Hazardous or toxic waste destruction or containment – Destruction or containment of radioactive waste – By fixation in stable solid media
Reexamination Certificate
2000-06-23
2002-10-01
Langel, Wayne A. (Department: 1754)
Hazardous or toxic waste destruction or containment
Destruction or containment of radioactive waste
By fixation in stable solid media
C588S252000
Reexamination Certificate
active
06459010
ABSTRACT:
FIELD OF THE INVENTION
This invention involves a process for containment of industrial wastes, particularly nuclear wastes, in apatite ceramics.
Apatite ceramics are valuable materials for use as matrices for the containment of industrial wastes, and particularly nuclear wastes and especially those with long half-lives such as fission products or certain actinides.
In irradiated nuclear fuel treatment plants at the end of the processing there are several actinide elements with long half-lives and some lanthanides which must be contained for long-term storage in highly resistant matrices.
The materials used for the matrices must have very high chemical stability, stability to radiation, and stability to temperature, to isolate the radioactive elements from the environment and keep them in this isolated state for very long periods, due to their long period of radioactivity.
STATE OF THE PRIOR ART
The matrix currently used for containment is glass, but recent studies have indicated that apatite ceramics have properties which are particularly suitable for long-term storage and could be used instead of glass as confinement matrices.
The apatites are compounds with the general formula:
Me
10
(XO
4
)
6
Y
2
(I)
in which Me is one or several metals, X represents P, V and/or Si, and Y represents one or several anions such as OH, Cl and F. Among these apatites, phosphocalcic hydroxy apatite:
Ca
10
(PO
4
)
6
(OH)
2
(II)
is the best known compound.
The apatites of formula (I) can have various substitutions, for the cationic sites (Me) as well as for the anionic sites (XO
4
and/or Y
2
).
The charge equilibrium, needed because of the introduction of elements which may be monovalent, divalent, trivalent or tetravalent, is established by various associated substitutions.
For example, divalent calcium can be replaced by a rare earth which is a trivalent element. This replacement can occur in several ways:
by the coupled exchange (Ca
2+
, OH
−
)⇄(Ln
3+
, O
2−
)
by the coupled exchange (Ca
2+
, PO
4
3−
)⇄(Ln
3+
, SiO
4
4−)
by the coupled exchange (2Ca
2+
)⇄(Ln
3+
, Na
+
)
This is just an example of the multiple substitutions which are possible.
As described by BROS R., CARPENA J., SERE V., BELTRITTI A, Radiochimica Acta, 74, 1996, pages 277-282 [1], the study of the OKLO natural reactor showed that apatites containing radioelements in their structures (actinides and/or fission products) are particularly stable, thermally and chemically, even in highly irradiating media.
These apatites are resistant in radioactive waste storage conditions to more than 1,000° C. They are chemically resistant in hydrogeological storage conditions, i.e. with a water pH which is neutral or basic. They can also withstand highly irradiating media because the radiation damage which they suffer is unstable at temperatures greater than 60° C. Phosphocalcic apatite, for example, can restructure itself as of 60° C.
The advantage of apatites for geological storage of industrial wastes, and particularly nuclear wastes with low, moderate or high activity, is evident, because they allow for strong chemical bonding in a matrix which is particularly stable with a continuity in geological media which has been demonstrated by studies of materials several million years old.
Massive polycrystalline shapes of these apatites are needed for containment of industrial wastes, and particularly radioactive wastes.
Until now, the preparation of massive apatite pieces containing wastes was done from powdery apatites subjected to sintering at high temperatures, i.e. greater than 1,000° C., possibly under high pressure.
The document FR-A-2 712 726 [2] describes a process for containment of actinides and/or lanthanides in apatite, involving the preparation of a mixture of powders including at least one phosphate chosen from among calcium, lanthanide and actinide phosphate, calcium fluoride, calcium carbonate, a silicon compound and possibly one or several lanthanide or actinide oxides, thermally treating the mixture to break down the calcium carbonate and calcinating the thermally-treated mixture at a high temperature (900 to 1,500° C.), possibly repeating the last calcination step several times after one or several intermediate grinding steps.
The document Inorganic Materials, volume 9, no. 4, 1973, pages 652-654 [3] also describes a process for manufacturing fluoroapatite silicates containing lanthanides by thermal treatment at a high temperature (1,200 to 1,350° C.).
Other processes for manufacturing of an apatite ceramic-based waste containment matrix involve the prior preparation of apatite power, its grading and sintering according to various processes such as natural sintering, pressure-assisted sintering, and sintering after using slip.
These techniques yield mass pieces with good mechanical properties, but they require high-temperature thermal treatments involving:
high costs of energy for preparation of the matrix
partial transformation of hydroxyapatite to oxyapatite, and
difficulties in enclosing species which are volatile at the temperature of the thermal treatment in the piece of apatite.
BRIEF DESCRIPTION OF THE INVENTION
This invention precisely involves a process for containment of industrial wastes in apatite ceramics which produces pieces with good mechanical properties but without the need for thermal treatment at high temperatures.
According to the invention, the process for containment of industrial waste in a apatite ceramic matrix involves the following steps:
a) preparing a homogeneous mixture of powders, comprising:
i) at least two calcium phosphates selected from the group consisting of Ca(H
2
PO
4
)
2
, Ca(H
2
PO
4
)
2
.H
2
O, Ca(HPO
4
), Ca(HPO
4
).2H
2
O, amorphous &agr;-Ca
3
(PO
4
)
2
, apatitic &agr;-Ca
3
(PO
4
)
2
, amorphous &bgr;-Ca
3
(PO
4
)
2
, apatitic &bgr;-Ca
3
(PO
4
)
2
, Ca
4
(PO
4
)
2
O; and optionally
ii) at least one compound selected from the group consisting of alkaline metal salts, alkaline metal phosphates, alkaline metal silicates, alkaline metal carbonates, alkaline metal halides, alkaline metal oxides, alkaline metal hydroxides, alkaline-earth metal salts, alkaline-earth metal phosphates, alkaline-earth metal silicates, alkaline-earth metal carbonates, alkaline-earth metal halides, alkaline-earth metal oxides, alkaline-earth metal hydroxides, and oxides of silicon,
wherein
the mixture is able to form a stoichiometric hydroxyapatite of formula (II)
Ca
10
(PO
4
)
6
(OH)
2
(II)
wherein
calcium is partly replaced by at least one member selected from the group consisting of alkaline metals and alkaline-earth metals;
phosphate anions are partly replaced by silicate anions; and
hydroxide anions are party replaced by halide anions;
b) putting the industrial wastes into the mixture;
c) compacting the mixture of powders containing the aforesaid waste at room temperature, under a pressure of 100 to 500 MPa, to yield a compacted piece; and
d) subjecting the compacted piece to hydrothermal treatment in a sealed chamber containing an aqueous medium, at a temperature of 100 to 500° C., for a period of at least 8 hours.
According to a first embodiment of the invention, particularly intended for containment of industrial wastes including at least one element chosen from the metals and halogens, steps a) and b) are done simultaneously by mixing the waste, during preparation of the mixture of powders, in the form of powders of oxides, hydroxides or salts of the metal(s) and/or alkaline or alkaline earth metal halide powder(s) so as to obtain a mixture corresponding to a hydroxyapatite as defined above, substituted by the metal(s) and/or halogens to be contained.
The metals could in particular be radioactive metals such as radioactive cesium, for example Cesium-135 and Cesium-137, Strontium-90, Technetium-99, the lanthanides, particularly Samarium-151, and the actinides. The halogens could be Chlorine-36 in particular.
According to a second embodiment of the invention mor
Carpena Joëlle
Dechambre Gérard
Donazzon Benoît
Lacout Jean-Louis
Commissariat a l'Energie Atomique
Langel Wayne A.
Nave Eileen E.
LandOfFree
Method for packaging industrial, in particular radioactive,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for packaging industrial, in particular radioactive,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for packaging industrial, in particular radioactive,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2934685