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Due to their extreme high hydrogen contents, high molecular weight (HMW-) and ultra-high molecular weight (UHMW-) polyethylene (PE) are a comprehensible choice as neutron radiation shielding material in casks for storage and transport of radioactive materials. But as a direct consequence of inserting radioactive material in such casks, gamma radiation occurs. Hence, the impact of gamma radiation on the molecular structure of polyethylene has to be taken into consideration. Consequently, PE has to withstand any type of gamma radiation induced degradation affecting safety relevant aspects in order to be applicable for long term neutron radiation shielding purposes during the whole storage period (in Germany, for instance, up to 40 years). The scope of our investigation comprises an estimation of the impact of gamma radiation and temperature on the molecular and supra molecular structure of the two types of PE used as neutron radiation shielding cask components. A further point which is worth exploring is to what extent these changes are detectable by conventional analysis methods. Therefore, thermoanalytical measurements were performed such as differential scanning calorimetry (DSC), thermo mechanical analysis (TMA), dynamic mechanical analysis (DMA), and thermo gravimetric analysis (TGA). Additionally optical and weighing methods were applied. With those methods it is possible to detect structural changes in polyethylene induced by exposure to gamma radiation. The observed amounts of changes of the irradiated material are not safety relevant for the application of polyethylene as neutron radiation shielding material; moreover, some properties actually improve via irradiation.
Abstract Hydrogenated nitrile butadiene rubber (HNBR) elastomer was thermo-oxidatively aged at different temperatures up to 150 °C. Fourier transform infrared spectroscopy (FTIR), compression stress relaxation (CSR) and international rubber hardness degree (IRHD) microhardness were used to characterise the chemo-mechanical changes of HNBR O-rings during thermo-oxidative ageing. FTIR shows the development of carbonyl, methyl and ester groups but the nitrile content was not affected by ageing. The effect of sample geometry during CSR was investigated. CSR data were converted through integrated kinetic laws. The conversion has proven its sensibility to detect heterogeneous ageing. This was confirmed by the IRHD measure-ments across the section of O-rings. The influence of compression during ageing was assessed through IRHD measurements across the section of compressed and uncom-pressed aged O-rings. The DLO effect was more pronounced in compressed O-rings. By applying the model of Wise et al., theoretical IRHD and oxidation profiles were determined on the basis of IRHD experimental data of compressed O-rings. Good agreements between the experimental and the theoretical IRHD profiles in the core region were obtained. However, near the edge, the theoretical IRHD values were overestimated.