Document: NRC Regulatory Guide
Document ID: 171a124d-54d4-4a50-8f5f-f063d9b686c9
Document Type: regulatory_guide
Title: Design-Basis Hurricane and Hurricane Missiles for Nuclear Power Plants + HISTORY - HISTORY 08/2012 – DG-1247 ,Proposed Revision 0
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1004/ML100480890.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.221
CFR Part: 
CFR Title: 

Content:
�� issued April 1974 (Ref. 3). The NRC based the original version of RG 1.76 on WASH-1300 ”Technical Basis for Interim Regional Tornado Criteria” (Ref. 4). WASH-1300 chose the design-basis tornado windspeeds so that the probability that a tornado exceeding the design basis would occur was on the order of 10-7 per year per nuclear power plant. WASH-1300 used only 2 years (1971 and 1972) of observed tornado intensity data to derive the conditional probability that, if a tornado were to strike a nuclear power plant, the maximum tornado windspeed would exceed a specified value. The probability that the tornado would strike a nuclear power plant (treated as a point) was based on more data. Estimates DG-1247, Page 3 of tornado intensity were regionalized to three regions of the contiguous United States in the original version of RG 1.76. Each region was assigned an associated set of design basis tornado characteristics, including maximum windspeed. In March 2007, the NRC issued Revision 1 of RG 1.76, “Design-Basis Tornado and Tornado Missiles for Nuclear Power Plants” (Ref. 5). This revised regulatory guide, which was based on Revision 2 of NUREG/CR-4461, “Tornado Climatology of the Contiguous United States,” issued February 2007 (Ref. 6), resulted in the modification of the regionalization presented in the original version of RG 1.76. The tornado database used in Revision 2 of NUREG/CR-4461 included information recorded for more than 46,800 tornado segments occurring from January 1, 1950, through August 31, 2003. More than 39,600 of those segments had sufficient information on their location, intensity, length, and width to be used in the analysis of tornado strike probabilities and maximum windspeeds. The second revision of NUREG/CR-4461 also relied on the Enhanced Fujita Scale which was implemented by the National Weather Service in February 2007. The Enhanced Fujita Scale is a revised assessment relating tornado damage to windspeed. The use of the Enhanced