A method for preparing a powder comprising an intimate mixture of U.sub.3O.sub.8 particles and PuO.sub.2 particles and which may further comprise particles of ThO.sub.2 or NpO.sub.2. The method comprises: preparing, via oxalic precipitations, an aqueous suspension S.sub.1 of particles of uranium(IV) oxalate and an aqueous suspension S.sub.2 of particles of plutonium(IV) oxalate; mixing the aqueous suspension S.sub.1 with the aqueous suspension S.sub.2 to obtain an aqueous suspension S.sub.1+2; separating the aqueous suspension S.sub.1+2 into an aqueous phase and a solid phase comprising the particles of uranium(IV) oxalate and the particles of plutonium(IV) oxalate; and calcining the solid phase to convert (1) the particles of uranium(IV) oxalate to particles of triuranium octoxide and (2) the particles of plutonium(IV) oxalate to particles of plutonium(IV) dioxide, whereby the powder is obtained.

The present invention relates to a process for sintering a compacted powder of at least one oxide of a metal selected from an actinide and a lanthanide, this process comprising the following successive steps, carried out in a furnace and under an atmosphere comprising an inert gas, dihydrogen and water: (a) a temperature increase from an initial temperature T.sub.I up to a hold temperature T.sub.P, (b) maintaining the temperature at the hold temperature T.sub.P, and (c) a temperature decrease from the hold temperature T.sub.P down to a final temperature T.sub.F, in which the P(H.sub.2)/P(H.sub.2O) ratio is such that: 500<P(H.sub.2)/P(H.sub.2O)≦50 000, during step (a), from T.sub.I until a first intermediate temperature T.sub.i1 between 1000° C. and T.sub.P is reached, and P(H.sub.2)/P(H.sub.2O)≦500, at least during step (c), from a second intermediate temperature T.sub.i2 between T.sub.P and 1000° C., until T.sub.F is reached.

A method for preparing a powder comprising an intimate mixture of U.sub.3O.sub.8 particles and PuO.sub.2 particles and which may further comprise particles of ThO.sub.2 or NpO.sub.2. The method comprises: preparing, via oxalic precipitations, an aqueous suspension S.sub.1 of particles of uranium(IV) oxalate and an aqueous suspension S.sub.2 of particles of plutonium(IV) oxalate; mixing the aqueous suspension S.sub.1 with the aqueous suspension S.sub.2 to obtain an aqueous suspension S.sub.1+2, separating the aqueous suspension S.sub.1+2 into an aqueous phase and a solid phase comprising the particles of uranium(IV) oxalate and the particles of plutonium(IV) oxalate; and calcining the solid phase to convert (1) the particles of uranium(IV) oxalate to particles of triuranium octoxide and (2) the particles of plutonium(IV) oxalate to particles of plutonium(IV) dioxide, whereby the powder is obtained.

An object of the present invention is to provide a method according to which a rare earth element can be efficiently recovered from a workpiece containing at least a rare earth element and an iron group element, and also wear and damage to the treatment container can be suppressed, allowing the container to be used repeatedly for a long period of time. The method of the present invention as a means for resolution is characterized in that in the heat treatment of an oxidation-treated workpiece in the presence of carbon, when the oxidation-treated workpiece is placed in a treatment container, a carbon substance is interposed between the oxidation-treated workpiece and the bottom surface of the container, and the heat treatment is performed in an inert gas atmosphere or in vacuum at a temperature of 1300 C. or more.

The present invention relates to a process for sintering a compacted powder of at least one oxide of a metal selected from an actinide and a lanthanide, this process comprising the following successive steps, carried out in a furnace and under an atmosphere comprising an inert gas, dihydrogen and water: (a) a temperature increase from an initial temperature T.sub.I up to a hold temperature T.sub.P, (b) maintaining the temperature at the hold temperature T.sub.P, and (c) a temperature decrease from the hold temperature T.sub.P down to a final temperature T.sub.F, in which the P(H.sub.2)/P(H.sub.2O) ratio is such that: 500<P(H.sub.2)/P(H.sub.2O)50 000, during step (a), from T.sub.I until a first intermediate temperature T.sub.i1 between 1000 C. and T.sub.P is reached, and P(H.sub.2)/P(H.sub.2O)500, at least during step (c), from a second intermediate temperature T.sub.i2 between T.sub.P and 1000 C., until T.sub.F is reached.