Abstract: A weld test plug has a flange; an actuator rod mounted to the flange, the actuator rod having: a part expander; an annular seal; and an axial stop; in which the actuator rod is connected to, during use, cause the part expander to actuate: the axial stop to move in a radially outward direction relative to the actuator rod; and the annular seal to radially expand and abut the axial stop. A method includes inserting an actuator rod into an open end of a vessel, the actuator rod carrying an annular seal, and an axial stop; sealing the open end of the vessel around the actuator rod; operating the actuator rod to actuate: the axial stop to move in a radially outward direction relative to the actuator rod; and the annular seal to radially expand to abut the axial stop and seal against an inner circumferential surface of the vessel.