Mineralogical investigations of the first package of the alternative buffer material test – I. Alteration of bentonites

Abstract

Bentonite, which is envisaged as a promising engineered barrier material for the safe disposal of highly radioactive waste, was and is investigated in different large scale tests. The main focus was and is on the stability (or durability) of the bentonite. However, most countries concentrated on one or a few different bentonites only, regardless of the fact that bentonite performance in different applications is highly variable. Therefore, SKB (Svensk Kärnbränslehantering) set up the first large scale test which aimed at a direct comparison of different bentonites. This test was termed the ‘alternative buffer material test’ and considers eleven different clays which were either compacted (blocks) or put into cages to keep the material together. One so-called package consisted of thirty different blocks placed on top of each other. These blocks surrounded a heated iron tube 10 cm in diameter. Altogether three packages were installed in the underground test laboratory Äspö, Sweden. The first package was terminated 28 months after installation and the bentonite had been exposed for the maximum temperature (130°C) for about one year.Almost all geochemical and mineralogical alterations of the different bentonites (apart from exchangeable cations) were restricted to the contact between iron and bentonite. The increase of the Fe2O3 content was attributed to corrosion of the tube. However, the typical 7 or 14 Å smectite alteration product was not found. At the contact of one sample, siderite was precipitated. Some samples showed anhydrite and organic carbon accumulation and some showed dissolution of clinoptilolite and cristobalite. IR spectroscopy, XRD, and XRF data indicated the formation of trioctahedral minerals/domains in the case of some bentonites. Even more data has to be collected before unambiguous conclusions concerning both alteration mechanisms and bentonite differences can be drawn.

Mineralogical, physical and chemical investigation of compacted Kunigel V1 bentonite in contact with a steel heater in the ABM test package 1 experiment, Äspö laboratory, Sweden

“…With regard to the distribution of exchangeable cations, the lateral gradient was basically insignificant, except for Mg 2+ concentrations in samples nearest to the steel heater. A small decrease occurred within 20 mm of the interface in the present study, although the increase in Mg at the contact was reported in the bentonites used in the ABM test packages (Kaufhold et al, 2013;Kumpulainen et al, 2016). No significant change was observed for the sum of exchangeable cations (total cations) within the JNB01 sample.…”

“…Unlike the XRD results with humidity control, the EPMA results do not support an Fe 2+ -for-Na + exchange by smectite near the I-B interface. An alternative reason for the Fe enrichment observed at this location might involve the precipitation of Fe(III) oxyhydroxides (Ishidera et al, 2008;Wersin et al, 2015) and/or the formation of siderite (Kaufhold et al, 2013;Wersin et al, 2015) resulting from the corrosion of the steel. Enrichment of Ca and Fe near the I-B interface may also be due to the precipitation of carbonate; thus the siderite formation should not be ignored.…”

“…Such interactions could impact repository safety because they might result in the conversion of smectite, which is an active component of bentonite buffer, to non-swelling silicates such as berthierine (Lantenois et al, 2005;Wilson et al, 2006;Perronnet et al, 2008;Sasamoto et al, 2010) and a patina consisting of an Fe-silicate (Kaufhold et al, 2015), and because potential radionuclide sorption sites in smectite might be saturated by ferrous ions anaerobically corroded from the iron overpack (Charlet & Tournassat, 2005;Géhin et al, 2007). In addition, neo-formation of expandable trioctahedral minerals converted from the dioctahedral smectite which would have less impact on repository safety have been also reported (Kaufhold et al, 2013;Svensson & Hansen, 2013;Svensson, 2015).…”

Distinguishing between more and less suitable bentonites for storage of high-level radioactive waste

“…In addition, Ca-rich bentonites do not tend to release colloidal particles as Na-bentonites do. Furthermore, the larger selectivity of Ca(Mg) towards the interlayer and the fact that most host-rock waters do contain some Ca will cause fast cation exchange if Na-bentonites are contacted with such solutions (Kaufhold et al, 2013;Dohrmann et al, 2013b). A general recommendation of the type of exchangeable cation initially present may be redundant because of the fast equilibration of the bentonite with the surrounding water.…”

Overview of the clay mineralogy studies presented at the ‘Clays in natural and engineered barriers for radioactive waste confinement’ meeting, Montpellier, October 2012

“…This model, therefore, will be further improved. 4) Kaufhold et al (2013) investigated one block of each different clay material used in the first package of the Ä spö hard rock laboratory ABM-test (alternative buffer material test). They found the different materials to have reacted differently and finally summarized all alteration mechanisms as FeÀ, C org , C inorg , SO 4 -accumulation and mineral dissolution.…”