a plug provided internally with a bore so that a wall of the plug surrounding the bore is sufficiently thin to allow the wall to expand outward slightly when the bore of the plug experiences high pressure to thereby partially fill a gap between an associated chamber member and the plug where an o-ring is located; and

a backup ring provided in a gap formed between the plug and its associated chamber member, said backup ring constructed of a material that has a higher coefficient of expansion than the materials from which its associated plug and chamber member are constructed so that the backup ring expands when exposed to high temperature to thereby partially fill the gap between the chamber member and the plug, a retaining disc provided attached to the plug so that an o-ring is held in the gap between the backup ring and the retaining disc, and said retaining disc provided with at least one opening extending through the disc to allow pressurization of the bore of the plug.