Measurement of important parameters determining aqueous phase diffusion rates through crystalline rock matrices

Abstract

One of the options being considered for the disposal of radioactive waste is deep burial in crystalline rocks such as granite. Recent theoretical studies on the migration of radionuclides in the groundwater circulating through the fissure networks, which are likely to be present to some extent in crystalline rocks, have indicated the importance of diffusion into the rock matrix as a retardation mechanism. Matrix diffusion will retard all radionuclides, but is of particular relevance to those exhibiting little or no sorption and which would otherwise migrate at or near the groundwater velocity. Diffusion measurements have been made on granites obtained from four different regions of the United Kingdom. In most of the experiments the weakly sorbed iodide ion was used as tracer, although the diffusion of Tc-99 (as the pertechnetate ion) has also been measured. Details of the experimental technique and method of analysis are set out and results given for the most important diffusion parameters, namely: D i, the intrinsic diffusion coefficient, α, the rock capacity factor, and ψ, a pore geometry parameter which is a characteristic of the rock type in question. In the pre-steady state region, a discrepancy between experiment and theory was observed. The cause of this has been interpreted as being due to the presence of two types of porosity; through-transport porosity, which carries the diffusion flux across the sample, and “dead-end” porosity, which comprises of cross-linking pores and pores closed at one end. A series of equations were set up and solved using this concept. From the analysis of experimental data in the pre-steady state region, estimates of the through-transport porosity, α +, and mean length of “dead-end” pores, l∗, were calculated. In addition, some initial results are given for diffusion through weathered fracture surfaces. Significant increases in both D i and α were measured. The implications of these results are discussed.