Chemistry Reference
In-Depth Information
Distribution ratios of ruthenium evolved irregularly with the dose absorbed. With
some amides, after a high increase (factors of 20-30 up to 1 MGy), the extraction
decreased, as shown for Pu(IV), but more drastically (factors of 10 to 300 from 1
to 2 MGy) (
188, 191
). Other authors indicated an absence of ruthenium extraction
(
D
Ru
< 10
−2
) over the entire range studied (0 to 1.8 MGy) (
194
). On the other hand,
the decontamination factors for U and Pu with respect to ruthenium(III) employing
irradiated amides were comparable to the corresponding values with TBP (
191
).
But with
N
-alkyl-caprolactams, the influence of the dose on the extraction of
ruthenium was strong, especially above 10 kGy, where a sharp increase in the extrac-
tion was observed (
201
).
The extraction of Zr increased with the absorbed γ-dose (factor of 20 up to 0.7-0.8
MGy for linear amides in
n
-dodecane (
191
) and factors of 4-6 up to 0.3-0.6 MGy
for shorter amides in benzene) (
192
). Beyond this threshold, the extraction decreased
slightly. This effect is more noticeable with
N
-methyl amides (
192
). As for Pu(IV),
the first step has been explained by authors by a synergistic extraction due to the
presence of carboxylic acids as degradation products (
191
). Nevertheless, the degra-
dation had a stronger effect on the decontamination factors of U and Pu with respect
to Zr(IV) than with TBP (
191
). Typically, decontamination factors were DF
M/Zr
= 7
and 12 for, respectively, U(VI) and Pu(IV) with TBP and 5 with the monoamide
DHOA at an irradiation dose of 300 MGy (
193
).
With cyclic amides (
N
-alkyl-caprolactams), Zr distribution ratios increased
with γ-irradiation in the range 0.1-10 kGy and decreased slightly beyond this. The
increase in the first step has been explained by the formation of large molecular com-
pounds like C
8
H
17
NHC
5
H
10
COOH, which have a better extracting capability (
201
).
For higher doses, this compound was supposed to be radiolyzed into smaller com-
pounds that would be soluble in the aqueous phase.
8.3.3.1.5 Removal of Degradation Products
The main degradation products of
N,N
-dialkyl-monoamides were easily removed
with dilute acid/water or during the extraction-scrub-strip sequence, unlike those of
TBP, which need specific alkali treatments (
187, 193
).
8.3.3.2 Malonamides
In the context of minor actinide partitioning from high-level radioactive liquid wastes,
malonamides have been proposed either as the single extractant in the DIAMEX
process (
202-210
) or in a mixture with dialkylphosphoric acid in the DIAMEX-
SANEX process (
156, 211, 212
). The semideveloped formula of the selected
malonamides is R(CH
3
)NCO(CHR´)CONR(CH
3
) (R and R´ are alkyl or oxyalkyl
groups). Degradation behavior was one of the main criteria selected when optimizing
the malonamide molecule's formula. To minimize the formation of surfactant com-
pounds, like long-alkyl-chain carboxylic acids, the number of carbon atoms was shared
between the radicals R and R´. The introduction of an oxygen in the central chain R´
was interesting because an additional cleavage became possible (
202, 213
). These
studies led to the selection of the reference molecule, DMDOHEMA (
N,N´
-dimethyl-
N,N´
-dioctyl hexyloxyethyl malonamide (C
8
H
17
(CH
3
)NCO)
2
CH(C
2
H
4
OC
6
H
13
)).
