CFD analysis is widely adopted for determination of design characteristics of major equipment with flexible geometry and flow. However, the accuracy of CFD analysis strongly depends on the numerical model, governing equation and simulation parameters. A thermal stratification phenomenon can lead to unanticipated damages of nuclear piping because of the stresses caused by different fluid densities due to stratified flow. In this paper, systematic CFD analyses are performed for surge line which is one of primary piping system by using representative commercial code to investigate key parameters; (1) mesh size and time step effect (2) turbulence parameter effect (3) material property approximation effect (4) conjugate heat transfer effect (5) insurge and outsurge flow effect. From numerical analysis results related to the items (1) through (3), the optimum CFD model as well as reasonable input parameters was determined. With regard to the item (4), thermal difference was bigger as 82∼208% than without considering conjugate heat transfer. On the other hand, for the item (5), stratified flows were come out clearer in outsurge flow. Based on the parametric CFD analyses to simulate stratified flows, most of numerical issues were resolved while further investigation is required for the conjugate heat transfer effect.Copyright © 2008 by ASME