Document: NRC Regulatory Guide
Document ID: 46049842-54a7-40a0-a0cc-ab115059f05e
Document Type: regulatory_guide
Title: Ultimate Heat Sink for Nuclear Power Plants + HISTORY - HISTORY DG-1275 , Proposed Revision 3, published 09/2013 (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1304/ML13043A624.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.27
CFR Part: 
CFR Title: 

Content:
et cooling tower is used as the UHS, the controlling parameter would be a wet-bulb temperature, and the critical time period may be on the order of several hours. Therefore, an acceptable design basis meteorological condition for this type of UHS would be the maximum observed (based on representative regional climatological information) wet-bulb temperature for the critical time period. As another example, the plant intake water temperature from a cooling pond used as the UHS may reach a maximum several days following a shutdown or DBA. This maximum pond temperature should be considered to coincide with the most severe combination of controlling meteorological parameters determined for the critical time period, (i.e. solar energy input, wind factors, evaporative cooling, convection/conduction, recirculation of plant heat load, etc). DG-1275, Page 5 Another critical time period is associated with ensuring the availability of a 30-day cooling supply. The meteorological conditions resulting in maximum evaporation should be the worst 30-day combination of controlling parameters specific to the design of the UHS. Natural or manmade features may provide the UHS safety functions. More than one water source may be involved in the UHS complex in performing these functions under different conditions. Because of the importance of the UHS to safety, these functions should be ensured during and following the design-basis events postulated for the site (e.g., the safe-shutdown earthquake (SSE); design-basis tornado, hurricane, flood, or drought). In addition, the UHS safety functions should be ensured during other applicable site-related events that may be caused by natural phenomena such as river blockage, river diversion, reservoir depletion, or, if applicable, accidents such as ship collisions, airplane crashes, nearby pipeline explosions, or oil spills and fires. Reasonable combinations of less severe natural and accidental phenomena or conditions also should be considered to