Environmental Engineering Reference
In-Depth Information
Four studies have been reported on the stabilization of cesium using Ceramicrete
technology. Cesium was added either as CsNO
3
or CsCl in the first three tests, while
the nature of cesium was not known for the fourth case but appeared to be a soluble
form detected in the wastewater. The TCLP results indicate that cesium, though
added as a nitrate or other soluble form, is immobilized. The leachability index is
not as high as Bamba et al. observed for a glass waste form,
11
but is higher than was
reported for a cement waste form.
12
Spent crystalline silicotitanate (CST) resin encapsulated in a Ceramicrete matrix
immobilized cesium well, but not as well as glass waste forms according to the PCT
and MCC-1 tests.
13
These Ceramicrete waste forms for loaded CST may not have
performed as well as glass because (1) PCT and MCC-1 tests are designed to test
glass, and (2) not adding fly ash resulted in a porous Ceramicrete matrix (23 vol%).
A Ceramicrete matrix that incorporates any form of ash, particularly fly ash, performs
better.
5,14
The amorphous silica in fly ash forms silico-phosphate glass in the matrix,
providing better strength and lower porosity.
6.2.4.3
Stabilization of Radioactive Elements
The oxides of actinides, even at high concentrations, are effectively encapsulated in
the Ceramicrete matrix.
6
Often actinide oxides are found in a lower oxidation state
such as oxides in the trivalent state (for example, U
2
O
3
or Pu
2
O
3
). Such oxides have
a higher solubility than their counterparts in a higher oxidized state. Once dissolved,
they are likely to be oxidized in the aqueous phosphate slurry during formation of
the stabilized matrix into insoluble oxidized compounds. X-ray diffraction patterns
proved Ce was present in the less-soluble oxidation state (CeO
2
), not the more-
soluble lower oxidation state (Ce
2
O
3
).
6
Radium-rich wastes from the Fernald Silos were stabilized at bench scale in a
Ceramicrete matrix with a loading of 66 wt%.
15
The total specific activity of the
waste was 3.85 μCi/g, with radium (
226
Ra) at 0.477 μCi/g. Waste form samples were
extracted per the TCLP test and the extract analyzed for specific activity. These data
were used to estimate the level of encapsulation in the matrix. The alpha and beta
activity of the leachate was 25 ± 2.5 and 9.81 ± 0.98 pCi/ml, respectively, compared
to a total activity of 127 pCi/ml and radium activity of 16 pCi/ml that would be
expected if all the activity leached during TCLP testing. Thus, most of the activity
was not extracted, but a significant amount did leach from the Ceramicrete waste
forms.
6.2.4.4
Salt Stabilization
The Ceramicrete slurry sets into a hard ceramic even in the presence of salts such
as nitrates and chlorides. Studies produced monolithic Ceramicrete solids from
concentrated sodium nitrate and sodium chloride solutions.
5,16
6.2.5
E
VALUATION
OF
R
ADIOLYTIC
G
AS
G
ENERATION
The Ceramicrete matrix material, MKP, contains 6 moles of water for every mole
of the magnesium potassium phosphate. Radiolytic decomposition of this water and
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