The design of the overpacks in drum packages, both in terms of thickness and materials of construction, greatly impact the ability of the package to accommodate heat source contents. The optimum overpack thermal protection needed is that whichresults in the lowest containment vessel temperature during both Hypothetical Accident Conditions (HAC) and Normal Conditions of Transport (NCT). For heat source packages, the use of very good or high efficiency insulating materials such as fiberboard and polyurethane results in high containment vessel temperatures during both NCT and HAC. Using a more modest or low efficiency insulating material would reduce the NCT and HAC containment vessel temperatures significantly. A material such as oak (lowefficiency) would maintain a containment vessel with a content of 100 watts at a fraction of the temperature reported for very good or high efficiency insulating materials. Four inches of oak can prevent the containment vessel from exceeding 500°F during both NCT and HAC with 100 watts of contents, whereas using a high efficiency material the vessel would exceed 1000°F.
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