Void fraction and pressure drop measurement in a reflooded single tube

Void fraction is a key parameter both to interpret emergency cooling experiments and to predict the clad temperature transient during a loss of coolant accident of PWR. However classical techniques to measure void fraction ask some problems, expecially in the case of large breaks for which both flooding rate and pressure are low and characterized by a wide range of void fraction associated with different two-phase flow regimes. A series of axial void fraction and pressure profiles was performed, respectively on a direct heated and an indirect heated reflooded single tube which inner diameter is very close to the hydraulic diameter of a 17 x 17 PWR assembly. This paper mainly deals with the neutron scattering technique used to investigate void fraction. Various aspects are discussed, such as radial distribution effect, energy of neutrons, measurements of scattered or transmitted neutron flux, counting technique, water temperature and axial void gradient effects. Typical results are presented as well as a comparison between experimental data and predictions of various void fraction models