Document: NRC Regulatory Guide
Document ID: e5306f4f-dc8c-4f60-a71c-af7ad4080384
Document Type: regulatory_guide
Title: Aquatic Environmental Studies for Nuclear Power Stations + HISTORY - HISTORY 12/2014 – DG-4023 -Proposed New Guide
Source: NRC Regulatory Guide Division 4
Source URL: https://www.nrc.gov/docs/ML1318/ML13186A085.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-4.24
CFR Part: 
CFR Title: 

Content:
sh racks, trash bars, intake screens, or some other physical component of the intake structure such that the individual cannot swim, walk, crawl, or otherwise move away from the structure. Impingement can harm or kill an organism through physical abrasion, starvation, exhaustion, asphyxiation, descaling, drowning, or other physical harms. Through-screen velocity is an important factor affecting impingement rates and impingement survivability of fish and shellfish species. However, mitigation measures, such as fish-return systems and modified Ristroph screens, may increase impingement survivability by providing a mechanism to deliver impinged organisms back to the natural water body. Finally, entrainment occurs when organisms pass through all components of the intake structure and enter the actual power plant with the cooling water. Entrained organisms typically are quite small since they pass through even mesh intake screens; usual entrained organisms include eggs, larvae, and spores, the absence of which may affect trophic cascades. Entrainment exposes organisms to many stressors, and the NRC generally assumes complete mortality for entrained organisms. Susceptibility to these three major types of water withdrawal effects varies by species, life history stage, and individual, as well as by temporal factors. Entrapment is largely a feature of the intake structure design, including the probability that an organism will enter a return system or that an organism is capable of returning to the source or receiving water body. For example, marine mammals would rarely be entrapped because their high intelligence allows them to swim easily around obstacles to return to their natural environment. However, exceptions exist particularly if a marine mammal swims into an intake pipe leading to an embayment or canal from which there is no escape route. Impingement typically affects fish and positively buoyant vegetation more than other biota, and some species are more susceptible