Subchannel void-fraction measurements in a 6 × 6 rod bundle using a simple gamma-transmission method

A relatively simple subchannel void-fraction measurement system was developed and applied to a scaled model of a BWR fuel assembly. The measurement system is based upon the gamma-transmission technique and uses a tomographic reconstruction procedure to calculate the void-fractions in the subchannels. Data pre-processing is used to remove positioning errors of the gamma source and detector. Measurements are performed under different operating conditions and radial power profiles. The measurements are compared with the drift-flux two-phase flow model. The void-fraction in the wall and corner subchannels is lower than average. By applying the drift-flux model to each subchannel separately and comparing this calculation with measurements the presence of lateral void-drift should be detectable. However, the data do not show evidence for lateral void-drift. By comparing two chordal void-fractions, one through the fuel rods and one in between the fuel rods, a flow pattern transition could clearly be seen between a void-fraction of 30% and 40%.