A Kinetic Model for Predicting Polymeric Neutron Shieldings Lifetime

Abstract

Polymers are widely used in radioactive material transport/storage casks. Their neutron shielding ability relies mostly on their concentrations in hydrogen atoms which slow down neutrons. However, in-service high temperatures are responsible for H abstraction decreasing polymers shielding capability. Therefore, their long-term properties must be accurately predicted. In this regard, a multi-year R&D program has been set up to elucidate oxidation mechanisms of neutron shielding materials and develop a nonempirical lifetime prediction methodology. This paper deals with accelerated aging tests and modeling of a vinylester based neutron shielding product, commercially known as TN Vyal B™. Samples are exposed to various temperatures up to 160 °C and O₂ partial pressure and analyzed using various experimental techniques. Basically, thin free films are fully oxidized while thick samples show the diffusion-limited oxidation (DLO): a superficial layer is oxidized whereas the core is not affected. Therefore, to investigate the long-term properties, it is obvious that DLO phenomenon must be taken into account by coupling both oxidation kinetic and oxygen diffusion. In our case, there is a good agreement between simulated weight losses and oxidation thicknesses. Based on degradation assessment and design temperature of dry storage casks, TN Vyal B™ excellent resistance can be confirmed.