Shallow Land Burial Facilities Research Articles

Evaluation of disposal limits for shallow land burial facilities: Application to the back end of nuclear fuel cycle

The limits on disposal rates and activity concentrations in radioactive wastes have been computed using different release scenarios for about 26 radionuclides commonly encountered in a shallow land burial facility originating from the back end of nuclear fuel cycle. These limits are then segregated for major specific radionuclides/groups of radionuclides on the basis of a critical scenario/critical pathway concept indicating signatory radionuclides for monitoring purposes. Disposal rates and activity concentrations of short lived ß, γ radionuclides and 137Cs are limited by the external exposure route to occupational workers. The well-water-drinking pathway limits the disposal rates and activity concentrations of 3H and long lived ß, γ radionuclides having low K d owing to their rapid migrational characteristics. The dwelling-inhalation pathway restricts activity concentration limits of α bearing wastes.

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 plantsmore » such as {sup 79}Se, {sup 99}Tc, and {sup 129}I in the vault facility need to be regulated for restricting the resultant dose within the apportioned dose limit prescribed for the facility.« less

Using a Multiregion Model to Study the Effects of Advective and Diffusive Mass Transfer on Local Physical Nonequilibrium and Solute Mobility in a Structured Soil

Waste management problems for shallow land burial facilities in the humid eastern United States are usually complicated by slow but continuous movement of wastes through the soil matrix and discrete but rapid pulses of wastes through macropores and fractures. Multiple‐pore‐region models employed to describe flow and solute transport in the soils usually consist of multiple mass transfer coefficients that cannot be measured experimentally, and their effects on subsurface mass transport are poorly understood. The objective of this research was to study the individual and concurrent effects of interaggregate advection and diffusion on mass transport in a structured soil. The interactions of these two mass transfer processes and local solute concentration equilibrium are examined for a heterogeneous soil. Pore region water retention, hydraulic conductivity, and dispersivities, obtained from independent measurements and published calibration results, were used to test a novel three‐pore‐region, one‐dimensional numerical model. Advective and diffusive mass transfer coefficients were estimated using mass transfer equations and fracture spacings published in the literature. The mass transfer coefficients were then varied systematically, and the sensitivity of local fluid pressure and solute concentration nonequilibrium to interregion mass transfer were analyzed. Our results indicated that time‐dependent interaggregate advection and diffusion were important processes controlling solute mobility in heterogeneous media. Under transient flow conditions, interaggregate advection may reduce the significance of interaggregate diffusion that otherwise dominates interaggregate mass transfer under steady state conditions. Nonetheless, the equilibrium of local solute concentrations was 20 times more sensitive to diffusive mass transfer than to advective mass transfer, which suggests that site characterization efforts should be directed more toward the former process. Unfortunately, characterization efforts of this type are not commonplace and if available are frequently ignored because they add a difficult reality to complex waste management problems. Since advective and diffusive mass transfer may be important processes limiting the efficiency of cleanup activities such as pump and treat, it is perhaps time to include the characterization of these processes and quantification of the timescale of physical nonequilibrium in site remediation efforts.

Novel Experiments for Understanding the Shallow Land Burial of Low-Level Radioactive Wastes

ABSTRACTData on the basic processes that occur in the shallow land burial of low-level radioactive wastes are needed to engineer facilities with guaranteed performance, to validate models for system predictions, and to provide input to models that consider contaminant pathways out of the facility. Two types of novel experiments that will provide experimental data on the basic processes in shallow land burial facilities are described in this paper. Generic experiments that give data on the movement of water and radionuclides and an experiment that is particularly important for semi-arid sites are described.