Measurement of the water-vapor void fraction in a 4x4 unheated rod bundle

In recent years, computational fluid dynamics (CFD) simulations have been used to evaluate the two-phase flow behavior inside fuel bundles for nuclear core design and accident management. Space - time distributions of the void fraction and interfacial velocity in bundle systems at high temperatures and pressures are important for validating CFD codes describing two-phase flow. However, such space - time distributions of the void fraction and interfacial velocity in bundle systems are difficult to obtain under high-temperature and high-pressure conditions. Thus far, we have developed an experimental apparatus with a 4 x 4 unheated rod bundle by adapting a through-rod wire mesh sensor to measure the void fraction and interfacial velocity distributions under high pressures and high temperatures. In this study, we measured distributions of the void fraction and interfacial velocity in a water-vapor system under high-pressure conditions up to 2.6 MPa and high temperatures, using the developed apparatus. We demonstrate that the experimental apparatus yields reasonable results for two-phase flow inside a fuel bundle. However, the analysis method used in this study could not properly estimate the interfacial velocity under some experimental conditions, highlighting the need to improve the method for analysis of the interfacial velocity.