Document: NUREG-0800
Document ID: ecf7a8d4-0097-4a93-8609-630545f03f70
Document Type: srp
Title: REGULATORY TREATMENT OF NONSAFETY SYSTEMS FOR PASSIVE
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1308/ML13081A756.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.3
CFR Part: 
CFR Title: 

Content:
he Westinghouse AP600 Standard Pressurized Reactor Design’, Related to Post-72 Hour Actions”. At that time the NRC was concerned with the ability of a severe hurricane to cause an extended loss of reliable offsite AC power for a period longer than 72 hours. Consequently, the NRC took the position that it was reasonable and practical to design post-72 hour SSCs (most notably nonsafety related diesel generators and their enclosure) to withstand a Category 5 hurricane and associated wind-borne missiles; but, these SSCs should not be required to withstand tornado loads and tornado missiles. Also at the time, tornado loads and missiles were considered generally to lead to more restrictive design requirements. Since this position was established in the mid-1990s , Regulatory Guide 1.76 has been revised using the Enhanced Fujita Scale, resulting in a significant decrease to the maximum design basis tornado wind speeds, and new guidance (Regulatory Guide 1.221) has been issued for addressing hurricanes and associated hurricane missiles. In addition, recent operating experience shows that tornado wind events can also cause an extended loss of reliable offsite AC power for more than 72 hours. Lastly, application of the guidance described in the memorandum referenced above could in some cases result in a level of treatment for nonsafety related SSCs which meet Criterion B for RTNSS that is higher than the level for safety-related SSCs. Therefore, the RTNSS missile protection guidance described in the memorandum is no longer appropriate. The NRC's position now is that RTNSS “B” SSCs should be protected from both tornados and hurricanes and missiles they might create, and that applicants should choose the design basis wind speeds for RTNSS “B” SSCs using the guidance in Regulatory Guides 1.76 and 1.221. 19.3-18 Draft Rev.0 – June 2013 The public has not had an opportunity to comment on this approach to treatment of the high winds hazard for certain RTNSS SSCs as it