MAS NMR Study on the Microstructural Changes of Zeolite-NaY under Neutron Irradiation
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MAS NMR Study on the Microstructural Changes of Zeolite-NaY under Neutron Irradiation J. Chen, L. W. Beck*, L. M. Wang, B. X. Gu**, R. C. Ewing Dept. of Nuclear Engineering & Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, U.S.A. * Dept. of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, U.S.A. ABSTRACT The results of high-resolution solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) indicate the bond angles of Al-O-Si decrease with the increase of neutronirradiation dose, but there are only minor changes for Si-O-Si bond angles in the current dose range studied. This suggests that zeolite with higher Si/Al ratio is more resistant to neutron irradiation damage. It is also found that [AlO6] formed at higher dose range. The ratio of Si/Al changed from 2.57 before neutron irradiation to 2.76 when the dose reached 2.25×1019 n/cm2, which shows that mild dealumination occurred during neutron irradiation. The strontium ion exchange experiments have been conducted for neutron irradiated, thermally treated and original zeolite samples, respectively. It is found that the ion-exchange ability of neutron-irradiated zeolite-NaY is between those of original and heat-treated zeolite samples. The distortion of the framework under neutron irradiation by the decrease of Al-O-Si bond angles and the formation of [AlO6] is partially responsible for the ion-exchange variability variation of zeolite-NaY. INTRODUCTION Research in zeolites have been fueled by their wide use in heterogeneous catalysis, adsorption and ion exchange on an industrial scale. Besides, zeolites are of considerable interest in nuclear industry being used as waste processing media, waste forms, and potential back-fill materials in nuclear waste repositories. For example, zeolites can be used for selective removal of nuclides, such as cesium, strontium, rare earths and actinides from the high-level liquid nuclear waste generated from nuclear fuel reprocessing, decontamination and decommissioning of nuclear facilities [1-3]. However, once zeolites incorporate radionuclides, they will be subjected to high radiation doses from radioactive decay. For commercial high-level nuclear waste forms with a 25wt% waste loading, the cumulative ionization dose from β-decay events can be as high as 1010 Gy after 100 years, and the anticipated dose from α-decay may reach 7×108 Gy in 1000 years. Our previous studies have shown that zeolites are susceptible to both radiation- and thermally induced amorphization [4, 5]. Complete amorphization can be induced by an ionization process with 1010-1011 Gy of ionizing energy deposition under electron irradiation, or by direct displacement damage processes at a cumulative dose as low as 0.1 dpa under ion beam irradiation [4]. It is beneficial for choosing or designing radiation tolerant materials to understand the microstructure changes upon neutron irradiation. In the current research, high-resolution solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) was emplo
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