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Interactions of Uranium and Neptunium With Cementitious Materials Studied by XAFS

Published online by Cambridge University Press:  10 February 2011

E. R. Sylwester ,
P. G. Allen ,
P. Zhao  and
B. E. Viani
E. R. Sylwester
Affiliation:
Glenn T. Seaborg Institute for Transactinium Science, Lawrence Livermore National Laboratory, P.O. Box 808, MS L-231, Livermore, CA 94551, sylwester1@llnl.gov
P. G. Allen
Affiliation:
Glenn T. Seaborg Institute for Transactinium Science, Lawrence Livermore National Laboratory, P.O. Box 808, MS L-231, Livermore, CA 94551
P. Zhao
Affiliation:
Glenn T. Seaborg Institute for Transactinium Science, Lawrence Livermore National Laboratory, P.O. Box 808, MS L-231, Livermore, CA 94551
B. E. Viani
Affiliation:
Geosciences & Environmental Technologies Division, Lawrence Livermore National Laboratory
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Abstract

We have investigated the interaction of U(VI) and Np(V) actinide ions with cementitious materials relevant to nuclear waste repositories using X-Ray Absorption Fine Structure (XAFS) Spectroscopy. The actinide ions were individually loaded onto untreated as well as hydrothermally treated cements. The mixtures were then equilibrated at varying pH's for periods of 1 month and 6 months.

In all cases uranium was observed to remain in the initial UO22+ form in the Near Edge (XANES) spectra. The uranium samples show evidence of inner-sphere interactions on both treated and untreated cements at all pH's, with the uranyl complexing with the mineral surface via sharing of equatorial oxygens. On treated cement near-neighbor U-U interactions are also observed, indicating the formation of oligomeric surface complexes or surface precipitates.

Neptunium was observed to undergo a reduction from the initial NpO2+ to Np4+ Calculated % reduction showed ca. 15% of Np(V) is reduced to Np(IV) after a 1 month equilibration time. After 6 months higher % reduction of between 40% and 65% was observed. No Np-Np interactions were observed in the EXAFS spectra, which suggests that surface precipitation of NpO2 is an unlikely mechanism for sorption.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 608: Symposium QQ – Scientific Basis for Nuclear Waste Management XXIII , 1999 , 307
Copyright
Copyright © Materials Research Society 2000

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References

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