A fuel assembly (1) for a nuclear reactor comprises a bundle of fuel rods (3) with a first group of fuel rods (5a) and a second group of fuel rods (5b). Each fuel rod comprises a cladding tube (15) and stack of fuel pellets (12) enclosed within the cladding tube. At least a main portion (13) of the fuel stack comprises a fissile nuclear material comprising Plutonium and Thorium, and is arranged with an axial hole (22). Said main portion (13) of the fuel stack of each fuel rod (5a) of the first group comprises a first axial hole (22a) constituting a first volume and said main portion of the fuel stack of each fuel rod (5b) of the second group comprises a second axial hole (22b) constituting a second volume. The first volume and the second volume are different.