The Fate Of Tc In A UK Intermediate-level
Nuclear Waste Repository
Author(s)
N. Evans, R. Hallam, S. Aldridge & P. Warwick
Abstract
In the UK, tetraphenylphosphonium bromide (TPPB) has been used to remove
technetium prior to disposal from some waste-streams by forming TPPBTc floc.
If this floc were to be disposed of in a cementitious repository the TPPBTc
would degrade by alkaline hydrolysis to triphenylphosphonium oxide, or by
radiolysis to triphenylphosphine, releasing the Tc into solution. Its chemistry
would be dominated by TcO4 in aerobic waters and sparingly soluble TcO2(s) in
anaerobic. Repository heterogeneity could mean that both Tc(VII) and Tc(IV)
are present simultaneously. If TcO4
- migrates into reducing conditions, ligands in
the waste may complex with Tc during reduction to form water-soluble
complexes. Also possible, is increased Tc solubility when organic ligands react
with TcO2(s). Comparisons of Tc solubilities at high pH starting from TcO2 and
from TcO4
- reduced in the presence of ligands were made. With EDTA and NTA
no difference was observed, suggesting that Tc(IV)-ligand complexes were
formed. For ISA and gluconic acid the Tc solubility starting from TcO4
- was
higher than in systems with TcO2 as the starting point. This suggests that the
TcO4
- was not reduced to TcO2, but an intermediate oxidation state complex was
formed, e.g. Tc(V). The conditional stability constant for the Tc(IV)-gluconic
acid complex has been determined to be log β = 26.6 ± 0.2.
Keywords: technetium, redox, high pH, nuclear waste repository, EDTA, ISA,
gluconic acid.
1 Introduction
The currently preferred UK option for the management of intermediate-level
radioactive waste (ILW) is to store it in a deep underground repository. This may
then be backfilled with a cementitious material. Once closed, the repository will
Keywords
technetium, redox, high pH, nuclear waste repository, EDTA, ISA,
gluconic acid.
Related Book
Other papers in this volume
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