Chemistry Reference
In-Depth Information
an average solvent DF of 154,000 over the course of the entire 48-hour test. As
expected, it was further demonstrated that heating of the strip section to greater
than 30°C increases stripping performance markedly. A steady-state CF of 14.4 was
achieved for Tank 37H/44F high-level waste. Uncertainties in the process flow mea-
surements prevented achievement of the target CF of 15. Adjustment of the process
flow in the latter hours of the high-level waste test allowed the CF to approach 15 to
within 5% without exceeding the maximum throughput of the contactors.
Several specific studies for the development of CSSX process were carried out
by the Oak Ridge team to correct some minor defects observed during tests or to
improve the reliability of the process.
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The solvent was loaded with
137
Cs and subsamples were stored on a shaker table
while in contact with the extract, scrub, or strip aqueous phases. Evidence of solvent
degradation was evaluated for exposure times of 83 days; this resulted in estimated
solvent doses of 1.24 Mrad, equivalent to the dose expected to be received during
16.5 years of operation at the SRS plant.
Distribution of cesium in the batch tests remained constant within experimental
error; in addition, no third-phase formation was observed. The solvent concentrations
of calix[4]arene-bis-(
tert-
octylbenzo-crown-6) and 1-(2,2,3,3-tetrafluoroproproxy)-
3-(4 -
sec
-butylphenoxy)-2-propanol remained constant within experimental error.
Solvent degradation with irradiation was evidenced by a decrease TOA concentra-
tion decrease and an degradation product (4-
sec
-butylphenol) increase in the solvent
phase. No decline in extraction or scrubbing performance of the irradiated solvents
was observed. The stripping performance of the solvent was seriously impaired with
irradiation; however, a mild caustic wash and replenishment of the TOA concentra-
tion restored the ability to strip the irradiated solvent.
Although solvent samples have been observed for approximately one year with-
out any solids formation, work was completed to define a new solvent composition
that was thermodynamically stable with respect to solids formation and to expand
the operating temperature with respect to third-phase formation.
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Chemical and
physical data as a function of solvent component concentrations were collected.
The data included BC6 solubility; cesium distribution ratio under extraction,
scrub, and strip conditions; flowsheet robustness; temperature range of third-
phase formation; dispersion numbers for the solvent against waste simulant, scrub
and strip acids, and sodium hydroxide wash solutions; solvent density; viscosity;
and surface and interfacial tension. These data were mapped against a set of pre-
defined performance criteria. The composition of 0.007 M BC6, 0.75 M 1-(2,2,3,3-
tetraluoropropoxy)-3-(4-
sec-
butylphenoxy)-2-propanol, and 0.003 M TOA in the
diluent Isopar L provided the best match between the measured properties and the
performance criteria.
Experiments suggested a potential cesium-stripping problem in the CSSX pro-
cess due to the presence of nitrite in the waste stimulant. The true reason for the
cesium-stripping problem was, in fact, the presence of surfactant (sodium mono- and
dimethyl naphthalene sulfonate) that can partition into the organic phase on extrac-
tion and then retain cesium upon stripping. To overcome this drawback, the authors
proposed a caustic wash with a moderate concentration of sodium hydroxide suffi-
cient to remove enough of these lipophilic anions to rejuvenate the solvent, therefore,
