Document: NRC Regulatory Guide
Document ID: de79d411-4fc9-456e-a5a0-f4a910ca4c9a
Document Type: regulatory_guide
Title: Assumptions Used for Evaluating a Control Rod Ejection Accident for Pressurized Water Reactors (Rev. 2)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2111/ML21119A157.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.77
CFR Part: 
CFR Title: 

Content:
s chemical releases coincident with the radiological consequences (e.g., a design-basis loss-of-coolant accident for plants that are vulnerable to both events simultaneously) and demonstrate that such coincident events do not produce an unacceptable level of risk. 3.3 Atmospheric Dispersion NUREG/CR-6210 documented that HABIT has two basic Fortran modules, i.e., EXTRAN and CHEM. The EXTRAN module is formulated for a Gaussian plume or puff dispersion model and DG-1387, Page 11 longitudinal, lateral, and vertical dispersions between the point of release to the intake of the CR. The CHEM module is calculated for the chemical concentration and exposure in the CR based on the ventilation system and associated air-cleaning installations. The EXTRAN also allows for the effect of building wakes and for additional dispersion in the vertical direction when the distance between the release point and the CR is small. When boiloff or a slow leak is analyzed, the effects of density on vertical diffusion may be considered if adequately substantiated by reference to data from experiments. For chemicals that are not gases at 100 degrees Fahrenheit at normal atmospheric pressure but are liquids with vapor pressures in excess of 10 torr, applicants and licensees should consider the rate of flashing and boiloff to determine the rate of release to the atmosphere and the appropriate time duration of the release. For gases that are heavier than air, the buoyancy effect should be considered for many parameters, such as density of the plume and roughness of the ground surface, in determining the dispersion characteristics. NUREG-2244, “HABIT 2.2: Description of Models and Methods,” incorporates both the U.S. Environmental Protection Agency’s DEnse GAs DISpersion Model (DEGADIS) code (Ref. 20) and the U.S. Department of Energy’s atmospheric dispersion model “SLAB” code (Ref. 21) for denser-than-air releases codes for dense gas transport phenomena. 3.4 Control Room Air Flow The