Method of manufacturing compounds of the monazite type, doped or

Details Method of manufacturing compounds of the monazite type, doped or Method of manufacturing compounds of the monazite type, doped or

1998-05-20

2000-02-08

Straub, Gary P.

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, 423304, 423325, G21F 900, C01B 2512

Patent

active

060230061

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method of manufacturing compounds of the Monazite type, doped or not doped with actinides as well as a method of packaging radioactive waste high in actinides and in lanthanides using these Monazite type compounds as a confinement matrix. Finally, this invention relates to a radioactive waste packaging block that includes a Monazite matrix containing radioactive elements.
Monazite is a natural phosphate mineral, containing a high level of light rare earth elements as well as a few per-cent of uranium and thorium. The general chemical formula of this mineral is LnPO.sub.4 where Ln represents the entire series of light rare earth elements from lanthanum to terbium. This mineral is commonly exploited as a source of rare earths.
This material when manufactured synthetically can have interesting applications, in particular, in the field of refractory materials and for the packaging of nuclear waste.
The known, currently used synthetic routes involve reacting one or more rare earth salts with a phosphate compound.
Hence, the document "MONAZITE" by L. A. BOATNER, J. C. SALES, Radioactive Waste Forms for the Future, C. EWING p. 495-564--ISBN 0-444-87104-7 (1988) describes a synthesis of Monazite which involves essentially cerium (III) nitrate hexahydrate of formula Ce(NO.sub.3).sub.3 .multidot.6H.sub.2 O and ammonium hydrogenphosphate or diammonium phosphate (NH.sub.4).sub.2 HPO.sub.4. More precisely, in this document, the preparation of lanthanide orthophosphate ceramics by precipitation with urea is described. This precipitation method can be applied to any lanthanide present naturally, but one can also prepare pure and doped YPO.sub.4 and ScPO.sub.4.
The mixture of lanthanide oxides and actinide oxides, which can be constituted notably by waste arising from the nuclear industry, is first of all dissolved, with heating, in an aqueous solution of nitric acid (1). Then cerium nitrate Ce(NO.sub.3).sub.3 .multidot.6H.sub.2 O (2) and water are added. Next, ammonium hydrogen phosphate (NH.sub.4).sub.2 HPO.sub.4 (3) is dissolved in this solution and the mixture (4) is homogenised. A metathesis reaction forms the orthophosphate of the mixed lanthanide waste, the latter being essentially made up of actinide compounds. Then, from this mixture, a precipitation (6) is carried out initiated by the addition of urea NH.sub.2 --CO--NH.sub.2 (5) in particulate form, and by heating the mixture to about 180.degree. C., until precipitation (6) occurs.
Water vapour, nitrogen, ammonia, hydrogen and carbon dioxide are released after heating to 400.degree. C.
The final stage of production of powders by this technique includes the transfer of the material into a ceramic crucible, for example, an alumina crucible, and the complete calcination (7) of the powder at 800.degree. C.
The powders thus obtained are then densified by sintering under a load (8).
The apparent or bulk density of the powders obtained is strongly dependent on the quantity of urea added during the controlled precipitation step.
This document also brings to the fore the influence of the molar ratio of urea/CePO.sub.4 on the bulk density.
In addition to this document, there are numerous publications which have dealt with the synthesis of compounds of the Monazite type: these syntheses are differentiated essentially by the starting materials utilised: in effect, a large number of rare earth salts such as carbonates, nitrates, oxalates, hydroxides, and phosphate compounds such as orthophosphoric acid, ammonium hydrogenphosphates and alkaline phosphates can be used.
The synthesis can also be carried out by a liquid route, that is to say by precipitation and reaction, or yet again by a hydrothermal synthesis, namely a synthesis under pressure and at a high temperature in a liquid medium.
All of these synthesis routes use a molar ratio of lanthanide/phosphorus close to 1, so as to obtain a final compound of the Monazite type of formula LnPO.sub.4 where Ln represents an element of the lanthanide series from lanthanum to terbium. In effect, beyond th

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