An open phase condition is a known phenomenon in the power industry and is now recognized to have adverse impact on the electrical power systems in several nuclear power plants. An open phase condition may result in challenging plant safety. Operating experience in different countries has shown that the currently installed instrumentation and protective schemes have not been adequate to detect this condition and take appropriate action. An open phase condition, if not detected and disconnected in a timely manner, represents design vulnerability for many nuclear power plants. It may lead to a condition where neither the offsite power system nor the onsite power system is able to support the safety functions, and could propagate to station blackout. The design of electrical power systems needs to be evaluated systematically and improved, where necessary, to minimize the probability of losing electric power from any of the remaining supplies as a result of single or double open phase conditions. The improved design should be coordinated with existing measures to ensure that the electrical power system is able to support the safety functions after the open phase condition is detected and disconnected. In this regard, the IAEA has developed a safety publication dealing with design vulnerability of open phase conditions. This paper summarizes the contents of the report, the rationale and criteria to enhance the safety of nuclear power plants by providing technical guidance to address an open phase condition vulnerability in electrical systems used to start up, operate, maintain and shutdown the nuclear power plant.

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