Coupled transport-reaction modeling of the long-term interaction between iron, bentonite and Callovo-Oxfordian claystone in radioactive waste confinement systems

Abstract

Appropriate geological disposals are required to manage the high level and intermediate level long lived radioactive waste. Several concepts have been developed in a number of nations. In France, different concepts have been investigated successively with time by Andra: either the steel waste package for vitrified waste was separated from the host rock (Callovo-Oxfordian (COx) claystone) by a bentonite engineered barrier or in the actual design, is in direct contact with the COx claystone. The long term interactions of the geological groundwater and steel overpack with the bentonite and/or claystone barriers were carried out by using the Kirmat 1D reactive transport code. Three different modeling approaches with the (T, t) conditions expected in the repository were implemented to (i) investigate the influence of the surface area of the primary minerals and the diffusion coefficient on the evolution of the iron–bentonite system and to (ii) compare the reactivity of the iron–bentonite and iron–COx claystone systems.For the iron–bentonite interactions, the simulated results showed significant differences depending on the different hypotheses used to calculate surface areas for minerals. When different sizes of mineral particles are assumed to estimate the surface areas of primary minerals, the system explored a larger change in porosity and more intense mineralogical modifications in the zone close to the bentonite/steel overpack interface. This result is likely related to the lower diffusion of aqueous corrosion products through the bentonite barrier. The comparison between iron–bentonite and iron–COx claystone interactions confirmed a clear different behavior of reactivity. For the iron–bentonite the alteration concerns two zones in direct contact with the geological groundwater and steel overpack but for the iron–COx claystone the mineralogical modifications are predicted to occur in the whole medium. The mineralogical evolution of the COx claystone barrier is mainly impacted by the dissolution of primary minerals, especially for total transformation of interstratified illite/smectite (I-Sm) and formation of secondary minerals such as illite and berthierine-FeII. The modeling results obtained from the sensitivity test showed a significant impact of the diffusion parameter on the evolution of the COx barrier, especially the changes in pH, porosity and the mineralogical modification of the zone in contact with the steel container.