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:
ermal-hydraulic transient codes have uncertaintie associated with their use for predicting reactor syster response. These uncertainties should be considered as part of the overall uncertainty analysis described in Regulatory Position 4. 2.1 Basic Structure of Codes 2.1.1 Numerical Methods A best-estimate code uses a numerical scheme for solving the equations used to predict the thermal hydraulic behavior of the reactor. The numerical scheme is, in itself, a complex process that can play an important role in the overall calculation. Careful numerical modeling, sensitivity studies, and evalu ations of numerical error should be performed to en sure that the results of the calculations are represen tative of the models used in the code. Numerical simulations of complex problems, such as those con sidered here, treat the geometry of the reactor in an approximate manner, making use of discrete volumes or nodes to represent the system. Sensitivity studies and evaluations of the uncertainty introduced by noding should be performed. Numerical methods treat time in a discrete manner, and the effect of time-step size should also be investigated. 2.1.2 Computational Models A best-estimate code typically contains equations for conservation of mass, energy, and momentum of the reactor coolant and noncondensible gases, if im portant (e.g., air, nitrogen). Energy equations are also used to calculate the temperature distribution in reactor system structures and in the fuel rods. The required complexity of these equations will vary de pending on the phenomena that are to be calculated and- the required accuracy of the calculation. NRC staff experience with its own best-estimate computer codes has indicated that separate flow fields for dif ferent fluid phases, or types, and calculation 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