Investigating annular flows and the effect of functional spacers in an adiabatic double-subchannel model of a BWR fuel bundle by ultra-fast X-ray tomography

Dryout of the coolant liquid film at the upper part of the fuel assemblies of a boiling water reactor (BWR), where annular flow prevails, represents a safety concern and an economical constraint. To be able to deal with this issue, annular flows must be well understood. Therefore we have investigated such flows in a double-subchannel model of a BWR fuel bundle using X-ray imaging at the Rossendorf Ultrafast Electron Beam X-ray Tomograph (ROFEX) facility. Adiabatic experiments slightly above atmospheric conditions using air-water annular flows have been carried out. In annular flows a significant portion of the liquid is present as droplets in the gas flowing in the middle of the conduit. Using functional spacer grids is the common method in nuclear technology to influence the annular flow such to enhance the deposition of droplets into the liquid film on the fuel pins thereby increasing dryout margins. The tests include, besides investigating the liquid film thickness in the plain channel, the investigation of the effect of a functional spacer on the liquid film. Using NaI as contrast agent in water enhances the image quality. The high time-resolution of the tomography enables to follow the wavy structure of the annular film flow. The paper aims at extracting quantitative information on the liquid film thickness distributions from the reconstructed tomographic images.