O/07A/3 Page 107 INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCE CLAYS IN NATURAL & ENGINEERED BARRIERS FOR RADIOACTIVE WASTE CONFINEMENT IMPACT OF IRON ON THE BENTONITE BUFFER WITIHIN THE KBS-3H DISPOSAL CONCEPT: RESULTS FROM REACTIVE TRANSPORT MODELLING P. Wersin 1 ,M. Bi rgersson 2 ,M. Snellman 3 , 1. NAGRA, Hardstrasse 73, CH-5430 Wettingen,Switzerland (paul.wersin@nagra.ch) 2. Clay Technology,S-223 70 Lund,Sweden (mb@claytech.se) 3. Saanio &RiekkolaOy, FI-00420 Helsinki,Finland (margit.snellman@sroy.fi) INTRODUCTION AND OBJECTIVES Steel components are unstable in EBS environments. They will corrode to fairly insoluble corrosion products, such asmagnetite, and also react with the smectitic matrix of the bentonite buffer. The latter reaction process, referred here as iron-bentonite interactions, may involve different processes including sorption, redox and dissolution/precipitation reactions, the details of which are not yet understood. The process of greatest relevance for the buffer’s performance is montmorillonite transformation in contact with reduced iron. This process is a very slow and experimentally difficult to investigate. Current data (e.g. Guillaume et al., 2003; 2004; Wilson et al., 2006) suggest thatthe transformation process may either lead to aFe-rich smectite (e.g. saponite) or to a non-swelling clay such as berthierine or chlorite. In addition, cementation effects resulting from precipitation of iron corrosion products or of SiO 2 induced by montmorillonite transformation need to be considered. In the KBS-3H (horizontal) disposal concept, which is being developed by SKB and Posiva, prefabricated modules of spent fuel copper-iron canisters surrounded by bentonite blocks and an outer thin perforated steel supercontainer (SC) are emplaced in horizontal tunnels. Upon tunnel closure,groundwater from the granitic host rock will saturate the buffer material by swelling,both inside and outside of the SC, which is expected to induce an effective seal around the canisters. On the other hand,corrosion processes leading to significant quantities of oxidized iron species and hydrogen will occur. A key question iswhetherthe released iron my affect the barrier performance of the buffer. In a preliminary study, this question was addressed with a very simple geochemical model using the Olkiluoto site as test case (Wersin et al., 2005). The results thereof indicated that the effect of the SC on the buffer was not significant.However,because of the non-consideration of clay mineral transformation reactions, no conclusions on the extent of the alteration zone could be drawn. In this contribution, we present the outcome of a follow-up study in which the impact of the SC on the bentonite was assessed with a reactive transport modelling approach. The interaction of the corroding iron source with the bentonitematrixwassimulated with a 1D diffusion-reaction model with aid of the geochemical codes PHREEQC and CrunchFlow. The Fe-bentonite interaction process included both Fe(II) sorption and dissolution/precipitation reactions of clay mineral phaseswhich were based on carefully evaluated thermodynamicand kinetic data. A number of limiting test caseswere run toexplore the sensitivity of the results to uncertainties in dataand model assumptions. RESULTS AND CONCLUSIONS The modelling results indicate thatthe flux of Fe into the bufferwill be small and the extent of the buffer zone affected by iron-bentonite interaction processeswill be limited. The largest Fe flux would arisefrom a relativelysoluble FeCO 3 source and concomitant low Fe(II) activity in the porewater of the unaffected buffer. But even forthis case and time scales of 10 5 years, no large detrimental effect on the bufferwere predicted. This is illustrated in Fig. 1 for a limiting test case with a high montmorillonite solubility, which shows the mineral volume fraction after 10 5 and 5·10 5 years.