Chemistry Reference
In-Depth Information
with similar chemical properties.
1-3
In contrast to the PUREX process, where
only uranium and plutonium are the center of attention, extraction of long-lived
radionuclides has currently become the priority task. The revived interest in fraction-
ating high-level waste is connected mainly with an increase of fuel burn-up and will
be especially important for mixed oxide (MOX) fuel and fuels for fast reactors.
After a few years of storage, the main radioactive heat emitters in HLW are
90
Sr
and
137
Cs. In addition, extremely long-lived actinides—neptunium, plutonium, ameri-
cium, and curium—should be collected for transmutation in the future. Therefore,
different flowsheets can be proposed for waste processing. It is possible to extract each
radionuclide in the special extraction (sorption) cycle, for example, uranium and plu-
tonium in the PUREX process, and after that, minor actinides (MAs) by the TRUEX
process,
4
strontium by the SREX process,
5,6
and cesium by sorption
7
or extraction.
8
The flowsheet of the UREX process, developed in the United States, includes the
following extraction cycles: (1) separation of uranium and technetium, (2) separation
of plutonium, (3) separation of cesium and strontium, (4) separation of MAs and
Rare Earth Elements (REE), and (5) group separation of MA from REE metals.
9,10
Flowsheet development in Europe
11
includes a modified PUREX process and, after
that, the DIAMEX process for separation of MAs and lanthanides, the SANEX pro-
cess for separation of MAs from lanthanides, and a special cycle for Am/Cm separa-
tion. Cesium and strontium will be in the raffinate of the DIAMEX process, and this
raffinate will be vitrified, or cesium can be preliminarily extracted.
12
All these stages can be realized, but the total cost of such multicyclic processing
will be very high. It needs special equipment and buildings for realization of each
cycle. It should be mentioned also that each cycle has some additional secondary liquid
wastes that need to be vitrified, leading to additional cost. Therefore, the idea to com-
bine the removal of some radionuclides in one extraction cycle seems to be very prom-
ising. Thus, an alternative to such multicycle schemes may be simultaneous extraction
of several radionuclides. For example, the process for simultaneous extraction and
separation of lanthanides and MAs using octyl(phenyl)-
N,N
-diisobutylcarbamoyl-
methylphosphine oxide (CMPO) (SETFICS process) was tested and shows positive
results.
13 -16
All of these nuclides can be extracted and group separation can be done in
the stripping stages. In this process, two separate processes, classical TRUEX and An/
Ln separation, are united. Such a combination results in a more complex flowsheet, but
no additional tanks for the intermediate product, An-Ln concentrate, are needed.
There are special extractants to extract each class of radionuclides: crown ethers
for cesium and strontium; and phosphine oxides, carbamoylmethylphosphine oxides,
and diamides for actinides, etc. It is unrealistic to have a single extractant that can
extract all target nuclides with nearly the same effectiveness. So, a promising tech-
nical decision is to mix extractants for different radionuclides and extract them
si mu lt a neously.
The goal of this work is to discuss the advantages and drawbacks of combined
extraction of different nuclides by extractant mixtures. It is not a review of synergis-
tic extraction as such, but rather only a technological view on the use of extractant
mixtures in radiochemistry.
Mixtures of extractants have been used for extraction many times. The use of
the PUREX extractant tri-
n
-butyl phosphate (TBP) in paraffinic hydrocarbons as
