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The Behavior of 99Tc in Doped-Glass/Basalt Hydrothermal Interaction Tests

Published online by Cambridge University Press:  25 February 2011

D.G. Coles  and
M. J. Apted
D.G. Coles
Affiliation:
Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352
M. J. Apted
Affiliation:
Rockwell Hanford Operations, P.O. Box 800, Richland, WA 99352
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Abstract

The release of polyvalent radionuclides from a nuclear waste repository located in basalt may be sensitively related to the redox potential (Eh) imposed by the basalt. A series of tests are reported here evaluating the effect of basalt on the concentrations of 99Tc released into solution from a borosilicate glass waste form. Crushed PNL 76-68 glass, doped with 0.7 mg 9 99Tc/g glass, was reacted with reference basalt groundwater under oxic hydrothermal conditions in a sampling autoclave, both alone and in the presence of crushed basalt. The steady state fluid concentrations of 99Tc and various stable species were determined from samples obtained at the test conditions of 200 °C, 30 MPa, and a initial solution to solid mass ratio of 10 for both tests.

In the glass + groundwater test, the 99Tc concentration rose rapidly to about 50 mg/L after only 200 hr of run time and remained at a value between 50 and 60 mg/L throughout the duration of the test. For the basalt + glass + groundwater test, the 99Tc concentration rose to an initial value of about 2.5 mg/L. At about 700 hr, the 99Tc concentration began to drop rapidly until a value near the analytical detection limit (approximately 0.005 mg/L) was reached after a test duration of 1,400 hr. It is concluded that the presence of basalt in these hydrothermal experimtents reduces the concentration of 99Tc in solution by nearly four orders of magnitude, probably by control of solution Eh and subsequent precipitation of a solid containing a reduced form of technetium. Reaction mechanisms are discussed that can account for these observations.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 26: Symposium D – Scientific Basis for Nuclear Waste Management VII , 1983 , 129
Copyright
Copyright © Materials Research Society 1984

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References

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