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The Effect of Degradation Products of Strong Acidic Cation Exchange Resins on Radionuclide Speciation: A Case Study with Ni2+

W. Hummel, L. R. van Loon

Nuclear Technology / Volume 128 / Number 3 / Pages 372-387

December 1999

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Radiolytic degradation experiments with acidic ion-exchange resins revealed oxalate and an unidentified ligand X to be the most strongly complexing ligands of the degradation products. The influence of these ligands on the Ni speciation in groundwater and cement pore water of a repository is assessed.

A complete and reliable thermodynamic database is built for this case study. Missing stability constants are estimated by chemical reasoning. Subsequent sensitivity analyses show whether these species are important or not. The backdoor approach used in this study addresses the following question: What concentrations must the ligand have to significantly influence the Ni speciation?

In the case of oxalate, the concentration necessary to complex 90% Ni will never be exceeded within the repository or in its environment due to precipitation of Ca-oxalate solids. Thus, a negative effect of oxalate on Ni speciation and sorption need not be considered in safety assessments.

In the case of ligand X, calculations demonstrate that Ni speciation is highly dependent on geochemical conditions and is occasionally ambiguous due to uncertainties in estimated stability constants. Hints are given to deal with these ambiguities in future safety assessments, and further experimental investigations are proposed to decrease uncertainties when necessary.

 
 
 
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