[en] The contribution under consideration reports on a gamma denitometric measurement of gas concentrations in a vertical heated tube bundle which is flowed around by a fluid. Two measurement positions, two flow rates of the circulating fluid, two subcooling values and eleven heat fluxes were selected for the measurement. The authors of this contribution describe the test facility, measurement methodology, results and their interpretation. The measurement uncertainty is described in detail.

[en] 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 somewhat 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 as 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 measuring of 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. (author)