Optimizing the performance of cold-neutron tomography for investigating annular flows and functional spacers in fuel rod bundles

•Annular flows w/wo functional spacers are investigated by cold-neutron imaging.•Performance of liquid film thickness measurement under different imaging modalities is evaluated and optimized.•MC simulations of the cold neutron imaging are performed to help quantifying the measurement accuracy.•Spectral, scatter corrections and minimizing the blur are necessary to avoid bias in film thickness estimate.•Empty channel referencing offers in general the highest accuracy and quality.

In a previous study we used cold-neutron imaging to examine annular flows and the influence of functional spacers on such flows in a fuel bundle model. Investigating adiabatic, air–water annular flows in a scaled-up model of two neighboring subchannels of a BWR fuel assembly, we have shown that cold-neutron tomography is a promising tool to study such flows in nuclear fuel bundles. Here we try to optimize the performance of the method and examine the influence of certain parameters and modalities of the imaging on the quality and accuracy of the results.