Abstract: A safety interlock for a pressure vessel having a shell portion, a door portion and an opening in one of the portions, the interlock including a collar affixed to the outside of the pressure vessel surrounding the opening, the collar having a bore therethrough with an enlarged section extending partway toward the pressure vessel opening, a resilient ring seal disposed within the enlarged section of the bore, a guide bushing positioned within the enlarged section of the bore in contact with the seal, a support bracket affixed to another portion of the pressure vessel, a movable shear bolt carried by the support bracket, and means for advancing the shear bolt when the bolt is aligned with the pressure vessel opening through the guide bushing, ring seal and collar into contact with the opening of the pressure vessel and maintaining the end of the bolt tightly against the pressure vessel opening so a pressure can selectively be maintained within the vessel.