Development of a new bubbly-slug interfacial friction model for RELAP5

Abstract

The development of a new bubbly-slug interfacial friction model for the Pressurized Water Reactor (PWR) safety code RELAP5 is described. The model is based on a set of best-estimate void fraction correlations which cover the full range of geometries and flow conditions encountered in PWR safety analysis. By exploiting the relationship between void fraction and interfacial friction that exists for steady, fully developed flow conditions, the correlations are converted into effective interfacial friction coefficients that can be applied in the code for transient as well as steady-state conditions. Assessments against separate effects tests indicate that the new model is more accurate than the existing model in many situations, particularly rod bundle geometries, and should never be significantly less accurate. The model has been implemented in a local version of RELAP5/MOD2 and in a pre-release version of RELAP5/MOD3 at Idaho National Engineering Laboratory (INEL).