Regulatory Guide 1.221, issued in 2011, describes new requirements for design basis hurricane winds in coastal regions of the United States. The motivation for the new guidance was a reduction in the regionalized design basis tornado wind speeds in Revision 1 of Regulatory Guide 1.76, issued in 2007.
The reduction in the tornado wind speeds makes it more likely that hurricane winds may govern the design of nuclear power plant structures in some locations. Differences in the assumed characteristics of the wind fields associated with tornadoes and hurricanes make a straightforward comparison of maximum wind speeds inconclusive. Structure dimensions can play a significant role in determining which wind case will govern, so wind pressure profiles must be examined on a structure by structure basis. Hurricane missile velocities also differ from tornado missile velocities.
The new requirements have potential significance when considered relative to the parameters previously used for both existing plants and new certified designs. This paper discusses the latest Regulatory Guide requirements, the characteristics of hurricane and tornado wind fields, and the different velocities considered for wind-borne missiles. The impact of the requirements is also discussed relative to the design basis for existing nuclear power plants and for recent new plant designs.
Javad Moslemian, Sara Dirks, Matthew Mathien - Sargent & Lundy