Hazardous or toxic waste destruction or containment – Destruction or containment of radioactive waste – By fixation in stable solid media
Patent
1995-11-16
1998-06-23
Mai, Ngoclan
Hazardous or toxic waste destruction or containment
Destruction or containment of radioactive waste
By fixation in stable solid media
588 14, 588 15, 588 16, 252625, 976DIG385, 423304, 423325, G21F 900
Patent
active
057714720
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the conditioning of radioactive waste and in particular long life radioactive waste such as actinides.
In irradiated nuclear fuel reprocessing installations, at the end of processing, a few long life actinide elements and certain lanthanides are left at the end of the treatment and which must be conditioned with a view to long-term storage in highly resistant matrixes.
The materials usable as a matrix must have chemical stability, radiation stability and very high temperature stability characteristics in order to isolate the radioactive elements from the environment and keep them in this isolated state for very long periods, due to their radioactive period.
At present, the matrix used for this conditioning iS glass, but new materials are continuously being sought which have even better characteristics for such conditioning purposes.
The present invention relates to a process for the conditioning of radioactive waste using as the confinement matrix a material having properties particularly appropriate for long-term storage.
According to the invention, the radioactive waste conditioning process consists of incorporating said waste into a confinement matrix based on phosphosilicated apatite.
The use of phosphosilicated apatite is very interesting, because apatites have the following remarkable properties.
These structures are very chemically and thermally stable in the natural medium. There are also natural silicated apatites containing actinides for hundreds of millions of years.
Apatites have a very limited solubility in water. In addition, their solubility decreases when the temperature increases, which is a positive point for the conditioning of the actinides, because their high activity involves an increase in the temperature of the matrix which will contain same.
Apatite structures are able to resist radioactivity, because the radiation damage which they suffer is unstable at a temperature exceeding 60.degree. C. Thus, apatite has the property of being restructured at temperatures exceeding 60.degree. C.
Apatite structures have the very interesting property of being able to integrate into their structure numerous metallic elements and in particular actinides and lanthanides.
Thus, natural apatites can be represented in the form of the following general formula: structure, numerous substitutions can be made and in particular the Ca.sub.2.sup.+ cations can be replaced by various metallic elements such as lanthanides, actinides and alkali metals, the PO.sub.4.sup.3- ions can be substituted by other anions such as SiO.sub.4.sup.4- and the F.sup.- anions can be substituted by the anions S.sup.2-, Cl.sup.-, Br.sup.-, I.sup.- or OH.sup.-.
According to the invention, the phosphosilicated apatite used complying with the formula:
M is at least one alkali metal cation,
Ln represents at least one cation chosen from among yttrium and lanthanides,
A represents at least one cation chosen from among actinides,
X represents S.sup.2-, 2F.sup.-, 2Cl.sup.-, 2OH.sup.-, 2Br.sup.- or 2I.sup.- and
t, u, x, y and z are such that Ca, Ln and A is equal to (26-u).
In this formula, the total number of negative charges is supplied by the anions PO.sub.4.sup.3-, SiO.sub.4.sup.4- and X.sup.2-. These charges are balanced by added cations among which the lanthanides and actinides can be in different states, e.g. divalent, trivalent or tetravalent states in the case of lanthanides and trivalent or tetravalent states in the case of actinides.
In order that said charge balance is respected, the quantities t, x, y and z are chosen as a function of the quantity u of anion PO.sub.4.sup.3- and the valency state of the lanthanides and/or actinides present in order to obtain neutrality. Therefore these quantities may or mav not be integers.
The alkali metals used in this apatite can e.g. be Na.sup.+, K.sup.+ and Cs.sup.+. The lanthanides can be of a random type and the actinides can e.g. be Np, Pu, Am, Cm, etc.
In the phosphosilicated apatites used in the invention, the choice of the PO.sub.4 :SiO.sub.4 ratio is
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Carpena Joelle
Lacout Jean-Louis
Commissariat a l''Energie Atomique
Mai Ngoclan
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