Characterization of the Second Package of the Alternative Buffer Material (ABM) Experiment — II Exchangeable Cation Population Rearrangement

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.

Interactions of bentonite with metal and concrete from the FEBEX experiment: mineralogical and geochemical investigations of selected sampling sites

“…Taking into account these corrections, the exchangeable Ca 2+ values remained around the value for the reference bentonite. In the long-term test of buffer material project (LOT), no such gradients of ECs were detected (Karnland et al , 2009), but various gradients were observed in the first package of the ABM project (ABM-I; Dohrmann et al , 2013; Wallis et al , 2015) and in the second package (ABM-II; Dohrmann & Kaufhold, 2017).…”

“…Geochemical and mineralogical changes of the bentonite caused by the heater and inflowing water over 18 years of operation may be identified only if reliable reference data of the bentonite before the experiment are available. Most of the changes identified in other large-scale tests, however, were restricted to the contacts with the hot metal or with the cement (Dohrmann et al, 2013(Dohrmann et al, , 2017Kaufhold et al, 2013Kaufhold et al, , 2017. Therefore, reference data may also be obtained from material derived from the centre of the blocks taken at a distance from the interfaces.…”

Thermally Induced Bentonite Alterations in the SKB ABM5 Hot Bentonite Experiment

“…The SKB (Swedish Nuclear Fuel and Waste Management Company, Stockholm, Sweden) has been testing the performance of bentonite barriers for many years in a granite formation at a depth of 500 m using the currently favored KBS-3 (kärnbränslesäkerhet, nuclear fuel safety) concept at a test site in the Äspö Hard Rock Laboratory [5]. The SKB conducted a series of six ABM (alternative 2 of 18 buffer material) experiments between 2006 and 2017 using a range of compacted bentonites simulating different temperatures and water saturation conditions [11][12][13][14][15], where the ABM3, ABM4 and ABM6 tests are still running and are expected to be excavated in 2024.…”

Geochemical and mineralogical characterization of smectites from the Ventzia basin, western Macedonia, Greece

“…Trioctahedral magnesian smectites may also be superior as barrier materials for high-level radioactive waste repositories. This is because Al-rich smectites may convert to trioctahedral smectites at the heater–bentonite interface in large-scale deposition tests (Plötze et al , 2007; Karnland et al , 2009; Kaufhold et al , 2013, 2017b; Dohrmann & Kaufhold, 2014, 2017; Svensson, 2015).…”