Experimental Studies of Packing Material Stability

摘要

The Basalt Waste Isolation Project is conducting experiments to assess the stability of bentonite (sodium montmorillonite) and crushed basalt as waste package packing materials in a nuclear waste repository in basalt. The experiments are designed to identify changes in physical, chemical, and mineralogical properties that these materials could undergo in the repository environment. A series of bentonite dehydration experiments showed that after 1 year at 370 exp 0 C the clay's structural and swelling properties were preserved and only reversible dehydration occurred. At 440 exp 0 C, however, irreversible dehydration, collapse of the clay structure, and loss of swelling ability took place. Hydrothermal tests using bentonite, or an equal mixture of bentonite and basalt, along with synthetic groundwater, were also conducted. A bentonite + water experiment showed negligible structural alteration at 200 exp 0 C, although some iron enrichment of the clay occurred. At 300 exp 0 C, partial conversion of the montmorillonite to an iron- and potassium-rich smectite occurred, along with the formation of secondary quartz and albite. An experiment in the basalt + bentonite + water system at 150 exp 0 C resulted in partial etching of the basalt grains, but no detectable change in rock or clay mineralogy. At 300 exp 0 C, the basalt was strongly etched. Furthermore, iron- and potassium-rich smectite apparently replaced montmorillonite as the dominant clay, and secondary silica, zeolites, and minor feldspar were formed. These mineral assemblages are thought to be metastable at 300 exp 0 C, based on natural analogs. The kinetics and reaction paths of further transformations of packing materials are important areas for continued study. 10 references, 5 figures, 1 table. (ERA citation 09:035228)