Environmental Engineering Reference
In-Depth Information
grouts. It can treat a wide range of radioactive waste streams at room temperature.
Studies have demonstrated that Ceramicrete waste forms were comparable to vitrified
waste forms in performance and can incorporate high loading of a wide variety of
waste streams. This section provides an overview of the Ceramicrete technology,
briefly discusses the solution chemistry behind chemical immobilization of radio-
active waste streams, and explores the mechanism of physical encapsulation that
isolates the contaminants from the environment.
6.2.2
T
HE
C
ERAMICRETE
P
ROCESS
AND
I
TS
C
HEMISTRY
The Ceramicrete process is based on an acid-base reaction between calcined mag-
nesium oxide (MgO) and a solution of potassium dihydrogen phosphate (KH
2
PO
4
).
Details of the calcination procedure of MgO have been published.
2
The dissolution
reactions of KH
2
PO
4
and MgO are given by the following equations:
3
KH
2
PO
4
= 2H
+
+ KPO
4
2-
(1)
MgO + 2H
+
= Mg
2+
(aq)+ H
2
O
(2)
The reaction product is magnesium potassium phosphate (MgKPO
4
.6H
2
O) that
is formed by dissolution of MgO in the solution of KH
2
PO
4
and eventual reaction
in the solution given by:
Mg
2+
(aq)+ KPO
4
2-
+ 6H
2
O = MgKPO
4
.6H
2
O
(3)
MgKPO
4
.6H
2
O (referred to as MKP hereafter) acts as a binder that can be used
as the matrix material to host any inorganic and to some extent even some organic
waste material. This is an exothermic reaction and releases a large amount of heat.
6.2.3
M
ECHANISMS
OF
I
MMOBILIZATION
Radioactive waste streams are mixed with the binder powders and water, and the
reaction is allowed to occur by mixing the components for 30 min in a concrete
mixer. The resulting slurry forms a smooth paste that can be poured into the storage
containers and allowed to set. Once the mixing is done, exothermic reaction between
the components starts heating the slurry. With this reaction the slurry thickens, and
when the temperature reaches 55°C the entire slurry sets into a hard mass. Even in
the hardened monolith, the exothermic reaction proceeds to produce additional heat.
Typically a maximum temperature of 60°C in 2-l-size samples prescribed for storage
of transuranic (TRU) waste streams, and 82°C in 55-gal-size monoliths used for
waste streams where there are no concerns of criticality, have been noted, as shown
in Figure 6.2.1. The samples attain sufficient structural integrity to be transported
in approximately 2 hours, but actual curing continues for several weeks. Waste
loadings of 40 to 80 wt% are typical, depending on the characteristics of a given
waste stream. Liquids and sludge are easily incorporated because the process
involves aqueous mixing.
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