Mechanismen der hydrothermalen Alteration von Zirkonolith und Mineralen der Pyrochlorgruppe und ihre Abhängigkeit vom Grad des radioaktiven Strahlenschadens
Final Report Abstract
The safe and long-term disposal of high-level nuclear waste, i.e., plutonium and other actinides, is one of the major concerns of the modern society. The project focused on investigating the suitability of two candidates for the sequestration of “surplus” plutonium: pyrochlore and zirconolite. The major issues investigated were: (1) What processes are involved in the hydrothermal alteration of zirconolite and pyrochlore? (2) Can the results from laboratory experiments be transferred to observations made on natural samples? (3) In how far does the degree of self-irradiation damage affect the aqueous stability of zirconolite and pyrochlore? The results revealed that one of the major mechanisms involved in the alteration of pyrochlore and zirconolite is a spatially and temporally coupled dissolution-reprecipitation process. During such a process, pyrochlore and zirconolite dissolve congruently and new stable phases or phase assemblages (e.g., rutile, zirconia) precipitate at an inward moving reaction front. A Ta-based pyrochlore (microlite) was epitaxially replaced experimentally by a defect-type pyrochlore; a reaction that has previously been described as a solid-state leaching process that involves ion exchange and hydration. Comparing the alteration of natural samples with experimental results, it could be shown that the results of nature and experiments correlate to a high degree. Thus, the results of laboratory experiments can be used to predict the performance of a nuclear waste form in a geological repository, if natural analogues are available. Experiments with a natural self-irradiated pyrochlore and a severly selfirradiated, synthetic 238Pu-doped zirconolite ceramic further indicated that the degree of self-irradiation damage, accumulated in a nuclear waste form over time, has a strong effect on its aqueous durability.
Publications
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(2005) Experimental hydrothermal alteration of crystalline and radiation-damaged pyrochlore. J. Nucl. Mater., 344, 17-23
Geisler T., Seydoux-Guillaume A.-M., Pöml P., Golla-Schindler U., Berndt J., Wirth R., Pollok K., Janssen A., and Putnis A.
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(2005) Experimental observation of an interface-controlled pseudomorphic replacement reaction in a natural crystalline pyrochlore. Am. Mineral., 90, 1683-1687
Geisler T., Pöml P., Stephan T., Janssen A., and Putnis A.
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(2005) Structural investigations and thermodynamics of nuclear waste form zirconolite ceramics. Beih. Eur. J. Mineral., 17, 104
Pöml P., Geisler T., and Konings R.J.M.
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(2006) 18O-tracing of the hydrothermal alteration of pyrochlore. Geochim. Cosmochim. Acta, 70, Suppl. 1, A34
Pöml P., Menneken M., Stephan T., Niedermeier D., Geisler T., and Putnis A.
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(2006) High-temperature heat capacity of zirconolite (CaZrTi2O7). J. Chem. Thermodynamics, 38, 1013-1016
Pöml P., Geisler T., and Konings R.J.M.
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(2007) Evaluation of the long-term performance of potential nuclear waste form materials. Frontiers in Mineral Sciences 2007, Programme and Abstracts, 197-198
Geisler T., Pöml P., Janssen A., Soman A., Menneken M., Plümper O.,Schepers A., Scheiter D., and Niedermeier D.R.D.
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(2007) Mechanism of hydrothermal alteration of natural self-irradiated and synthetic crystalline titanate-based pyrochlore. Frontiers in Mineral Sciences 2007, Programme and Abstracts, 198-199
Pöml P., Geisler T., Menneken M., Stephan T., Niedermeier D.R.D., and Putnis A.
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(2007) Mechanism of hydrothermal alteration of natural self-irradiated and synthetic titanate-based pyrochlore. Geochim. Cosmochim. Acta, 71, 3311- 3322
Pöml P., Menneken M., Stephan T., Niedermeier D., Geisler T., and Putnis A.
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(2009) Fluid-mediated phase transformations in nuclear waste form materials. Book of Abstracts of the 33rd International Symposium "Scientific Basis for Nuclear Waste Management” in St. Petersburg, Russia, 65
Geisler T.
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(2009) Hightemperature heat capacity of Gd-pyrochlore (Gd2Ti2O7). J. Chem. Thermodynamics, 41, 1049-1051
Janßen A., Pöml P., Beneš O., Geisler T., and Konings R.J.M.
