Ion diffusion from Sellafield OPC paste formulations

Abstract

The disposal of nuclear waste is highly regulated and the disposal option will be dependent on the radionuclide content of the waste. The encapsulation of nuclear waste to prevent migration of radionuclides into the environment and as a safe means of long term storage and disposal can be achieved using ordinary Portland cement (OPC) and various additives such as blast furnace slag (BFS) or pulverised fly ash (PFA). Treated radioactive wastes in this manner are characterised by good thermal, chemical, physical stability and compressive strength. In addition the alkaline chemistry of concrete renders most radionuclides highly insoluble. The ultimate destination of some of these encapsulated wastes is in a Deep Geological Facility (GDF), where for many years the wastes will remain inert to their environment. In the longer-term the environmental conditions will change and the inertness of these waste forms could be affected from the seepage of water into the facility along with microbial activity. The diffusivity or leaching behaviour of cement encapsulated radioactive waste is crucial to ensure the overall safety of a storage/disposal system.
The research presented in this thesis evaluates the diffusivity of strontium, caesium and cobalt when added as inactive forms to BFS:OPC and PFA:OPC formulation as their chlorides and for strontium when added as chloride and carbonate. The cylindrical cement paste samples (CPS) having diameter of 3.2 cm and height 5.3 cm were immersed in re-circulating test solutions consisting of de-ionised water, concentrated Sellafield pore water (CSPW), diluted Sellafield pore water (DSPW) and bacterial inoculated water, John Innes Soil Solution (JISS). Strontium carbonate was selected to determine the influence of a water insoluble compound on diffusivity of the cation. Freshly cured and aged BFS:OPC samples were also studied to evaluate the impact of carbonation on cation diffusivity. Chloride salts were used, as these would be benign to microorganisms, i.e. would not stimulate or support growth unlike nitrate or sulphate anions. The outcome of this study indicate that the make-up water composition affected the segregation of inherent and added cations in the cement paste samples and also both the bleed water volume and physical characteristics of the cement paste samples. Strontium when added as a soluble salt to the make-up water influenced the rate of diffusivity. Depending on the type of formulation (BFS:OPC, PFA:OPC), a direct correlation was observed between diffusivity of Sr2+ and total amount of Ca2+ present in the CPS. The rate of diffusivity and the depth of cation diffusion was significantly higher in 3% SrCl2 PFA:OPC having lower concentration of Ca2+ compared to its BFS counterpart. The concentration of the added salt to the make-up water also affected the diffusivity. The difference in the diffusivity was observed between closed and open diffusivity system. The solubility limits were not a factor in open circuit which was comparable with the pH values; contrary to the closed circuits. The concentration of cations and anions in the test solution influenced strontium and caesium diffusivity. The diffusivity of sulphate was influenced by the nature of the cation added to the make-up water. Strontium had the greatest effect on lowering the diffusion primarily due to the formation of sparingly soluble strontium sulphate. The pH values of the circulating JISS test solutions from all the contaminated cement samples were lower in comparison with control, which was comparable with viable population in the circulating system. There was no significant viable population measured in the JISS from control CPS. The JISS test solution composition retard strontium diffusivity but accelerated caesium diffusion in comparison with distilled water values, this retardation could be due to the inherent sulphate content (≈8600 ppb) of the JISS test solution.
This work provides fundamental understanding of the physic-chemical factors influencing the diffusivity of cations from BFS:OPC and PFA:OPC formulations. The scheme i.e. closed circuit recirculation adopted in this research would be more fitting of the real situation i.e. stagnation followed by percolation and therefore diffusivity of ions will be greatly influenced by the test solution chemistry and composition.