Organic materials are a key component of Transuranic (TRU) waste stored in the Waste Isolation Pilot Plant (WIPP). These materials will be radiolytically changed in the repository for many hundreds of years. The first and the most important step of post-irradiation reactions is detachment of hydrogen atoms to form organic radicals. The next step depends on the kind of the molecule. In present work, this process will be discussed using polypropylene as an example. The polypropylene radicals undergo a secondary reaction with oxygen from the air to form peroxy radicals. Gravimetric analysis, performed on several electron beam irradiated samples, showed that reaction with oxygen is a chain reaction and can last for long periods of time.