United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-86: Lightning Strikes at Nuclear Power Generating Stations

                                                          SSINS No.: 6835 
                                                             IN 85-86 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                              November 5, 1985

Information Notice No. 85-86:   LIGHTNING STRIKES AT NUCLEAR POWER 
                                   GENERATING STATIONS 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 

Purpose: 

This notice is provided to alert recipients of a potentially significant 
problem of reactor trips and instrument damage caused by lightning strikes. 
It is expected that recipients will review the information for applicability
to their facilities and consider actions, if appropriate, to preclude a 
similar problem occurring at their facilities. However, suggestions 
contained in this notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

The NRC is continuing to evaluate pertinent information. Recipients of this 
notice will be notified of additional information or if specific actions are
required. 

Description of Circumstances: 

A number of plant trips and instrumentation problems attributable to 
lightning have occurred over the past 6 years. Since solid state circuitry 
designs are being increasingly employed in safety related systems, the 
impact of lightning induced line surges on those circuits is emphasized in 
this notice. Descriptions of several of the more significant events are 
presented below. Events involving lightning strikes of switchyards and the 
consequential impact on power distribution systems are not covered by this 
notice. However, INPO SER 76-84 covers this latter subject as well as 
summarizing earlier INPO documents dealing with lightning strikes at nuclear
power plants. 

Zion Power Station Units 1 and 2 

On August 17, 1979, both units tripped simultaneously during a severe 
lightning storm. Investigation indicated that a lightning strike in close 
proximity to the plant caused either a momentary surge or interruption in 
the ac power supply circuits to the rod control power supply cabinets. This 
transient tripped the overload protection devices for the dc power supply 
cabinet, resulting in a power interruption to the control rod stationary 
gripper coils, 


8511010020 
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which caused the rods to drop into the core. The resulting high, negative 
flux rate initiated the reactor trip signal. Tests verified that noise 
induced on the ac input to one power supply would actuate the overvoltage 
protection trips on the main and auxiliary power supplies. In addition to 
the noise spikes, one Unit 2, 24-V positive power supply was damaged by the 
lightning strike and had to be replaced. The following corrective actions 
were initiated: 

o    The control rod system neutral was isolated from the station ground. 

o    The overvoltage protection trip setting was changed from 27 to 29 V. 

o    A low-pass filter was installed on the input to each 24-V positive 
     power supply. 

o    A volt trap (a voltage suppressor circuit designed to reduce large 
     voltage surges and noise induced by lightning strikes) was installed 
     across the 50-ohm motor generator neutral resistor. 

o    A volt trap was installed across the power feed to the auxiliary power 
     supply. 

o    The power feed for the auxiliary power supply was changed from the 
     480-V system to the control rod drive (CRD) motor generators. 

Zion Unit 2 experienced additional reactor trips attributed to lightning on 
April 3 and July 16, 1980, before the above listed corrective actions were 
implemented. In these cases it was determined that the transient tripped the
overload protection devices, as was the case in the trip of both units on 
August 17, 1979. However, no power supplies or other equipment were damaged 
during the latter two trips. 

Again, on December 2, 1982, Zion Unit 2 reactor tripped from 100% power 
during an electrical storm. It was concluded that lightning induced a 
disturbance in the electrical system causing a reactor trip from a generator
trip. Additional lightning protection for the containment building was 
provided and the static wire associated with the 345-kV line was isolated 
from the power station structural steel. 

Salem Power Station Unit 1 

On June 9, 1980 the reactor tripped during an electrical storm. Lightning 
struck at the south penetration area of reactor containment causing a 
transient on seven main steam pressure transmitters. Two of these pressure 
transmitters were damaged and had to be replaced. The transient caused a 
high steam line pressure differential reactor trip signal and a safety 
injection signal. The licensee believes the lightning strike hit main steam 
vent pipes which extend above the penetration area roof and the surge was 
carried into the building via piping connections. 
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Kewaunee Power Station 

On August 19, 1980, two of four instrument busses lost power during an 
electrical storm. This resulted in a spurious safety injection (SI) 
actuation signal, and the unit tripped from full power. In addition to 
inducing the instrumentation transients, the inverter fuses were blown. 
These fuses were replaced, and no other evidence of equipment failure was 
observed. 

Byron Power Station Unit 1 

On July 13, 1985, when lightning struck the Unit 1 containment, the reactor 
tripped from approximately 11% power because of induced voltage surges in 
instrument and control cables in one of four containment penetration areas. 
The induced voltage caused failure of four rod drive power supplies, 
including 1 redundant pair. The failure of the redundant supplies resulted 
in 10 control rods dropping into the core. A power range negative-flux-rate 
reactor trip resulted from the rod insertion. In addition to the reactor 
trip, damage occurred to 30 plant instruments. The following systems were 
affected by the damaged instrumentation: protection channel II, one train of
the 48-volt power supply for the solid state protection system, the 
meteorological tower, control rod drive, and loose-parts monitoring. 

A review of cable routings showed that a significant common denominator 
existed in containment penetrations. All damaged instruments were associated
with cables passing through penetrations located in one containment region. 
In addition to the damaged instrumentation, the lightning damaged a 
significant amount of security equipment. 

The licensee determined that an improved lightning protection system was 
required to prevent recurrence of a similar incident. By installing copper 
conductors, external to containment, from the roof mounted lightning rods 
directly to ground rods in the earth, a low impedance path to ground was 
provided for future lightning strikes. This modification is similar to the 
Zion modification described above. 

Arkansas Power Station Unit 2 

On August 5, 1985, the reactor tripped from 100% power on a low departure-
 from-nucleate-boiling ratio (DNBR) signal as the result of a lightning 
strike transient induced in two of the core protection system channels. The 
licensee's followup investigation revealed no damage to the plant's 
electrical equipment or instrumentation measuring systems. 
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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 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 Contact:  Ray S. Love, RIII
                    (312) 790-5593

                    Vincent D. Thomas, IE
                    (310) 492-4755

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