The 9975 is a double containment shipping package used to transport plutonium bearing materials for the US Department of Energy. The 9975 is also used for long term storage of plutonium bearing materials at the Savannah River Site. The package utilizes a fiberboard overpack to protect against fire and impact events. The 9975 has been shown to maintain containment during a hypothetical facility accident fire even though the facility fire is hotter and longer than the regulatory transportation fire. Fiberboard aging and degradation has been investigated using both laboratory and field surveillance data. This information is used to evaluate an aged 9975 used for nuclear material storage. Variations in fiberboard thermal properties due to aging were shown to have modest effects on the maximum component temperatures, while the package geometry variations due to aging and degradation have a larger effect on the maximum component temperatures. Specifically, the air gap between the upper fiberboard assembly and the drum lid increases during storage due to the deterioration of the lower fiberboard assembly and slumping of the package containment vessel. A limiting air gap distance has been determined via thermal fire analysis, which may be used to estimate a storage life.