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INTRODUCTION Reversible cation exchange is one of the most important properties of zeolites. This is the basis for using zeolites in the selective removal of radionuclides, such as cesium, strontium, rareearths, and actinides, from the high-level liquid nuclear waste [1]. Zeolite is also a potential waste form and back-fill material in nuclear waste repositories. As zeolites are intended to retain radionuclides, they will receive considerable radiation doses over time, and the cumulative doses in zeolites utilized as exchange media, waste forms, or near-field back-fill can be substantial. A number of studies have reported that zeolites are susceptible to various types of radiation damage. At room temperature, analcime, natrolite and zeolite-Y, become completely amorphized at an ionizing dose in the range of 3.2x10'° to l.6xl0"Gy, and the dose required for amorphization decreases with increasing temperature due to the thermal instability of the zeolites [2]. Based on these results, zeolites in the near-field of a waste repository may be amorphized within 1,000 years after waste emplacement. Temperature also affects the structure and properties of zeolites. Most zeolites become dehydrated upon heating and many undergo the crystalline-to-amorphous transition at temperatures above 400-900*C, depending on the type of zeolite. In the case of nuclear waste, significant heating is possible after waste emplacement in a repository. Heat generated from decay the of fission products can result in an initial storage temperature as high as 600*C, and the temperature may still be as high as 300°C after 100 years of storage [3]. The cation exchange behavior of zeolite depends on the nature of the cation species, size, and charge, as well as the structure of the zeolite. Radiation- or thermally-induced amorphization dramatically changes the structure, which in turn leads to a redistribution of cations and a change in the framework charge. The collapse of the framework structure may also reduce the number of accessible cation sites by blocking the open channels and reducing the effective surface area. Consequently, the ion exchange and retention behavior of zeolites may be affected significantly by the amorphization process. Extensive experimental studies of ion exchange of radionuclides in zeolites have been performed over the last few decades [4,5]. However, these studies have been focused only on the effects of time, temperature, solution chemistry and pH. The effects of radiation- or thermally-induced amorphization on ion exchange capacity have not been considered in these studies. In the present study, the radiation and thermal effects on the structure of zeolite-NaY were investigated and the changes in the ion-exchange and retention capacity of Cs as a result of the structure change were studied. 493 Mat. Res. Soc. Symp. Proc. Vol. 608 0 2000 Materials Research Society

EXPERIMENTAL Materials Zeolite-NaY (Si/A1=2.55) that contained 13.0 %(wt) Na 2O was supplied by Zeolyst International Company in the form of powder with the s