A high temperature, high pressure chemical digestion vessel comprising an inner container within which digestion is to take place under high temperature and high pressure conditions. The container is provided with a vent. A replaceable, rupturable pressure release disk is provided having a predetermined rupture pressure for being sealingly interposed in covering relation to the vent to seal the container and thereby enclose a substance to be digested in the container and to rupture and release pressure within the container if pressure within the container reaches the predetermined rupture pressure. The pressure release disk comprises a flat sheet material. A container cap is provided for holding the pressure release disk in sealing relation to the vent in the container. An outer pressure resisting-casement is provided for receiving the container therein and providing support to the container.