Probabilistic analysis of faults affecting multiple trains of the electrical power supply system of nuclear power plants

Abstract

Faults simultaneously impairing multiple redundant trains of the electrical power supply system of NPPs recently received growing attention by the nuclear community. This was triggered by events at several different NPPs including Byron in the U.S. or Forsmark in Sweden. Such events have generally not been included in PSAs of NPPs yet. Therefore, GRS has initiated a research project aiming at a comprehensive and in-depth analysis of events characterized by fault states of multiple trains of the electrical power supply system (including but not limited to open phase conditions) and at the development of modelling and quantification methods to include them in PSAs. The project consists of different interacting efforts. Firstly, the possible causes of faults affecting multiple trains of the electrical power supply system and their consequences are assessed from an operating and modelling perspective. The second step comprises the development of a detailed dynamic model of the electrical power supply system of a German PWR and the investigation of the cause and the propagation of such faults. Then, a current PSA model of a German PWR is extended to allow for the modelling of the phenomena identified in the previous steps. This includes adding relevant equipment not modelled before and new failure modes of equipment already modelled. The additional reliability parameters and frequencies of initiating events required to quantify the extended PSA model are estimated. Finally, the additional failure mechanisms considered in the extended PSA model are evaluated quantitatively.