Radionuclide Release Rates Research Articles

Diffusional transport of radionuclide chains in subseabeds

The diffusion-controlled migration of radionuclides released from a leaky canister located in a subseabed sedimentary layer, bounded at the top by the ocean and at the bottom by an impermeable basalt zone, is analyzed to determine the transport rate to the ocean floor. The unsteady-state diffusion equations for radionuclide decay chains have been solved by integral transform techniques to obtain analytical solutions for the concentration distribution, flux at the ocean-sediment interface and discharge rate to the ocean floor for each chain member. By means of a parametric study the effects of the major parameters on the release rate of radionuclides to the ocean floor are elucidated.

Preferential Dissolution Phenomena of Nuclear Waste Materials

ABSTRACTPreferential dissolution of polyphase nuclear waste materials in short term leach tests can exaggerate radionuclide release rates when extrapolated to the lifetime of the waste form. Possible preferential leach phenomena are associated with the presence of cracks, intergranular phases and readily soluble phases. The rate of dissolution and the microstructural connectivity of the most soluble phase determine the period over which perferential dissolution is observable. The connectivity of phases is amenable to control during processing by altering the starting green density of the precursor powders.

Leach Rates of High Level Waste and Spent Fuel. - Limiting Rates as Determined By Backfill and Bedrock Conditions.

The release rate of radionuclides from a degraded canister containing radioactive waste is controlled by diffusion through the backfill and by diffusion into the water flowing past the canister. In low porosity low permeability bedrock like the Swedish granites the amount of water which can be contaminated may be very small. Two sample cases are calculated. One uses the conditions for a repository for vitrified high level waste, the other applies to a repository for spent fuel. The small amount of water which can carry the waste in combination with the low solubilities for the major longlived actinides limit the release rate from the repositories to very small values.

Radionuclide Retardation and Release Rates for Oceanic Sediments and Clay

The concept of emplacing nuclear waste in the argillaceous sediments covering extensive regions of the ocean floor is being given serious consideration by a group of interested nations.1 For such a subseabed repository to be acceptable the effectiveness in isolating the radioactive material must be demonstrated. Since the waste form cannot be relied upon to retain its integrity in the suboceanic environment, the subseabed repository relies principally on the containment qualities of the sediment that forms the geologic barrier impeding the spread of radionuclides.

Development of a Backfill for Containment of High-Level Nuclear Waste

ABSTRACTSodium and calcium bentonites, pressed to densities between 1.9 and 2.2 g/cm3, have hydraulic conductivities in the range of 10−11 to 10−13 cm/s. Batch sorption distribution ratios (Rd) indicate that Sr, Cs, and Am are strongly sorbed on bentonites and zeolites, that Np and U are moderately sorbed on bentonites and zeolites, and that Am, Np, U, I, and Tc are strongly sorbed on charcoal. Sorption results with basalt and tuff ground waters are similar; however, iodine in tuff ground water sorbs more strongly on bentonites Thermal diffusivity measurements for dry, compacted (p ∼ 2.1 g/cm3) sodium bentonite indicate that the thermal conductivity of a high density bentonite backfill should be roughly similar to that of silicate host rocks (basalt, granite, tuff). These results indicate that a bentonite backfill can significantly delay the first release of many radionuclides into the host rock and that by forming a diffusion barrier a bentonite backfill can significantly decrease the longterm release rate of radionuclides from the waste package.

The Interaction of Borosilicate Glass and Grawodiorite at 100°C, 50 Mpa: Implications For Models Of Radionuclide Release

An essential component of any assessment of HLRW geological disposal options is the quantitative prediction of radionuclide release rates from the near-field over time spans of the order of 103-106 years. Fundamental to this assessment is the investigation of the interaction of potential wasteforms with groundwater under repository conditions of temperature, pressure, and groundwater flow-rate. Consequently, many studies world-wide have been initiated to examine the kinetics of wasteform dissolution over a wide range of physical and chemical conditions. Although these studies have provided a considerable amount of invaluable data on wasteform-fluid interactions, they have tended to focus on breakdown of the wasteform itself, and not on the fate of released waste components in the nearfield. For example, effects of saturation of species in solution, precipitation of secondary minerals or amorphous gels, and the effect of host rock chemistry on the products (solid and fluid) of waste-fluid interaction have largely been ignored or even specifically excluded in laboratory experiments. This is despite growing evidence from source term modelling studies which suggest that the above processes may well be the chief factors in governing rates of radionuclide release from the near-field, bearing in mind the limited availability of ground

Migration of Radionuclide Chains in Groundwater

Expressions are developed for the maximum concentrations and discharge rates of radioactive nuclide chains migrating through an adsorbing medium. These expressions are presented in terms of nondimensional parameters. Conditions are established that quantify the reconcentration effort of radionuclide chain migration in terms of the non-dimensional parameters. This approach provides a relatively simple mechanism for determining an upper bound to the concentrations or release rates of parent and daughter radionuclides in groundwater.