United States Nuclear Regulatory Commission - Protecting People and the Environment

Susceptibility of Nuclear Stations to External Faults (NUREG/CR-7175)

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Publication Information

Manuscript Completed: July 2012
Date Published:
September 2014

Prepared by:
Michael Villaran and Meng Yue

Nuclear Science and Technology Department
Brookhaven National Laboratory
Upton, New York 11973-5000

Darrell Murdock, NRC Project Manager

Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code N6838

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Abstract

The offsite electric power supply, delivered via the electrical transmission grid and nuclear power plant (NPP) switchyard, is the preferred source of power for normal and emergency NPP shutdown. Since the deregulation of the electric power industry, NPP electrical distribution systems have become more vulnerable to the effects of external transmission system faults because most of those switchyards and transmission lines are no longer owned, operated, or maintained by the same companies that own and operate the nuclear plants. Also, with the exception of the North America Electric Reliability Corporation (NERC) standard NUC-001-2, there is a lack of detailed industry-wide technical standards for (1) the interface between NPPs and transmission/subtransmission networks; (2) the protection systems for the interface; and (3) the maintenance of the primary and secondary equipment in the interface.

As part of a research program sponsored by the NRC Office of Nuclear Regulatory Research, the effects that electrical faults and other disturbances originating on the electric power grid can have on the availability of offsite power sources and the performance of the NPP are studied. A review of NPP switchyard configurations, transmission grid interface configurations, and their electrical protection systems was undertaken to better understand the dynamics of the interconnection between the NPP onsite and offsite power systems.

Several simulation models were developed based upon actual NPP power distribution systems, their transmission system interfaces, and electrical protection systems using power system analysis software. An event tree type approach was followed in developing the simulation study scenarios and contingencies in the analyses. The importance of maintenance on the response of the electrical protection systems to external fault events was considered.

Conclusions and observations are presented for improving the response of electrical protection systems to an external fault in order to minimize the occurrence of a loss-of-offsite power and nuclear plant trip.

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