A cuboid pressure vessel comprises a top wall, a bottom wall, and an external wall. The external wall comprises a main body including a first side wall having a first opening, and a second side wall having a second opening; a first lid secured to the first side wall so as to occlude the first opening; and a second lid secured to the second side wall so as to occlude the second opening. The bottom wall comprises a main body; and a reinforcement rib provided in an upright manner on the outer surface of the main body. The reinforcement rib is provided in an upright manner on the outer surface of an area, within the main body, which contains a central section of the main body, and which excludes four corner sections, a region adjacent to the first lid and a region adjacent to the second lid.