Modelling of two-phase critical flow in steam generator tube guillotine rupture scenario with Apros for the C-FLOW separate-effect-test facility

Abstract

The purpose of this master’s thesis is to investigate two-phase critical mass flow rate in a separate-effect-test facility proposed to be built in LUT University’s Nuclear laboratory under C-FLOW project funded by SAFER2028 National Research Programme. The facility focuses on critical flow discharges in a primary-to-secondary leak scenario when steam generator tube undergoes a guillotine break. For this purpose, a test facility is being built, consisting of an upstream pressure vessel, a larger diameter measuring pipe with volume flow measurement and shut-off valve, tube for discharging steam/water mixture to atmosphere with different length-to-diameter ratios. The main idea of the facility is to use different L/D ratio discharge tubes and to examine effects of the very long upstream discharge tube on to critical mass flow rate. L/D ratio larger than 302 has not been found in the literary and the separate-effect-test in C-FLOW will broaden this with representative steam generator tube sizes of VVER-440 and EPR. This work includes a theoretical examination of critical mass flow in terms of mathematical models. Different mathematical models are examined and how they describe one- and two-phase critical mass flow rate. The work consists of a review of the most important SETs used in the prior studies and few examples of newer experimental work that can be considered state-of-the-art. Additionally, a simulation model using the Apros system code is created to model the test facility. By using the results obtained from the simulation, it is possible to find out the critical pressure ratio where the two-phase critical flow conditions are met for tubes with different L/D ratios. Furthermore, the simulation results provide valuable insights into various critical parameters, including void fraction along the discharge tube, void fraction at the end of the discharge tubes, pressures at the end of discharge tubes, mass flow rates, and velocities for both phases. These comprehensive findings enhance our understanding of the system's behaviour and aid in optimizing its performance, ensuring safer and more efficient operation.