As far as I know all materials are changed or destoryed by radiation over time, Ed.

What is your point?

PP is a great stable solid to use as a radiation shielding as it can be converted into so many other useful things.

Have you read: The Gamma Radiation Tolerance of Polypropylene: Measurement and Enhancement or Post irradiation degradation of polypropylene radiation durability of polypropylene stabilized with phenolic stabilizer (II)

The second's abstract:

Post irradiation degradation of 60Co gamma irradiated polypropylene stabilized with a phenolic type stabilizer has been followed up to two years after being irradiated. The effectiveness of the stabilizer at various concentrations on the polypropylene samples irradiated up to 150 kGy has been studied and the buildup of carbonyl group in the irradiated samples measured. The obtained results indicated that the oxidative reactions are retarded by addition of stabilizer. The variation of bend strength of the irradiated samples with and without stabilizer was also followed. This property was found to be greatly improved for the stabilized samples irradiated at low doses. However at high doses the effectiveness of the stabilizer declined. In order to correlate these results with the change in the molecular weight of polymer, trends of the change in rheological behaviour such as melt viscosity (μa) and flow behavior parameters (K, n) of both stabilized and unstabilized samples have been investigated. Results showed that at low doses, addition of the stabilizer greatly reduces the extent of chain scission, and therefore, the mechanical properties of the irradiated polymer are retained.