Mass transport properties of mature wasteform grouts

6 Release of gases from intermediate level radioactive wastes encapsulated in cement-based wasteform grouts is a 7 major concern during the interim storage, transportation and operational stage of a phased geological repository 8 system. The ability of the wasteform grout to retain radioactive gases depends on its mass transport properties, 9 which evolve with age, water saturation degree and any degradation. This paper reports the gas diffusion, gas 10 permeation and water absorption (sorptivity) coefficients of two mature wasteform grouts containing 75% pul11 verised fuel ash or 75% blast furnace slag. Some of the slag grouts were found to be affected by microcracking, 12 and these were subsequently examined using backscattered electron microscopy and quantified using a specifi13 cally developed image analysis procedure. Prior to transport testing, samples were conditioned in a realistic 14 manner to a range of water saturation levels (10-100%). The mass transport properties were found to be highly 15 dependent on the degree of water saturation, in particular for samples conditioned at relative humidity greater 16 than 55%. The microcracks have a significant effect on permeability, but less influence on diffusion and water 17 absorption. Implications of the results on the effectiveness of these grouts for radioactive waste containment are 18 presented. 19