The XY toy model used for validation. Color code: yellow = sodium (plenum or rod followers), red = inner fuel, orange = outer fuel, green = fertile (lower axial blanket, plate in the inner core), purple = absorbers, blue = reflector. Boundary conditions: specular reflection (left), vacuum (right, top and bottom). The dashed white line encloses the regions represented in Figure 2 and Figures 4 to 8.