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:
ion of non equilibrium between phases may be required to calcu late some important phenomena (e.g., countercurrent flow, reflood heat transfer) to an ac ceptable accuracy. The NRC staff has also deter mined that certain phenomena require that the equa tions be solved in multiple dimensions. However, one-dimensional approximations to three dimensional phenomena will be considered accept able if those approximations are properly justified. Other basic code features include equations of state and other material properties. Sensitivity studies and comparisons to data should be performed to deter mine the importance of the simplifications used. 3. BEST-ESTIMATE CODE FEATURES 3.1 Initial and Boundary Conditions and Equipment Availability The heat generated by the fuel during a loss-of coolant accident depends on the power level of the reactor at the time of the loss-of-coolant accident and on the history of operation. The most limiting initial conditions expected over the life of the plant should be based on sensitivity studies. It is not necessary to assume initial conditions that could not occur in com bination. For example, beginning-of-life peaking fac tors together with end-of-life decay heat do not re quire consideration. Given the assumed initial conditions, relevant factors such as the actual total power, actual peaking factors, and actual fuel condi tions should be calculated in a best-estimate manner. The calculations performed should be represen tative of the spectrum of possible break sizes from the full double-ended break of the largest pipe to a size small enough that it can be shown that smaller breaks are of less consequence than those already consid ered. The analyses should also include the effects of longitudinal splits in the largest pipes, with the split area equal to twice the cross-sectional area of the pipe. The range of break sizes considered should be 1.157-4 sufficiently broad that -the system response as a func tion of break size is well enough