United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-100: Loss of Offsite Power to Vital-Buses at Salem 2

                                                            SSINS No.:  6835
                                                            IN 86-100 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                            WASHINGTON, DC 20555

                              December 12, 1986

Information Notice No. 86-100:  LOSS OF OFFSITE POWER TO VITAL-BUSES AT 
                                   SALEM 2 

Addressees: 

All holders of an operating license or a construction,permit for pressurized
water reactors (PWRs) or boiling-water reactors (BWRs). 

Purpose: 

This notice is intended to alert licensees of an event that occurred at Unit
2 of the Salem Nuclear Generating Station after loads had been added to 
vital and nonvital buses without performing adequate dynamic analyses. The 
event resulted in actuation of relays that provide undervoltage protection 
for equipment and detect loss of power to vital AC power buses, multiple 
transfers of vital buses from one station power transformer (SPT) to 
another, operation of reactor coolant pumps without cooling of their thermal 
barriers and motor bearings, reliance on natural convection to cool the 
reactor core, and repeated opening of a pressurizer relief valve. 

It is expected that recipients review this notice for applicability to their
reactor facilities and consider actions, if appropriate, to preclude 
occurrence of similar problems at their facilities. Suggestions contained in 
this notice do not constitute NRC requirements; therefore, no specific 
action or written response is required. 

Description of Circumstances: 

On August 26, while Unit 2 was operating at 100% of full power, an 
instrument technician inadvertently grounded Instrument Vital Bus C causing 
a voltage spike on the bus. The voltage spike generated spurious signals 
which indicated that pressure was low in two steam generators and that the 
circuit breaker for a reactor coolant pump was open. In response to the 
spurious circuit breaker signal, the reactor protection system tripped the 
reactor. The control system then tripped the turbine and dumped steam to the 
condenser and the atmosphere. Spurious low steam generator pressure signals 
in conjunction with actual high steam flow signals caused a safety injection 
signal and safety injection. In response to the safety injection signal, 
Diesel Generators A and C started. Diesel Generator B was out of service for 
maintenance. 

8612090400 
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                                                       IN 86-100 
                                                       December 12, 1986 
                                                       Page 2 of 4 

Because of the turbine trip, the nonvital buses transferred automatically 
from the auxiliary power transformer to the station power transformers 
(SPTs) causing a transient decrease in voltage at the SPTs. In a 15-second 
time span, Vital Bus A transferred from SPT 22 to SPT 21 and back again. 
Vital Bus B transferred from SPT 21 to SPT 22, and Vital Bus C transferred 
from SPT 22 to SPT 21. At the end of that time span, two of the vital buses 
satisfied the "blackout" logic, and a station "blackout" signal was 
generated although off-site power to the station was normal. The station 
"blackout" logic generates a "blackout" signal if 2 of 3 vital 4160 VAC 
buses are below 91% of rated voltage for more than 13 seconds. The station 
"blackout" signal automatically disconnected the vital buses from the SPTs, 
shed vital bus loads, connected the vital buses to the diesel generators, 
and then sequenced safety-related loads onto the vital buses. 

The reactor coolant pumps ran throughout the undervoltage transient. By 
design, power to the component cooling water pumps, which are on the vital 
buses, was not restored automatically because the reactor coolant pumps 
would not be operating during a valid station blackout. With the component 
cooling water pumps stopped, the thermal barriers and motor bearings for the 
reactor coolant pumps were not being cooled. In accordance with procedures 
and training, the operators secured the reactor coolant pumps within 5 
minutes to avoid damage to the pumps and established natural circulation 
through the reactor core. After the reactor coolant pumps were stopped, a 
power-operated relief valve on the pressurizer opened and closed repeatedly 
to relieve pressure in the reactor coolant system. The pressurizer safety 
valves were not challenged. In accordance with emergency procedures, the 
safety injection signal was reset after its cause was determined, component 
cooling water pumps were restarted, safety injection was terminated, and 
Vital Bus B was reconnected to the SPT. 

Within an hour of trip, two reactor coolant pumps were operating and normal 
control of pressure in the reactor coolant system had been established. 
Within the next hour, the other vital buses were reconnected to the SPTs, 
and the diesel generators were secured. Four hours after trip, the unit was 
stable and in hot standby. 

Discussion: 

The Salem plant has two reactor units. Each unit (see Figure 1) has four 
4.16 kV nonvital (group) buses that supply power to those components that 
are required for normal operation of the power plant and are not required 
during recovery from an accident. Loads on these buses include the reactor 
coolant pumps. The nonvital buses can be supplied with power from either (a) 
the reactor via the turbine generator and the auxiliary power transformer or 
(b) offsite via a pair of 13.8-4.16 kV SPTs. Two nonvital buses are normally 
supplied by each SPT. Nonvital buses for the other reactor unit are supplied 
by another pair of SPTs. The usual plant practice was to supply power to the 
nonvital buses from the auxiliary power transformer. Therefore, turbine 
trips caused the nonvital buses to transfer automatically from the auxiliary 
power transformer to the SPTs. Block loading the SPTs in this alignment 
caused temporary degradation of voltage.  

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                                                       IN 86-100 
                                                       December 12, 1986 
                                                       Page 3 of 4 

Each unit has three 4.16 kV vital buses that supply power to safety-related 
loads. Each vital bus is supplied by one of the SPTs. If voltage for a vital
bus drops below 91% for 10.5 seconds, an undervoltage transfer signal is 
generated. The bus automatically disconnects from the SPT that is.supplying 
power to it and connects to the other SPT. If voltage for two of the three 
vital buses drops below 91% for 13 seconds, then a station "blackout" signal 
is generated for the unit, the vital buses are automatically disconnected 
from the SPTs, loads are shed from the vital buses, the diesel generators 
are started, and loads are sequenced onto the diesel generators via the 
vital buses. The undervoltage transfer and station blackout signals are 
provided by sets of relays with time relays. If voltage drops below 70%, 
other under-voltage relays instantaneously initiate a station blackout 
signal. 

Before the event and after the last dynamic analysis of SPT loads, the 
licensee added 8200 and 2200 kVA to SPTs 21 and 22, which normally supply 
Unit 2, and 3200 and 2200 kVA to SPTs 11 and 12, which normally supply Unit 
1. Each of these changes includes 800 kVA added to vital buses, with the 
remaining loads added to the nonvital buses. The additional loads on the 
nonvital buses include condensate, heater drain, and circulating water 
pumps, which were installed to improve the performance of the plant. 
Further, to facilitate repairs to the Unit 1 auxiliary power transformer, a 
Unit 1 condensate pump was temporarily connected to a Unit 2 nonvital bus 
that transfers to SPT 21 on turbine trip. The static effects of these loads 
on the performance of the buses had been determined, but the dynamic effects 
had not. When the turbine for Unit 2 tripped, the heavily loaded nonvital 
buses were transferred without sequencing from the auxiliary power 
transformer to the SPTs. The SPTs were unable to maintain rated voltage and 
a voltage transient resulted. 

The last dynamic analysis of SPT loads was performed in 1980. Before restart
of Unit 2 after the recent event, the licensee reduced the loads on the SPTs
consistent with that analysis, revised operating procedures based on this 
reduction in available equipment, and decided not to supply power to the 
non-vital buses from the auxiliary power transformer. Similar changes were 
made for Unit 1. Further, Unit 1 was derated to 85% of full power and Unit 2 
to 80%. To determine the cause of the multiple vital bus transfers from SPT 
to SPT and to provide a basis for corrective action, the licensee has 
initiated a comprehensive design review of the electrical distribution 
system for the plant, including review of transients and analysis of the 
vital bus under-voltage transfer and station blackout relaying schemes. 

Three concerns result from this event: 

(1)  adding loads to vital and nonvital buses without performing a dynamic 
     analysis of their effect on the buses and power supplies 

(2)  hunting by nonvital buses for a source of adequate power during an 
     undervoltage condition 

(3)  the consequences of generating a blackout signal when in fact there is 
     no blackout, e.g., operation of the reactor coolant pumps without 
     component cooling water 

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                                                       IN 86-100 
                                                       December 12, 1986 
                                                       Page 4 of 4 

This notice requires no specific action or written response. If you have any
questions regarding this matter, please contact the Regional Administrator 
of the appropriate regional office or this office. 




                                   Edward L. Jordan, Director
                                   Division of Emergency Preparedness
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contacts:  Lee H. Bettenhausen, RI 
                     (215) 337-5291 

                     Roger W. Woodruff, IE 
                     (301) 492-7205 

Attachments: 
1.   Salem Electrical Distribution 
2.   List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013