Environmental Management Science Program Project ID Number 73762 (continued from project 54691) Radiation Effects on Sorption and Mobilization of Radionuclides during Transport through Geosphere

Site restoration activities at DOE facilities and the permanent disposal of nuclear waste inevitably involve understanding the behavior of materials in a radiation field. Radionuclide decay and the associated radiation fields lead to physical and chemical changes that can degrade or enhance important material properties. Alpha-decay of the actinide elements and beta-decay of the fission products lead to atomic-scale changes in materials (radiation damage and transmutation). The radiation exposure due to the release and sorption of long-lived actinides and fission products (e.g., Cs and Sr) may cause changes in important transport properties (e.g., sorption and cation exchange capacity) in geological materials, such as colloidal clays and zeolites, along transport pathways. Thus, the effect of radioactive decay on soils and geologic materials during transport (e.g., through the vadose zone) are an important aspect of understanding the migration and retention of radionuclides in the geosphere. During the previous funding period of this project, we have evaluated radiation effects in selected near-field materials with accelerated laboratory experiments utilizing energetic electrons and ions and in situ transmission electron microscopy (TEM) during irradiation at the HVEM/IVEM-Tandem National Facility at Argonne National Laboratory. The materials irradiated included zeolites and layered silicates (mica and clays). We have found that all of these materials are susceptible to irradiation-induced solid-state amorphization. Amorphization can either be induced by ionization processes (β− or γ− irradiation) or by direct displacement damage processes (α−decay events). The critical doses for complete amorphization of these phases are as low as <0.1 displacement per atoms (dpa) or 10 Gy in ionization energy deposition (a dose expected in a zeolite with 10 wt.% loading of Cs in 400 years). Experiments on thermally-induced amorphization have shown that even partial amorphization will cause a dramatic reduction (up to 95%) in ion-exchange and sorption/desorption capacities for radionuclides, such as Cs and Sr. Because the near-field or chemical processing materials, e.g., zeolites or crystalline silicotitanate (CST), will receive a substantial radiation dose after they have incorporated radionuclides, our results suggest that radiation effects may, in some cases, retard the release rate of sorbed or ion-exchanged radionuclides. These results have a direct bearing on repository performance assessments (e.g., the extent to which zeolites can retard the release of radionuclides) and on the technologies used to process high-level liquid wastes (e.g., separation of Cs from HLW using CST at the Savannah River Site). Because the dose rate of available gamma sources is too low to achieve significant effects in a reasonable period (e.g., requires 50 years to reach the critical amorphization dose for zeolite-NaY), heavy ions or energetic electrons in an electron microscope were used in our previous studies. Although these experiments are adequate for studying the radiation effects on the microstructure of the target material, the radiation affected volume is too small for studying changes in bulk properties, such as sorptive or ion-exchange capacities of clays and zeolites which is of critical importance because these phases play an important role in the retardation of radionculides in the geosphere. During the last two years, we have continued our study of radiation effects on the sorption and ion-exchange capacities of two important groups of materials: clays and zeolites with newly designed experimental methods (proton and neutron irradiation).