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Chemical Effects at the Reaction Front in Corroding Spent Nuclear Fuel

Published online by Cambridge University Press:  19 October 2011

Jeffrey A. Fortner ,
A. Jeremy Kropf ,
James L. Jerden  and
James C. Cunnane
Jeffrey A. Fortner
Affiliation:
fortner@cmt.anl.gov, Argonne National Laboratory, Chemical Engineering, CMT/205, 9700 S. Cass Avenue, Argonne, IL, 60439, United States, 630-252-5594
A. Jeremy Kropf
Affiliation:
kropf@cmt.anl.gov, Argonne National Laboratory, Chemical Engineering, CMT/205, 9700 S. Cass Avenue, Argonne, IL, 60439, United States
James L. Jerden
Affiliation:
jerden@cmt.anl.gov, Argonne National Laboratory, Chemical Engineering, CMT/205, 9700 S. Cass Avenue, Argonne, IL, 60439, United States
James C. Cunnane
Affiliation:
cunnane@cmt.anl.gov, Argonne National Laboratory, Chemical Engineering, CMT/205, 9700 S. Cass Avenue, Argonne, IL, 60439, United States
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Abstract

Performance assessment models of the U. S. repository at Yucca Mountain, Nevada suggest that neptunium from spent nuclear fuel is a potentially important dose contributor. A scientific understanding of how the UO2 matrix of spent nuclear fuel impacts the oxidative dissolution and reductive precipitation of Np is needed to predict the behavior of Np at the fuel surface during aqueous corrosion. Neptunium would most likely be transported as aqueous Np(V) species, but for this to occur it must first be oxidized from the Np(IV) state found within the parent spent nuclear fuel. In this paper we present synchrotron x-ray absorption spectroscopy and microscopy findings that illuminate the resultant local chemistry of neptunium and plutonium within uranium oxide spent nuclear fuel before and after corrosive alteration in an air-saturated aqueous environment. We find the Pu and Np in unaltered spent fuel to have a +4 oxidation state and an environment consistent with solid-solution in the UO2 matrix. During corrosion in an air-saturated aqueous environment, the uranium matrix is converted to uranyl (UO22+) mineral assemblage that is depleted in Np and Pu relative to the parent fuel. The transition from U(IV) in the fuel to a fully U(VI) character across the corrosion front is not sharp, but occurs over a transition zone of ∼ 50 micrometers. We find evidence of a thin (∼ 20 micrometer) layer that is enriched in Pu and Np within a predominantly U(IV) environment on the fuel side of the transition zone. These experimental observations are consistent with available data for the standard reduction potentials for NpO2+/Np4+ and UO22+/U4+ couples, which indicate that Np(IV) may not be effectively oxidized to Np(V) at the corrosion potential of uranium dioxide spent nuclear fuel in air-saturated aqueous solutions.

Keywords

actinideextended x-ray absorption fine structure(EXAFS)corrosion
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 985: Symposium NN – Scientific Basis for Nuclear Waste Management XXX , 2006 , 0985-NN01-03
Copyright
Copyright © Materials Research Society 2007

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References

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