An Analysis of the Effect of Hydraulic Parameters on Radionuclide Migration in an Unsaturated Zone

Decommissioning of a retired Training, Research, Isotope production, General Atomics (TRIGA) research reactor facility has been performed for the reuse of the decommissioned site. The residents adjacent to the decommissioned site should be safe from radiation exposure. Radionuclides around the decommissioned site were first distributed on the site surface, and then later moved from the surface to the groundwater table. Finally, radionuclides around the decommissioned site have been found mostly to be located in the unsaturated zone. Therefore, it is necessary to develop a technology for interpreting radionuclide-migration in an unsaturated zone to secure the radiation safety of the decommissioned site. The Multimedia Environmental Pollutant Assessment System (MEPAS) [1] is a physics-based environmental analysis code that integrates source-term, transport, and exposure models for endpoints such as concentration, dose, or risk. Developed by the Pacific Northwest National Laboratory, MEPAS was designed for site-specific assessments using readily available information. Endpoints are computed for chemical and radioactive pollutants. The Multimedia Contaminant Fate, Transport, and Exposure Model (MMSOILS) [2] was used to estimate the human exposure and health risk associated with releases of contamination from hazardous waste sites. The methodology consists of a multimedia model that addresses the transport of a chemical in groundwater and surface water, the soil erosion, the atmosphere, and the accumulation in the food chain. The RESidual RADioactive (RESRAD) family of computer codes [3] was developed by Environmental Science Division (EVS) to provide useful tools for evaluating human health risk at sites contaminated with radioactive residues. The RESRAD methodology was cited in a Department of Energy (DOE) order for dose assessment and determination of guidelines for clean-up of radiologically contaminated sites. The potential effect of rainfall and evapotranspiration on contaminant migration in the unsaturated soil zone was studied using a deterministic dynamic modeling approach at the University of California [4]. Recently, researchers at the University of Tubingen, Germany, investigated the transport of fate of reactive compounds in the unsaturated vadose zone using numerical simulation of steady-state and transient flow scenarios [5]. Also, researchers at the University of Liege, Belgium, A One-Dimensional Water Flow and Contaminant Transport in Unsaturated Zone (FTUNS) code has been developed in order to interpret radionuclide migration in an unsaturated zone. The pore-size distribution index (n) and the inverse of the air-entry value (α) for an unsaturated zone were measured by KS M ISO 11275 method. The hydraulic parameters of the unsaturated soil are investigated by using soil from around a nuclear facility in Korea. The effect of hydraulic parameters on radionuclide migration in an unsaturated zone has been analyzed. The higher the value of the n-factor, the more the cobalt concentration was condensed. The larger the value of α-factor, the faster the migration of cobalt was and the more aggregative the cobalt concentration was. Also, it was found that an effect on contaminant migration due to the pore-size distribution index (n) and the inverse of the air-entry value (α) was minute. Meanwhile, migrations of cobalt and cesium are in inverse proportion to the Freundich isotherm coefficient. That is to say, the migration velocity of cobalt was about 8.35 times that of cesium. It was conclusively demonstrated that the Freundich isotherm coefficient was the most important factor for contaminant migration.