Thermal decomposition of radio-oxidized Polymers and impact on radioactive material transportations

In case of accidental conditions of transportation, the temperature of packages can reach 150°C. The accumulation of the gases formed may affect the integrity of the package. Therefore, it is important to predict the gas quantity released by different organic materials and measure the kinetic of this decomposition.The purpose of this work is to present the results of a multi-year study performed on four polymers (polyethylene, cellulose, polyurethane, polyvinyl chloride). The effect of dose, atmosphere and degradation temperature on the quantity of gas release was investigated.ExperimentalThe polymers were gamma-irradiated under air atmosphere at room temperature at different doses (from 0 to 4 MGy). After irradiation, the thermal degradation of polymers at different temperatures (from 60°C to 150°C) during 48 hours under two types of atmosphere (air or inert) was analyzed. For that purpose, a known amount of polymer was conditioned in a small hermetic chamber (made on stainless steel or glass) equipped with a pressure gauge. The pressure elevation during the thermal treatment was recorded. An identification and a quantification of the gas phase were also realized using high resolution mass spectrometry.Results and discussionIn contrast to the non-irradiated polymers, which are mostly stable even at 150°C, a rapid increase of the pressure during the first hours after heating and a stabilization or a slow increase after is observed for radio-oxidized materials. The production of volatile species is almost proportional to the dose and is thermally activated. The main degradation products are water and carbon dioxide. The production of hydrogen and inflammable gases is rather limited. Under air atmosphere, the pressure increase is partially compensated by oxygen consumption.ConclusionsThe thermal decomposition of pre-oxidized polymers is rapid. The majority of the gases is released in the first hours. Water and carbon dioxide are the predominant species. Although irradiation reduces the thermal stability of the polymers, our results show that the risk of inflammation is not exacerbated.