Abstract

—Bentonites are candidate materials for encapsulating radioactive waste within barrier systems in crystalline rocks. In the ‘Alternative Buffer Material’ (ABM) test in the hard rock laboratory in Äspö, Sweden, six packages of eleven different buffer materials (mainly bentonites) with various exchangeable cation populations were packed vertically with an iron tube used as a heater in the center. After installation, the second ‘ABM package’ (ABM-II) was first allowed to saturate with water for approximately 2.5 years. The blocks were then exposed to a temperature of up to 141°C for approximately 3–4 years. The hypotheses for the present study were: (1) no horizontal gradient of the cation exchange population was present in the individual blocks of ABM-II because ABM-II had a longer reaction time in comparison to the ABM-I package, which did not have horizontal gradients; (2) the exchangeable cation Ca2+:Na+:Mg2+ ratio was equal in all blocks of ABM-II and was independent of block position in the package. As expected from ABM-I, all blocks in the ABM-II experiment showed large differences between the measured values of the reference materials and the reacted samples. The exchangeable Na+ and Mg2+ values in ABM-II were reduced by up to 55% to 59% in comparison to the reference material. Contrary to the first hypothesis, horizontal gradients were observed in ABM-II; and, contrary to the second hypothesis, the exchangeable cation ratios differed markedly in the different reacted buffer materials. The largest total Na+ loss was observed in the middle part (–67%), whereas Mg2+ values decreased by 79% in the upper part. The exchangeable Ca2+ values increased strongly in ABM-II, particularly in the upper part. The most useful parameter to distinguish between ion exchange equilibria of ABM-I and ABM-II was the Na+/Mg2+ ratio. This ratio was constant in ABM-I (3.0) and had a similar ratio (3.5) in the lower part of ABM-II; however, the ratio strongly increased (5 – 10) in the upper part of the ABM-II package. The large Na+/Mg2+ ratios in the upper part of ABM-II could possibly be explained by water loss into the rock (caused by a pressure drop and boiling) and subsequent water uptake.

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