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Working reports


Posiva's working reports are used to publish research results related to the practical aspects of the final disposal of spent nuclear fuel. Approximately one hundred working reports are published every year, usually in Finnish or English. The reports are written by Posiva employees or specialists from third party organisations. The working reports are an important tool for distributing the results of research work. All working reports are published on the Posiva website.



Workreport 2010-72

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Name:

Titanium Alloys as Alternative Material for the Supercontainer Shell in the KBS-3H Concept: A Preliminary Ti-Clay Interaction Study

Writer:

Wersin, P., Grolimund, D., Kumpulainen, S., Kiviranta, L., Brendlé, J. & Snellman, M.

Language:

English

Page count:

52

Summary:

Ti alloys have been proposed as alternative materials to steel for the supercontainer shell surrounding the bentonite buffer in the KBS-3H disposal concept. Ti-based materials display high strength and are known to behave chemically inert under a variety of conditions. This preliminary study addresses the suitability of titanium as supercontainer material with regard to the performance of the clay buffer. Thus, possible titanium-bentonite interactions which may adversely affect the buffer's safety functions are evaluated by means of a literature and a preliminary experimental assessment.

Titanium metals display very low corrosion rates (< 1 nm/a) over a large range of pH and Eh conditions. The corrosion behaviour is governed by the low solubility of tetravalent TiO2 which forms a passive surface corrosion layer under both oxic and reducing conditions. The interactions between titanium and clay have been barely studied so far. Preliminary long-term data obtained by Prof. Olefjord and co-workers from Chalmers (S) in the 1980ies (as part of SKB's canister program) suggests similar corrosion rates in compacted bentonite compared to those measured in water, i.e. ?1 nm/a. So far, no work on reaction products from this interaction process has been carried out. Even the speciation of Ti in natural clays is uncertain. In principle, four possible reaction products resulting from Ti-clay interactions are possible: (i) Ti sorbed to the clay surface via cation exchange or specific adsorption, (ii) Ti incorporated in the octahedral or tetrahedral clay structure, (iii) Ti precipitated as separate TiO2 or mixed (Fe, Ti) oxide, (iv) Ti precipitated as separate silicate phase and (v) polymerized as cross-linked TiO2 units in the interlayer (Ti pillared clay). The latter two transformation products would have the strongest impact on the buffer, but are improbable on the basis of current knowledge.

A preliminary batch-type investigation has been carried out to shed more light on Ti-clay interaction processes and on the Ti species resulting from these interactions. Purified MX-80 bentonite was mixed with metallic Ti nanopowder and foil at different pH and temperature conditions. After several months, solid and solute samples from the first set of tests were analyzed by wet chemistry and quantitative X-ray fluorescence analysis. The chemical speciation of Ti was analyzed with XAS. In addition to reacted samples, a number of reference and starting materials (e.g. MX-80, Rokle bentonite, Opalinus Clay, Illite du Puy) were characterized by XRF and XAS. Preliminary results can be summarized as:
•    Natural clay materials contain significant but variable amounts of Ti. The standard purification procedure for bentonites to remove accessories does not or only barely remove Ti.
•    The Ti in the natural clay materials Rokle bentonite, Opalinus Clay and Illite du Puy occurs as microcrystalline TiO2 (presumably as anatase). On the other hand, the Ti spectra in MX-80 suggest the presence of structural Ti in the smectite, but the evidence is not conclusive so far.
•    The exposure of purified MX-80 to titanium powder at room temperature within a period of five months did not lead to measurable additional Ti in the clay.

A second series of experiments with synthetic "Ti-free" montmorillonite at higher temperatures (80?C) was conducted for a period of 4.5 months in order to increase Ti transfer rates from the metal surfaces to the clay. As revealed from spectroscopic analysis (micro-XRF and XANES) on the reacted materials, the use of synthetic montmorillonite combined with the increase of temperature allowed to partly overcome the experimental problems noted in the first series. Thus, Ti background concentrations were effectively very low and Ti transfer to the clay could be identified with spectroscopic analysis. On the other hand, the spectroscopic data highlighted further weaknesses in the setup. Thus the use of Ti nanopowder is not recommendable because of difficult separation from the clay.

It is recommended to carry out a third experimental series of "accelerated tests" with an optimized experimental setup.

This report is a result of a joint project between Posiva and SKB. The report will also be printed as SKB Report R-10-51.

Keywords:

KBS-3H, supercontainer shell, titanium, Ti-clay interactions.

File(s):

Titanium Alloys as Alternative Material for the Supercontainer Shell in the KBS-3H Concept: A Preliminary Ti-Clay Interaction Study (pdf) (634.9 KB)


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