Left: top-view on the arrangement of fuel rods (yellow), cladding tubes (green) as well as control rods (blue) in case of a complete fuel assembly. Each element can be specified unambiguously based on its slot position given by x_id, y_id. Right: same as left figure, but several elements along the diagonal are missing (x_id ∕y_id= 1, 5, 9). This configuration is investigated in more detail in Section 3.2.