A vessel assembly is disclosed for high pressure high temperature chemistry. The vessel assembly includes a cylindrical vessel and a cylindrical vessel seal cover. The vessel and the seal cover have respective surfaces that, when engaged, define a circumferential interior passage between the vessel and the seal cover. A pressure release opening in the seal cover is in fluid communication with the circumferential passage. A retaining ring surrounds the vessel and the seal cover at the position where the vessel and the seal cover meet to maintain an outer circumferential engagement between the seal cover and the vessel when pressure forces gases in the vessel to flow into the circumferential passage and then outwardly from the pressure release opening.