Neutron imaging of annular flows in a tight lattice fuel bundle model

High conversion light water reactors (HCLWRs), featuring tight lattice fuel bundles, could fulfill the transition between present-day and future, fast reactor based technology allowing an improved fuel utilization. Nevertheless, the cooling of the tight lattice and the thermal-hydraulic feasibility should be thoroughly examined. We have studied annular flows and the influence of functional spacer on them in a tight lattice fuel bundle model using cold-neutron imaging, a non-intrusive technique. The investigation focused on adiabatic, air-water annular flows in a scaled-up model of four neighboring subchannels of a tight lattice fuel bundle model. Liquid film thickness distributions have been measured for different flow conditions with and without spacer to quantify the influence of the latter. The film thickness is determined with a minimal bias and a reasonable statistical uncertainty (7-10%) at a high spatial resolution (∼60 μm). The entrained liquid hold-up fraction and the effect of the spacer on it is given as well.