Coupling of CFD and SYS TH codes in BEPU approach, Case study : two-phase PTS scenario

BEPU (Best Estimate Plus Uncertainty) is a new modern approach for nuclear reactor safety and used in accident analysis. In this area, BEPU implies the application of standard and qualified procedures to survey the effect of each phenomenon on safety margin. System TH and CFD codes are the main tools in this way utilizing appropriate model for each phenomenon. However, models and simplifications employed in each code limit the application of them. As a part of BEPU, identification and selection of the best tool for investigation of a phenomena have effects on safety margin. But some TH scenarios and industrial concerns are beyond the ability of individual category of codes, and code coupling should be considered as main strategy for capturing all features of those scenarios. For different nodalization and models considered in system TH and CDF codes, correct coupling of them requires more attention about preparing suitable boundary condition for each type. TH step of Two-Phase PTS scenario is one of those events: the coupling of system TH and CFD codes is needed to predict imposed temperature profile on the downcomer. The maturity of system codes such as RELAP is sufficient to obtain plant response to the initial event. On the other hand, produce necessary input data for structural mechanical analyses may include 3D effects and local phenomena which are outside the capabilities of systems codes. Therefore, CFD codes should be employed to bridge the gaps. The results of RELAP simulation before injection point of ECCS is considered as input of CFD part. CFD codes is employed for mixing and stratification zone of cold leg needing 3D nodalization for prediction of temperature profile. In these zones, calculation of interfacial heat transfer, based on turbulence kinetic energy and dissipation rate, plays a significate role to predict temperature profile. But, traditional eddy viscosity model need special treatment near the free surface for prediction of turbulence characteristics. In this paper, a new model for turbulence based on source function of turbulence kinetic energy and eddy frequency was employed to investigate the effects of improvement CFD turbulence models on safety margin as main goal of BEPU for two-phase PTS. The result of this coupling platform is compared with whole simulation of RELAP code for demonstration of coupling effect and improvement of turbulence prediction in stratified zone.