Document: NRC Regulatory Guide
Document ID: 5f799693-27fd-4e13-a5e1-4c02f393d90a
Document Type: regulatory_guide
Title: Best-Estimate Calculations of Emergency Core Cooling System Performance + HISTORY –HISTORY 04/2013 – Periodic Review of Revision 0 – Reviewed with issues identified for future consideration 03/1987 – Draft RS 701-4, Proposed Revision 0
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003739584.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.157
CFR Part: 
CFR Title: 

Content:
under consideration, a best-estimate heat transfer model should be used. This model should include the effects of any flow blockage calculated to occur as a result of cladding swelling or rupture. 5. Thermal-hydraulic models that do not calcu late multiple channel effects should be com pared with applicable experimental data or more detailed calculations to ensure that all important phenomena are adequately calculated. 3.14 Boiling Water Reactor Channel Box Under Spray Cooling Following the blowdown period, heat transfer from the channel box and wetting of the channel box should be based on the calculated fluid conditions on both sides of the channel box and should make use of best-estimate heat transfer and rewetting models that have been compared with applicable experimen tal data. 3.15 Special Considerations for a Small-Break Loss-of-Coolant Accident in Pressurized Water Reactors The slower small-break loss-of-coolant accident leads to fluid conditions characterized by separation of the fluid phases versus the more homogeneous fluid conditions that would result from rapid large break loss-of-coolant accident transients. Phenomena ~ that would occur in a PWR during a small-break loss of-coolant accident would, therefore, be significantly different from those phenomena that would occur during a large-break loss-of-coolant accident. The distribution of liquid throughout the reactor system, in addition to the total liquid inventory, is of in creased importance for the small-break loss-of coolant accident. A number of special factors must be given increased consideration in small-break loss of-coolant accident calculations to correctly predict phenomena influenced by the liquid inventory distribution. Break flow may be greatly influenced by the loca tion and specific geometry of the break. For a break in a horizontal pipe containing stratified flow, the quality of the break flow will be a strong function of the assumed location of the break on the pipe (e.g., top or