Consequences of lightning strikes on nuclear power generating stations

Direct or indirect lightning strikes have caused reactor trips in nuclear power generating stations. If nuclear power generating stations do not have adequate lightning protection and grounding, such stations may be exposed to greater risk of loss of offsite power, fire, and damage to redundant trains of engineered safeguard equipment. The loss of offsite power is an important initiating event for design basis accidents. I studied the effects of lightning strikes on reactor trips or misoperation of the equipment from January 1984 through September 2000. From this study, I found that most of the lightning strikes operated the overvoltage protection devices for the control rod drive system. In others, the lightning strikes had partially damaged the main/startup transformers, tripped the switchyard breakers or tripped the transmission lines and, by that, affected the availability of the required stable offsite power sources to the nuclear power generating stations. Direct lightning strikes, electrostatic discharges, lightning electromagnetic impulse can disrupt the electrical and electronic equipment used in the reactor protection system and the other safety systems. The use of the integrated protection and microprocessor-based systems in simplified passive advanced light water reactors and the digital systems replacing the existing analog systems may be more vulnerable to a lightning strike. The protection from lightning could include the installation of lightning dissipation arrays, installation of ferrite shields, installation of a secondary shield or the addition of a short time delay on the power supply monitoring card to allow the transients to dissipate. The instruments and computer grounding system may be grounded through a separate radial grounding system that may be connected to the station grounding grid at only one point to avoid circulating currents. Advanced surge protection devices that include gas discharge tubes, Zener diodes for accurate and fast( picoseconds) voltage control, and metal oxide varistors can be used to protect the electrical and electronic safeguard equipment from high voltage transients associated with lightning.