way in which this sequence could occur is if, after the loss of power
occurred, a safety relief valve lifted and failed to properly reseat resulting
in a loss of reactor coolant inventory and a LOCA initiation signal.  

Properly designed safety systems will respond appropriately to all credible
sequences of a loss of offsite power and LOCA.  The design of the control
logic, the electrical power and control systems, and the fluid systems all
contribute to ensuring that safety systems respond appropriately to these
events. 
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                                                            IN 93-17
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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.


                                            ORIGINAL SIGNED BY


                                        Brian K. Grimes, Director
                                        Division of Operating Reactor Support
                                        Office of Nuclear Reactor Regulation

Technical contact:  James Lazevnick, NRR
                    (301) 504-2782

Attachment:  List of Recently Issued NRC Information Notices

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