Mass transport properties of mature wasteform grouts

Release of gases from intermediate level radioactive wastes encapsulated in cement-based wasteform grouts is a major concern during the interim storage, transportation and operational stage of a phased geological repository system. The ability of the wasteform grout to retain radioactive gases depends on its mass transport properties, which evolve with age, water saturation degree and any degradation. This paper reports the gas diffusion, gas permeation and water absorption (sorptivity) coefficients of two mature wasteform grouts containing 75% pulverised fuel ash or 75% blast furnace slag. Some of the slag grouts were found to be affected by microcracking, and these were subsequently examined using backscattered electron microscopy and quantified using a specifically developed image analysis procedure. Prior to transport testing, samples were conditioned in a realistic manner to a range of water saturation levels (10–100%). The mass transport properties were found to be highly dependent on the degree of water saturation, in particular for samples conditioned at relative humidity greater than 55%. The microcracks have a significant effect on permeability, but less influence on diffusion and water absorption. Implications of the results on the effectiveness of these grouts for radioactive waste containment are presented.