Serpent model of TAPIRO: vertical section (left) and horizontal section (right). Zoom on the core and reflector regions. The dimensions of the depicted regions are 100 × 100 cm. The figure on the bottom show some specific regions of the core (on the left) and of the reflector (on the right) where neutron spectra have been evaluated with Serpent (Fig. 2). Each mf label is associated with a zone with a different fuel composition.