Degradation of polyethylene foam under irradiation and chemically aggressive environments: Part I: visual observations, weight evolution and leaching of organics

In the Belgian radioactive waste inventories, certain low- and intermediate-level short-lived waste forms are present that are susceptible to swelling, especially due to alkali-silica reactions or delayed ettringite formation. From the point of view of their radiological content, these waste packages would be admissible to surface disposal. However, they do not meet the safety criteria due to their swelling potential. Given the significant volume of such waste and the cost difference between geological disposal (as the alternative disposal solution) and surface disposal, ONDRAF/NIRAS is actively developing strategies to allow the disposal of this waste in a surface repository. One explored strategy is to modify the engineered barrier system by introducing compressible materials, specifically polyethylene (PE) foam, to accommodate the pressure buildup from expansive reactions. It is, however, required that the mechanical properties of this foam would not be affected by the conditions in the facility and that its degradation would not lead to the release of potential radionuclide complexing organic compounds in the disposal facility. In the current study, two screening tests were performed to assess the effect of multiple factors relevant to a disposal facility: temperature, irradiation time and dose rate, anoxic vs oxic conditions, and humidity or chemical environment, on the degradation of polyethylene (PE) foam, i.e., Plastazote® HD60. The extent of the degradation was assessed by evaluating multiple parameters, including mechanical properties, chemical modifications of the polymer, weight changes, and organic release. In the present report, all details concerning the experimental work and the evaluation of the effect of the tested factors on the weight of the polymer foam and the organic release are discussed. The other results are part of a second report. The results indicate that significant water absorption by PE foam can occur when irradiated under wet conditions. This water absorption is linked to in-diffusion of water rather than radiolytic processes. The longer the submersion time, the larger the water uptake. Moreover, an elevated temperature also leads to higher water absorption, likely caused by accelerated diffusion of water through the closed-cell foam and/or by additional thermal degradation, rendering the polymer more hydrophilic. Under dry irradiation conditions, only small weight changes were observed, linked to the combination of radiolytic and thermal degradation, and their effect depends on the temperature and oxygen availability. Furthermore, the production of soluble organics by radiolytic, thermal, and hydrolytic degradation of PE foam was found to be minimal under all conditions of both screening tests. Aside from the sample size and the elevated pH, none of the other factors consistently affected the organic release. This lack of a clear and consistent effect can be attributed to the complex interplay of the different degradation reactions occurring or thriving under different conditions, which do not uniformly result in the formation of soluble organics. On the other hand, the apparent effect of the pH environment can be attributed to additional hydrolysis reactions taking place at pH 13.5. In contrast, at an initial pH of 9, the NPOC (Non-purgeable Organic Carbon) release was either not detectable or could be attributed to radiolytic and/or thermal degradation rather than additional hydrolytic processes.