A pressure vessel (10) includes a tank (12) containing fluid under pressure, and includes a closure cap (14) for closing the tank (12). The closure cap (14) is a unitary part made entirely of a homogeneous material. The unitary part (14) includes a rupturable closure wall (20) which, when ruptured, permits fluid to flow outward from the tank (12). The unitary part (14) also includes a conduit (74) for directing fluid to flow outward from an outlet opening (120) when the closure wall (20) is ruptured. The closure wall (20) has oppositely facing inner and outer side surfaces (98,100) and includes a stress riser (96). The side surfaces (98,100) have contours which direct the fluid pressure to maintain the stress riser (96) under compressive stress and shear stress, rather than tensile stress, so as to resist creep throughout the time that the fluid is contained under pressure in the tank (12).