EXAMINATION OF RADIONUCLIDE TRANSPORT IN THE VADOSE ZONE USING FIELD LYSIMETERS

Determining safe disposal methods for radioactive waste created as a result of nuclear weapons testing and commercial nuclear power production is important for environmental risk mitigation. There are many factors required to effectively assess the fate and transport of radionuclides in the environment. While lab-scale experiments can provide thermodynamic data that is essential for predicting the behavior of radionuclides, field-scale data must be also be considered. This research studied the long-term transport of ²³⁹Pu (as Pu(IV) and Pu(V)), ²³⁷Np (as Np(IV) and Np(V)), ¹³⁷Cs, ⁶⁰Co, ¹³³Ba, and ¹⁵²Eu in conditions that more closely mimic vadose zone conditions. The data will provide valuable information that is relevant to radioactive waste storage and can be used to reduce the uncertainty associated with geochemical models used in performance assessments. Field lysimeters (24” x 4” PVC pipe) were packed with a sandy loam soil, with a single radionuclide source placed approximately 12” from the base. Quarterly collections of rainwater effluent were analyzed using liquid scintillation counting, gamma spectroscopy and inductively coupled plasma mass spectroscopy. The concentrations of selected major ions, pH and dissolved oxygen were also measured. After one year, measureable concentrations of both ²³⁷Np and ⁶⁰Co have been released from the respective lysimeters. During the most recent sampling event, approximately 150 Bq of activity was found to have been released from one of the lysimeters containing ²³⁷Np as Np(V). Total released activities have varied from approximately 150 to 150,000 Bq in the nine lysimeters containing ⁶⁰Co. The observed mobility of ²³⁷Np(V) and ⁶⁰Co is consistent with the limited retardation commonly reported for batch and saturated column transport experiments. The immobility of Pu(IV), Pu(V), Np(IV), and Eu(III) is consistent with the strong sorption and resulting high retardation factors often observed in batch experiments. Future research will include continued effluent monitoring for one year and one-dimensional ²³⁷Np and ⁶⁰Co transport modeling.