The figures in the two upper rows show transmission-radiographic images of longitudinal scans of a.) an empty generic model (top row) as well as b.) a generic model filled with 18 out of 19 possible fuel assemblies (middle row) where the central fuel compartment remained empty. The bottom row shows corresponding difference images, a − b. The color code represents the ratio of muons that were detected in both detectors over the total number of muons. Different assumptions on the initial muon directions are considered from left to right and range from a fixed initial muon direction with θ_in = 0° (left) to an angular distribution with respect to θ_in of 0° to 25° (right). The x- and y-coordinates refer to the muon positions at the exit of the incoming detector.