The performance of bentonite barriers for high level radioactive waste (HLRW) disposal is currently being tested in various real- and up-scale disposal tests. One of the disposal tests, the ABM test (ABM = alternative buffer material), was conducted by SKB (Svensk Kärnbränslehantering) as a medium-scale experiment at the Äspö hard rock laboratory in Sweden. The present study deals with the second parcel (ABM-II), which was retrieved after 6.5 years with 2.5 years of water saturation and 3–4 years of heating up to 141°C. Nine different bentonites and two marine clays were tested to investigate the performance. The aim of the study was to provide a detailed characterization of the mineralogical and chemical changes that took place in ABM-II, compare the findings with ABM-I (the first of the six test parcels), and try to draw some general conclusions concerning the use of bentonites in such geotechnical barriers. The ABM-II test parcel revealed a set of reactions that a HLRW bentonite might undergo. The most prominent reaction was the rather complete exchange of cations, which was discussed in a second part to this publication (II – cation exchange; Dohrmann and Kaufhold, 2017). The corrosion of the Fe in metal canisters was observed, but no discrete corrosion product was identified. At the interface of bentonite and the metal canister, the formation of smectite-type trioctahedral clay minerals was observed. In contrast to the ABM-I test, anhydrite was present in many of the bentonite blocks of the ABM-II test. In most concepts used for HLRW disposal in crystalline rocks, a temperature below 100°C at the canister surface was applied to avoid boiling. In the ABM-II test, boiling of water was possibly observed. Throughout the experiment, a pressure/water loss was recorded in the upper part of the geotechnical barrier and water was added to maintain pressure in the bentonite. As a result of evaporation, NaCl crusts might have formed and the barrier was partly disintegrated. These results demonstrated that a reasonable assumption is that no boiling of water occurs in disposal concepts in which a pressure loss can occur.

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