Impact of failure of uninterruptible power supplies on nuclear power generating stations

Uninterruptible power supplies in nuclear power generating stations are designed to provide a stable, and uninterruptible vital AC power to the safety related and non-safety related instrumentation and control systems. Generally, a loss of instrumentation and control system results in undesirable system conditions and plant transients. This results in an unnecessary actuation of the safety systems such as the Reactor Protection System and engineered safeguard system. Such system disturbances and transients have safety implications and safety significance since they result in challenges to the plant operation. The study revealed that component failure (electrolytic capacitors, followed by transformers, SCRs, and transfer switches) is the root cause of the failure of the uninterruptible power supplies, followed by voltage transients and human error. The licensee should consider monitoring the temperature and humidity in the inverter enclosures and to have adequate and proper maintenance. Since, solid state circuitry designs (including digital) are being increasingly used in the Reactor Protection System and other systems, the chances of misoperation of low voltage equipment due to system disturbances have increased. The low voltage circuitry may be more vulnerable now because of the integrated protection and microprocessor-based systems being used in the Passive Advanced Light Water Reactors that will be built into the next millennium.