Method for preparing a mixture of powdered metal oxides from nit

Details Method for preparing a mixture of powdered metal oxides from nit Method for preparing a mixture of powdered metal oxides from nit

1999-03-02

2000-08-29

Mai, Ngoclan

Chemistry of inorganic compounds

Radioactive

Uranium compound

75369, 75344, 423 3, 423261, 423251, 423252, C01G 4301

Patent

active

06110437&

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The invention relates to a process for obtaining a mixture of pulverulent metal oxides, of appropriate reactivity, belonging to the nuclear industry, from their nitrates which are in the form of an aqueous solution or of a mixture of solids which are naturally pulverulent or made pulverulent.
The invention relates more particularly to a process for the production of a mixture of pulverulent metal oxides from their nitrates, using at least two metal elements which can be involved in the formation of a nuclear fuel, such as uranium, plutonium, thorium (U, Pu, Th) and other elements which are fissile, fertile, or permitting simulations, such as cerium (Ce).
The invention also relates more particularly to a process for the production of a mixture of pulverulent metal oxides from their nitrates, using at least one of the metal elements such as uranium, taken in combination with at least one of the rare-earth elements such as gadolinium (Gd), in order to control the fission of fuels based on mixed sintered oxides.
As a base material to be treated according to the process of the invention, the elements involved in the form of nitrates and leading by their decomposition to mixtures of pulverulent metal oxides can have various origins: they may or may not have undergone irradiation and, consequently, have any isotopic composition. In the particular case of uranium, this element may be of natural origin, or may have been enriched beforehand, for example by gaseous diffusion, by ultracentrifuging or by laser way, may have been irradiated, and, in this case, issue from the reprocessing of irradiated fuels after cooling and separation of plutonium (Pu) and of fission products.
For the purposes of the present invention: such as the uranium oxide (UO.sub.3), plutonium oxide (PuO.sub.2), thorium oxide (ThO.sub.2), caesium oxide (Cs.sub.2 O), ruthenium oxide (RuO.sub.2) and rare earths oxides such as, for example, gadolinium oxide (Gd.sub.2 O.sub.3) and cerium oxide (CeO.sub.2), of which the favoured pairs are U-Pu, U-Th, U-Cs, U-Ru, U-Gd and U-Ce. define the most suitable specific surface with which it is desirable to provide for the metal oxides to optimize their suitability for conversions subsequent to the denitration, such as, for example: before its subsequent reduction); using hydrofluoric acid; be converted into nuclear fuels by compaction and sintering of the compacted pellets. nitrate, mixed with at least one of the nitrates of plutonium, thorium, caesium, ruthenium and of rare earths such as, for example, gadolinium and cerium.
In the case of the direct reduction of UO.sub.3 to UO.sub.2, followed by hydrofluorination of UO.sub.2 to UF.sub.4, the reactivity of UO.sub.2 is correlatively related to the specific surface of UO.sub.3 resulting from the denitration. It is found to be desirable that the suitabilities for the direct reduction of UO.sub.3 to UO.sub.2 and for the hydrofluorination of UO.sub.2 to UF.sub.4 should be regulated on demand during the denitration of uranyl nitrate by the production of a uranium oxide UO.sub.3 having a specific surface adapted to each technology.
In the case of the production of metal oxide powders of ceramic quality, which are intended for the production of nuclear fuels, [such as uranium dioxide (UO.sub.2) or its mixture with at least one of the oxides of plutonium (PuO.sub.2), thorium (ThO.sub.2), caesium (Cs.sub.2 O), ruthenium (RuO.sub.2), gadolinium (Gd.sub.2 O.sub.3) and cerium (CeO.sub.2)] it is well known that the sintering of such powders makes it possible to obtain high-density pellets, on condition that powders of high specific surface are selected and that an optimum sintering protocol is followed for each value of specific surface (treatise on inorganic chemistry by Paul Pascal, volume XV, pages 300 to 304, 1961 edition).


STATES OF THE ART

Numerous processes for the production of mixtures of metal oxides are known, by precipitation or by decomposition of their nitrates: they are generally intended for the preparation of nuclear fuels contai

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patent: 4364859 (1982-12-01), Ohtsuka et al.
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EP 0143726--English Abstract.
EP 0328742--English Abstract.
DE 3802047--English Abstract.
WO 8908610--English Abstract.
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