A safety lock for a pivoted door (16) closing an opening in a pressure vessel (10) to prevent the door from being opened when pressure in the vessel is above a selected level. A handle (44) is pivotably supported relative to the opening and movable to a locked position in which the door cannot be opened. A pressure lock housing (50) extends from the pressure containment vessel, the housing having a cylindrical opening therein, the axis of which intersects the plane of pivotation of the handle. A piston member (102) is received in the cylindrical opening (98) and has a portion extending externally of the housing. A tubular member (122) carried by the handle is engaged by the piston member when the handle is in the locked position, and pressure within the containment vessel forces the piston member outwardly to thereby prevent the handle from being moved towards the unlocked position. The piston member is automatically retracted when the pressure in the containment vessel is below a safe level.