NEUTRON SHIELDING MATERIALS LONG TERM PERFORMANCE: A NEW APPROACH FOR MODELING THERMO-OXIDATIVE AGING

Polymeric matrix composites (PMC) are widely used in radioactive material transportation and storage casks. The PMC neutron shielding ability relies on the concentration in hydrogen atoms (to decelerate neutrons) and neutron-absorbing atoms such as boron. However, throughout the service life of the casks and due to severe environments (high temperature for instance), the concentration of hydrogen atoms might fall, and consequently, decrease PMC shielding capability. Therefore, their long term behavior must be accurately predicted. In this regard, Arrhenius law-based models are commonly applied. First, accelerated aging tests (at various temperatures and oxygen pressure) are carried out. Then, without rigorous explanations, short-term experimental data are extrapolated to long periods. To overcome obvious limitations of these models, TN International has launched a multi-year R&D program in order to elucidate the oxidation processes and develop a non-empirical methodology. The “diffusion-limited” character of oxidation reported in several studies is well known. Basically, thin films exposed to oxidative environments become fully oxidized. However, thick samples present a heterogeneous process: a superficial layer is oxidized, whereas the core is not affected. Therefore, in our new numerical model, oxidation kinetics is coupled with oxygen diffusion. It simulates more confidently PMC weight losses (which are then converted into a loss of hydrogen atoms) and oxidation profiles. In this paper, our new model is briefly described, followed by a comparison of simulated and experimental data.