Evolution of Onsite and Offsite Power Systems in US Nuclear Power Plants

The AC electric power system is the source of power for station auxiliaries during normal operation and for the reactor protection system and emergency safety features during abnormal and accident conditions. Since the construction of early plants in US, the functional adequacy and requirements of the offsite power systems, safety and non safety related onsite electric power systems have changed considerably to ensure that these systems have adequate redundancy, independence, quality, maintenance and testability to support safe shutdown of the nuclear plant. The design of AC systems has evolved from a single train to multiple (up to four) redundant trains in the current evolutionary designs coupled with other auxiliary AC systems. The early plants were designed to cope with a Loss of Offsite Power (LOOP) event through the use of onsite power supplies only. However operating experience has indicated that onsite and offsite power AC power systems can fail due to natural phenomena (earthquakes, lightning strikes, fires, geomagnetic storms, tsunamis, etc.) or operational abnormalities such as loss of a single phase, switching surges or human error. The onsite DC systems may not be adequately sized to support plant safe shutdown over an extended period if AC power cannot be restored within a reasonable time. This paper will discuss the requirements to improve availability and reliability of offsite and onsite alternating current (AC) power sources to U.S. Nuclear Power Plants. In addition, the paper will discuss the requirements and guidance beyond design basis events. (author)