Chemistry Reference
In-Depth Information
SC-CO
2
Spent fuel
Fission
products
TBP-HNO
3
extractant
UO
2
(NO
3
)
2
(TBP)
2
TBP recovery
UO
2
FIGURe 11.4
A “dry” process for the reprocessing of spent nuclear fuel using SC-CO
2
.
the conventional PUREX process. Such an arrangement has several potential advan-
tages over its conventional analog, among them faster mass transport, higher rates of
extraction, and tunable metal ion extraction efficiency.49
49
In addition, replacement by
SC-CO
2
of the normal paraffinic hydrocarbons typically used eliminates the need
for disposal of degraded organic solvent, thus, reducing secondary waste generation.
Obviously, however, since nitric acid is required to dissolve the spent fuel, this pro-
cess would still produce acidic, high-level aqueous waste. In the second approach,
referred to as “dry SF-PUREX” (Figure 11.4), the need for an initial dissolution of
the spent fuel in nitric acid would be eliminated by direct solubilization of uranium
and plutonium from their respective oxides into SC-CO
2
. That such an approach
may be feasible was suggested by the results of a series of earlier studies by Wai and
coworkers. These investigators had shown that UO
2
(NO
3
)
2
(TBP)
2
has an unexpect-
edly high solubility in SC-CO
2
(ca. 0.42 M at 40°C and 200 atm).
45
In a subsequent
report, they showed that UO
3
could be dissolved in SC-CO
2
by employing a syner-
gistic combination of HTTA and TBP, forming UO
2
(TTA)
2
(TBP)
2
:
50
UO
3 S
+ 2HL
SCF
+ TBP
SCF
→ UO
2
(L)
2
TBP
SCF
+ H
2
O
(11.4)
where HL is a fluorinated ß-diketone. Although this reaction was not found to
be effective for the dissolution of UO
2
, other related work showed that nitric acid
itself forms a highly soluble 1:1 complex with TBP in SC-CO
2
,
51
a complex read-
ily prepared simply by mixing TBP with a concentrated nitric acid solution in the
appropriate proportions.
49
Subsequent investigation demonstrated that this com-
plex can oxidize UO
2
to the hexavalent state, leading to formation of highly soluble
UO
2
(NO
3
)
2
(TBP)
2
.
52
Thus, a TBP-HNO
3
complex can effect the direct dissolution of
UO
2
in supercritical carbon dioxide:
3UO
2 S
+ 8TBP ⋅ HNO
3 SCF
→ 3UO
2
(NO
3
)
2
(TBP)
2 SCF
+ 2TBP
SCF
+ 2NO + 4H
2
O
SCF
(11.5)
