Document: NRC Regulatory Guide
Document ID: da2e0703-3488-44b0-b6d0-089aac7cae3d
Document Type: regulatory_guide
Title: Format and Content of Plant-Specific Pressurized Thermal Shock Safety Analysis Reports for Pressurized Water Reactors
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003740028.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.154
CFR Part: 
CFR Title: 

Content:
y. References 19 through 24 describe the results o f recent mixing analyses and experiments. Reference 19 i d e n t i f i e s a useful s t r a t i f i c a t i o n c r i t e r i o n t o determine which overcooling transients w i l l require the additional mixing analysis. Particular attention should be given t o scenarios t h a t involve H P I under very low flow o r stagnant loop conditions. When stagnation i s p a r t i a l (i.e., not a l l loops stagnate), s t r a t i f i c a t i o n i s expected only w i t h i n the cold legs o f the stagnant loops. However, scenarios involving complete loop stagnation w i l l require the evaluation o f a transient cooldown i n the presence o f s t r a t i f i e d layers both i n the cold legs and i n a portion o f the downcomer. The mixing model should include the e f f e c t o f metal heating on the mixing behavior, p a r t i c u l a r l y i n a stagnant flow situation. Also, the e f f e c t o f noncondensible gases, i f present, should be included. References 19 through 23 describe tools t h a t have been used f o r such an analysis. I _ This section should also document the heat transfer correlations applied i n the mixing analysis. The research e f f o r t s described i n References 18 through 23 indicated t h a t the downcomer heat transfer coefficients generally exceeded 300 Btu/hr-ft2-OF. These values o f heat transfer c o e f f i c i e n t were generally high enough t o keep the vessel wall surface temperatures w i t h i n a few degrees o f the downcomer f l u i d temperature. Furthermore, because the vessel wall cool- down was controlled by conduction processes rather than convection processes, the vessel wall surface temperatures were insensitive t o heat transfer coef- f i c i e n t variations due t o changes i n flow and heat transfer regimes. 4.5 Thermal-Hydraulic Analysis Results This section should present graphs o f the best-estimate downcomer pressures, f 1 u i d temperatures, and heat transfer coefficients and