Abstract: The core of a pressurized water nuclear reactor is partly formed from dismantlable assemblies (10), whereof the rods contain mixed UO.sub.2 -PuO.sub.2 oxide pellets, said assemblies (10) being subdivided into concentric zones (12,14,16) with a plutonium concentration decreasing from the inside to the outside. At the end of each irradiation cycle, the rods (C.sub.3) located in peripheral zone (16) are discharged and the rods (C.sub.2,C.sub.1) located in the other zones (12,14) are transferred towards the outside of assembly (10) into zone (14,16) adjacent to that which they previously occupied. New rods (C.sub.4) with a single enrichment are then introduced into the thus freed central zone (12).