Abstract: In a pressurized water reactor, the pressure vessel contains a plurality of fuel wrappers of parallelepipedal section supported on a lower grid which carries projecting sleeves equal in number to the wrappers. Each sleeve penetrates into a bore of a wrapper base plate and has a higher coefficient of thermal expansion than the base plate so that its external diameter is smaller than that of the bore in the cold state and equal thereto at the operating temperature of the reactor. The bore is thus clamped against the sleeve as a result of thermal expansion, the fuel assemblies being secured within the wrappers by means of spring-type elastic devices.