Chemistry Reference
In-Depth Information
liquors. Only the systems that have been developed up to the implementation of cold,
spiked, or highly active countercurrent tests at laboratory scale or in industrial pilot
plants will be described: first, the two-cycle processes, and second, the single-cycle
processes.
3.3.1 t
w o
-
C y C l e
P
r o C e S S e S
3.3.1.1 First step: An(III)
+
Ln(III) separation from the Rest
of the Fission Products
In each country concerned with the reprocessing of spent nuclear fuel arising from
civil or military applications, the highly active waste (HAW) that has been gen-
erated for decades by the PUREX or similar processes is still currently stored
on the reprocessing plant sites, either as a liquid sludge contained in tanks or as
a solid in vitrified or cemented matrices. HAW usually contains large quantities
of alpha emitters (minor actinides, such as Np and TPEs) that increase its long-
term radiotoxicity. Therefore, the solvent-extraction processes developed in the
early 1980s aimed at achieving high minor-actinide DFs (e.g., American TRUEX
and Russian-American UNEX), but not at separating trivalent actinides from lan-
thanides: they involved highly efficient bidentate and chelating extracting agents
bearing oxygen-donor atoms. On the contrary, other solvent-extraction processes
based also on oxygen-donor extracting agents, such as Chinese TRPO, Japanese
DIDPA (first cycle), and French DIAMEX processes, were developed as head-end
steps of An(III)/Ln(III) partitioning schemes. The concept feasibility of all these
processes has already been validated on genuine highly active feeds, and their
recent developments mainly deal with the following issues:
•
Process comprehension, such as the study of aggregation phenomena to
predict third-phase formation.
Minor experimental modifications, such as the use of hydrophilic complex-
•
ants to ease the scrubbing and stripping of the loaded solvents and simplify
the flowsheets.
Optimization of the extractant skeleton or the diluent nature to improve
•
extraction or separation yields.
These developments will hereafter be described for each family of extractants
from NOPCs to amino compounds, and for each topic, results will be sorted chrono-
logically by country.
3.3.1.1.1 Phosphate and Phosphonate Derivatives
The enthalpies of extraction of U(VI) and Am(III) nitrates by neutral organophosphate
extractants, such as tri-
n
-butyl phosphate (TBP), tri-
n
-amyl phosphate (TiAP),
t r i -
sec
-butyl phosphate (TsBP), tri-
iso
-amyl phosphate (TiAP), and tri-
n
-hexyl
phosphate (THP) have been determined in
n
-dodecane over the temperature range
283−333 K (
88
). Am extraction becomes more exothermic in the following order:
T H P~Ti A P < TAP < TsBP < TBP, highlighting the steric hindrance caused by
branched substituents compared with linear homologues and long-chain substituents
