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:
f any flow blockage calculated to occur as a result of cladding swelling or rupture. Heat transfer calculations that account for two-phase conditions in the core during refilling of the reactor vessel should be justified through com parisons with experimental data. Best-estimate mod els will be considered acceptable provided their tech nical basis is demonstrated through comparison with appropriate data and analyses. The FLECHT-SEASET tests (Refs. 40, 45, and 46) should be considered when establishing an ac ceptable set of relevant data. Reference 7 contains extensive information regarding a large amount of ex perimental reflood heat transfer data. This informa tion should also be considered when developing and assessing models. The results from the 2D/3D pro gram are particularly relevant. 1.157-10 3.13 Convective Heat Transfer Coefficients for Boiling Water Reactor Rods Under Spray Cooling Models will be considered acceptable provided their technical bases can be justified with appropriate - data and analyses. These models should contain the following: 1. Following the blowdown period, convective heat transfer coefficients should be deter mined based on the calculated fluid condi tions and heat transfer modes within the bun dle and on the calculated rod temperatures. 2. During the period following the flashing of the lower plenum fluid, but prior to ECCS initiation, heat transfer models should in clude cooling by steam flow or by a two phase mixture, if calculated to occur. 3. Following initiation of ECCS flow, but prior to reflooding, heat transfer should be based on the actual calculated bundle fluid condi tions and best-estimate heat transfer models that take into account rod-to-rod variations in heat transfer. 4. After the two-phase reflood level reaches the level 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.