Near-Field and Far-Field Migration of Radionuclides from a Shallow Land Burial Facility

Mathematical models have been developed to predict the spatial and temporal profiles of radionuclide concentration in the near field and far field of a shallow land burial facility. The disposal facility considered is a reinforced concrete vault located 2.9 m above the water table. The source term is derived from leaching considerations based on diffusion-controlled kinetics for the transfer of radionuclides from the cylindrical waste form into the surrounding pore water of the backfill material. The concentration of radionuclides in the backfill at the bottom boundary of the vault is converted to a release rate into the near field using the outgoing water flux. The delay and decay during transit in the sand and soil layers are taken into account while evaluating the concentration and release at the near field-water table boundary. Using this release rate as the inlet flux, the concentration of radionuclides in the groundwater has been computed using a two-dimensional model. Results indicate that the vault facility with cement as the backfill material could contain almost all the radionuclides commonly encountered in the waste stream generated during the operation of nuclear power plants. However, the storage of some of the long-lived radionuclides produced in fuel reprocessing plants such as ⁷⁹Se, ⁹⁹Tc and ¹²⁹I in the vault facility need to be regulated for restricting the resultant dose within the apportioned dose limit prescribed for the facility.