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The annular flow is the predominant flow regime in the upper part of Boiling Water Reactor (BWR). It consists of a thin liquid coolant film on the fuel rods and a fast moving gas core between the fuel rods. The liquid phase extends to the gas core due to whisps and entrained droplets. As the liquid film removes heat from the fuel rods by evaporation cooling, it loses mass due to the phase transition. Droplets entrained in the gas core hardly contribute to the heat removal. Therefore functional spacers have been used in the recent decades to counteract the thinning of the film, mitigating the occurrence of dryout. This doctoral thesis builds around the new DoToX facility that reproduces the dryout in a lab scale manner. Within an undisturbed subchannel between four heating rods an organic model liquid is evaporated to the point of forming annular flow. A generic functional spacer induces a swirl in the subchannel. By varying the flow rate of the coolant at constant heating power the point of dryout is reached in a steady state downstream of the spacer. The test channel can be rotated such that the recordings of a common X-ray machine allow to compute a three dimensional tomographic reconstruction. With advanced post processing methods the local, time averaged Liquid Film Thickness (LFT) and the liquid holdup of entrained droplets in the gas core can be extracted from the reconstruction data. In summary this thesis validates the concept of annular flow and dryout investigation by means of X-ray tomography in fuel bundle geometry. Local distributions of the LFT are visualized and thus dryout patches with high spatial resolution. The evolution of the holdup fraction of the gas core downstream of the spacer is resolved in sections of the subchannel. The additional droplet deposition of the spacer vanes might not always and everywhere compensate for the liquid film thinning due to increased shear stress downstream of the functional spacer. The data acquired and to be acquired with the DoToX provides a valuable source for CFD validation. The well known temperature and flow boundary conditions allow for tailored simulations that, in a later stage, can transfer the findings of DoToX to real reactor conditions.