Experimental investigation of the internal structure of boiling two-phase water flow under LWR core operating conditions

• First-of-a-kind radial measurements of boiling two-phase flow under LWR conditions were performed at KTH. • Commissioning tests validated the HWAT facility performance against established models. • Advanced multi-sensor unit enables local measurements of void fraction, velocity and interfacial length scales. • Advanced post-processing reveals polydispersed bubble and droplet distributions. • Resulting data supports closure law development and validation of thermal–hydraulic models. An experimental setup has been designed and manufactured at the Royal Institute of Technology (KTH) in Stockholm to investigate the internal structure of boiling two-phase water flow under prototypical Light Water Reactor (LWR) core conditions, including those relevant to PWR, BWR and SMR designs. The setup is based on the High-pressure WAter Test (HWAT) loop, designed for 25 MPa pressure, 1 kg/s water mass flow rate and 1 MW thermal power. The facility has been updated with a new test section and advanced instrumentation systems to enable measurements in both forced convection and natural circulation, under steady-state and transient operations. This novel experimental setup allows for the first-time measurements of radial distributions of local two-phase flow parameters under high-pressure LWR core conditions. The resulting data is intended to enhance the fundamental understanding of boiling two-phase flow phenomena, contribute to the development of closure laws (including for polydispersed flow) and support the validation of computational codes (1-D and 3-D). The paper presents the loop design, the updated instrumentation with associated uncertainties, and data post-processing methods (including the derivation of dispersed phase length scales). Results from commissioning tests, such as heat balance tests and single-phase tests, are presented. Examples of high-pressure boiling two-phase flow measurements are presented and discussed. Fundamental behavior and associated key parameters, including radial distributions of void fraction, mixture velocity, interfacial length scales and polydispersed characteristics, are identified and quantified.