Validation and uncertainty quantification for two-phase natural circulation flows using trace code

The work presents validation of the TRAC/RELAP Advanced Computational Engine (TRACE) for the natural circulation two-phase flow in vertical annulus. The natural circulation experiments were recently conducted for vertical internally heated annulus at the Multiphase Thermo-fluid Dynamics Laboratory (MTDL) at University of Illinois [1]. The experimental matrix consists of 107 experiments with system pressure in a range of 145 − 950kPa and heat flux up to 275kW/m². The void fraction, gas velocity and interfacial area concentration were measured in 5 axial locations along the test section with 6 measurements of bulk liquid temperature and pressure. To validate the capability of the TRACE code under natural circulation flow conditions, the complete model of the facility was created and validated using forced convection and single-phase natural circulation data. The sensitivity and uncertainty quantification were performed. The sensitivity to important simulation parameters was studied using Solol’ variance decomposition [2] and Morris screening method [3]. The sensitivity of boundary conditions on void fraction measurement, was investigated. The sensitivity study has shown significant differences in model sensitivity between different experimental conditions. With heat flux being the most influential parameter for the high-pressure cases without flashing and pressure, temperature and heat flux having combined strong effect in case of low-pressure experiments when flashing occurs. Additionally, higher uncertainty in void fraction prediction was observed for experimental conditions at low-pressure with flashing.