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A Study of the Hydrothermal Stability of Copper for Use as a Container Material for Nuclear Waste

Published online by Cambridge University Press:  28 February 2011

Paul I. Lazaar
Affiliation:
Dept. of Geology, Temple University, Philadelphia, Pa. 19122
G. C. Ulmer
Affiliation:
Dept. of Geology, Temple University, Philadelphia, Pa. 19122
D. E. Grandstaff
Affiliation:
Dept. of Geology, Temple University, Philadelphia, Pa. 19122
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Abstract

The hydrothermal stability of copper has been studied to assess its suitability as a container material for disposal of nuclear waste in the proposed repository site at Hanford, Wa. The experiments (Cohassett basalt, synthetic Grande Ronde #4 groundwater, and copper powder with a water:rock:Cu-powder mass ratio of 20:1:1) were conducted at 300°C, 30 MPa, using Dickson rocking autoclaves for periods up to 3000 hours. Redox was calculated from dissolved H2 measured by gas chromatograph and He-ionization detector.

After ca. 100 hours the solution Cu concentration stabilized at 2–3 ppm, near Cu saturation, and did not vary significantly during the remainder of the experiment. The in situ solution pH was slightly alkaline. The copper powder showed little evidence of etching. The Cu concentration did not reflect oxide-coating spallation effects such as those described by Johnston et al.[15]. Within 48 hours, the log fO2 values decreased rapidly toward the magnetite-hematite phase boundary (−31 at 300°C); this is well within the stability field of native copper. SEM and EDX analysis of the reaction products revealed a copper-iron sulfide. One experiment used copper powder containing ca. 5% cuprite. Oxygen released by the cuprite overwhelmed the buffering capacity of the basalt. The resulting log fO2 values stabilized near the copper-cuprite phase boundary (−23.1 at 300°C) with the solution remaining within the copper stability field. Copper purity is important as oxygen contamination or oxidation of the copper containers may strongly affect the repository redox and mobility of radionuclides.

Type
Research Article
Copyright
Copyright © Materials Research Society 1988

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