Abstract: An improved burst disc assembly for use in pressure relief conduits for high vacuum and ultra-high vacuum containment vessels, the assembly having a body with inlet and outlet openings connected by an internal passage, a burst disc disposed within the internal passage and connected therein to hermetically isolate the inlet and outlet openings. The assembly further has a mounting structure connected to the body and outwardly extending therefrom that may be clamped between adjacent faces of a pair of metal seal flanges provided in the relief conduit to secure the assembly in position and hermetically seal the assembly in the conduit in a manner prohibit the passage of gas molecules from the surrounding environment into the containment vessel through the relief conduit. The mounting structure is formed from a soft metal material having substantially lower hardness value than that of the metal seal flange faces to enable adequate sealing.