Major task of Division „Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung (BAM) is safety evaluation of storage containers for radioactive waste. Most important in this context is the sufficient sealing function of bolted lid systems for a safe confinement of the radioactive inventory during transport, interim storage and final disposal. In case of containers for radioactive wastes with negligible decay heat dissipation, representing low and intermediate level wastes, elastomer O-rings are widely used whose sealing function is maintained by the exerted force of the compressed elastomer O-ring.
In order to improve the predictive capability of the long-term stress relaxation of elastomers during thermo-oxidative ageing, a new method to separate reversible and irreversible processes was developed. The separation is performed through the analysis of compression set after tempering. On the basis of this separation, a user subroutine (UMAT) in the finite element code ABAQUS® is used to develop a numerical model for long-term stress relaxation during homogeneous ageing. The model consists of an additive contribution of physical and chemical relaxation. Computer simulations of compression stress relaxation were carried out for long ageing times and, based on the experimental data (at 75 °C, 100 °C, 125 °C and 150 °C for up to 170 days), the results were validated with the so-called „Arrhenius” approach, a kinetic analysis and the results of a time-temperature shift. Simulations based on physical and chemical processes allowed the calculation of degradation rates and the description of irreversible chemical relaxation processes even in low temperature ranges, in which only limited experimental results are available as the required time to observe these effects is very long. To model the chemical relaxation, two decay functions are introduced, each with its own activation energy and its own degradation process. The two processes could be attributed to shorter and longer times, respectively. A degradation rate-based model for the evolution of each process and its contribution to the overall homogeneous ageing of the system is proposed. The main advantage of the model is the ability to quickly determine an interpolation at lower temperatures within the range of slower chemical processes without having to rely on long measurements to determine a linear „Arrhenius” extrapolation.
Erroneous or Arrhenius: A Degradation Rate-Based Model for EPDM during Homogeneous Ageing
Maha Zaghdoudi, Anja Kömmling, Matthias Jaunich, Dietmar Wolff
erschienen in Polymers, Band 9, Heft 12, Seite 2152 ff
BAM division Safety of Storage Containers