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:
ometry 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 bottom). Small-break loss-of-coolant accident calculations should, therefore, include various as sumed break locations in the spectrum of breaks ana lyzed. The assumed operating state of the reactor coolant pump will also influence the distribution of liquid throughout the system and the amount of liquid lost through the break. The pump operation assumptions used in the cal culations should be the most likely, based on operat ing procedures, with appropriate consideration of the uncertainty of the pump operation during an actual event. Level depression in the core region and subse quent core heatup may be influenced by liquid holdup in the steam generator tubes, manometric ef fects of liquid in the piping and loop seal region, and liquid levels relative to vent paths for steam through upper plenum bypass flow paths and vent valves. Steam generator heat transfer under "reflux" or "boiler-condensor" modes of operation may also strongly influence core inventory through level de pression and the effect on total system pressure and, thus, on ECCS flow. These phenomena should be carefully considered in the calculation. Sensitivity studies of the importance of these effects should be performed for use in the uncertainty evaluation. Heat transfer from an uncovered core under high-pressure conditions typical during a small-break loss-of-coolant accident may include contributions from both convective and radiation heat transfer to the steam. Models will be considered acceptable pro vided their technical basis is demonstrated through comparison with appropriate data and analyses. Spe cific guidance regarding uncovered bundle heat trans fer is given in Regulatory Position 3.9.3. 3.16 Other Features of Best-Estimate Codes No list of best-estimate code features could be