Method for preparing a powder comprising particles of triuranium octoxide and particles of plutonium dioxide

Abstract

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.

Claims

1. A method for preparing a powder comprising particles of triuranium octoxide and particles of plutonium dioxide, comprising the steps of: a) preparing, by 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; b) mixing the aqueous suspension S.sub.1 with the aqueous suspension S.sub.2 to obtain an aqueous suspension S.sub.1+2 comprising particles of uranium(IV) oxalate and particles of plutonium(IV) oxalate; c) separating the aqueous suspension S.sub.1+2 into an aqueous phase and a solid phase, the solid phase comprising the particles of uranium(IV) oxalate and the particles of plutonium(IV) oxalate; and d) 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 the particles of plutonium(IV) dioxide, whereby the powder is obtained; and wherein steps b) and c) are performed simultaneously or successively.

2. The method of claim 1, wherein step a) comprises: placing an aqueous solution A.sub.1 comprising nitric acid and uranium(IV) nitrate in contact with an aqueous solution A.sub.2 comprising a first precipitating agent to form a first reaction medium in which uranium(IV) is precipitated in a form of uranium(IV) oxalate, the first precipitating agent being oxalic acid, a salt or an alkylated derivative thereof; and placing an aqueous solution A′.sub.1 comprising nitric acid and plutonium(IV) nitrate in contact with an aqueous solution A′.sub.2 comprising a second precipitating agent to form a second reaction medium in which plutonium(IV) is precipitated in a form of plutonium(IV) oxalate, the second precipitating agent being oxalic acid, a salt or an alkylated derivative thereof.

3. The method of claim 2, wherein a concentration of nitric acid in the aqueous solutions A.sub.1 and A′.sub.1 is between 0.5 mol/L and 5 mol/L.

4. The method of claim 2, wherein a concentration of the uranium(IV) nitrate in the aqueous solution A.sub.1 and a concentration of plutonium(IV) nitrate in the aqueous solution A′.sub.1 are between 0.001 mol/L and 1 mol/L.

5. The method of claim 2, wherein a concentration of the first precipitating agent in the aqueous solutions A.sub.2 and a concentration of the second precipitating agent A′.sub.2 are between 0.05 mol/L and 1 mol/L.

6. The method of claim 2, wherein the first precipitating agent is present in the first reaction medium in excess with regard to stoichiometric conditions for an oxalic precipitation of uranium(IV) and the second precipitating agent is present in the second reaction medium in excess with regard to stoichiometric conditions for an oxalic precipitation of plutonium(IV).

7. The method of claim 2, wherein the aqueous solution A′1 further comprises uranium(VI) nitrate.

8. The method of claim 1, wherein step c) comprises a vacuum or pressure filtration of the aqueous suspension S.sub.1+2.

9. The method of claim 1, wherein steps b) and c) are performed simultaneously.

10. The method of claim 1, wherein step d) comprises a treatment of the solid phase at a temperature of at least 550° C. and in an oxidizing atmosphere.

11. The method of claim 1, wherein the powder further comprises particles of an actinide(IV) dioxide, the actinide(IV) dioxide being thorium dioxide or neptunium dioxide, and wherein the method further comprises the steps of: a′) preparing, by oxalic precipitations, the aqueous suspension S.sub.1 of particles of uranium(IV) oxalate, the aqueous suspension S.sub.2 of particles of plutonium(IV) oxalate and an aqueous suspension S.sub.3 of particles of actinide(IV) oxalate; b′) mixing the aqueous suspensions S.sub.1, S.sub.2 and S.sub.3 with each other to obtain an aqueous suspension S.sub.1+2+3 comprising the particles of uranium(IV) oxalate, the particles of plutonium(IV) oxalate and the particles of actinide(IV) oxalate; c′) separating the aqueous suspension S.sub.1+2+3 into an aqueous phase and a solid phase, the solid phase being formed by the particles of uranium(IV) oxalate, the particles of plutonium(IV) oxalate and the particles of actinide(IV) oxalate; and d′) calcining the solid phase to convert (1) the particles of uranium(IV) oxalate to the particles of triuranium octoxide, (2) the particles of plutonium(IV) oxalate to the particles of plutonium dioxide, and (3) the particles of actinide(IV) oxalate to the particles of actinide(IV) dioxide, whereby the powder is obtained; and wherein steps b′) and c′) are performed simultaneously or successively.

12. The method of claim 1, wherein the powder further comprises particles of an actinide(IV) dioxide, the actinide(IV) dioxide being thorium dioxide or neptunium dioxide, and the method comprises the steps of: a″) preparing, by oxalic precipitations, an aqueous suspension S.sub.1 of particles of uranium(IV) and actinide(IV) double oxalate, and the aqueous suspension S.sub.2 of particles of plutonium(IV) oxalate; b″) mixing the aqueous suspension S.sub.1 with the aqueous suspension S.sub.2 to obtain an aqueous suspension S.sub.1+2 comprising particles of uranium(IV) and actinide(IV) double oxalate and particles of plutonium(IV) oxalate; c″) separating the aqueous suspension S.sub.1+2 into an aqueous phase and a solid phase, the solid phase being formed by the particles of uranium(IV) and actinide(IV) double oxalate and the particles of plutonium(IV) oxalate; and d″) calcining the solid phase to convert (1) the particles of uranium(IV) and actinide(IV) double oxalate to the particles of triuranium octoxide and actinide(IV) dioxide, and (2) the particles of plutonium(IV) oxalate to the particles of plutonium dioxide, whereby the powder is obtained; and wherein steps b″) and c″) are performed simultaneously or successively.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a flow diagram of the assembly used to prepare an aqueous suspension S.sub.1 of uranium(IV) oxalate particles and an aqueous suspension S.sub.2 of plutonium(IV) oxalate particles in the example of embodiment of the method of the invention described below.

(2) FIG. 2 is a flow diagram of the assembly used to mix the aqueous suspension S.sub.1 of uranium(IV) oxalate particles with the aqueous suspension S.sub.2 of plutonium(IV) oxalate particles, and for the near-simultaneous filtration of suspension S.sub.1+2 obtained from this mixture in the example of embodiment of the method of the invention described below.

(3) FIG. 3 illustrates the laser particle size distribution of particles of uranium(IV) oxalate and plutonium(IV) oxalate in the aqueous suspension S.sub.1+2 obtained in the example of embodiment of the method of the invention described below; the diameter of the particles, denoted D and expressed in μm, is shown along the X-axis: the frequency in volume of the particles, denoted Fv and expressed in %, is given along the right Y-axis whilst the cumulative volume of the particles, denoted Vc and expressed in %, is given along the left Y-axis.

(4) FIGS. 4A and 4B illustrate X-ray diffractograms of the oxalate particles of the aqueous suspensions S.sub.1, S.sub.2 and S.sub.1+2 obtained in the example of embodiment of the method of the invention described below, after filtration and dewatering of these suspensions, FIG. 4B giving an enlargement of FIG. 4A at the peak located at 2θ=14° in this Figure; in each of FIGS. 4A and 4B, the diffractogram denoted 1 corresponds to the particles of uranium(IV) oxalate in the aqueous suspension S.sub.1, the diffractogram denoted 2 corresponds to the particles of plutonium(IV) oxalate in the aqueous suspension S.sub.2 whilst the diffractogram denoted 3 corresponds to the particles of uranium(IV) oxalate and plutonium(IV) oxalate in the aqueous suspension S.sub.1+2.

(5) FIG. 5 illustrates the changes in time, denoted t and expressed in minutes on a logarithmic scale, of the concentrations of uranium(IV) and plutonium(IV), denoted [C] and expressed in mg/L, of a sample of the aqueous suspension S.sub.1+2 obtained in the example of embodiment of the method of the invention described below that was left to age for 15 hours.

(6) FIG. 6 gives a photograph of the aqueous suspension S.sub.1+2 obtained in the example of embodiment of the method of the invention described below, taken under scanning electron microscope (SEM) after filtering and dewatering this suspension.

(7) FIG. 7 gives an X-ray diffractogram, denoted 1, of the powder of triuranium octoxide and plutonium dioxide obtained in the example of embodiment of the method of the invention described below; for comparison, this Figure also gives the computed diffractogram for particles of triuranium octoxide denoted 2, and the computed diffractogram for particles of plutonium dioxide denoted 3.

(8) FIG. 8 is a photograph of the powder of triuranium octoxide and plutonium dioxide obtained in the example of embodiment of the method of the invention described below, taken under SEM.

EXAMPLE OF EMBODIMENT OF THE METHOD OF THE INVENTION

(9) This example relates to the preparation of a powder composed of a mixture of U.sub.3O.sub.8 particles and PuO particles, from an aqueous solution A.sub.1 of uranium(IV) nitrate and an aqueous solution A′.sub.1 of plutonium(IV) nitrate and uranium(VI) nitrate.

(10) The aqueous solution A.sub.1 comprises 0.15 mol/L of uranium(IV) nitrate or uranous nitrate, of formula U(NO.sub.3).sub.4, 2.5 mol/L of nitric acid and 0.06 mol/L of hydrazinium ions N.sub.2H.sub.5.sup.+ (supplied in the form of hydrazinium nitrate N.sub.2H.sub.5NO.sub.3), whilst the aqueous solution A′.sub.1 comprises 0.15 mol/L of plutonium(IV) nitrate of formula Pu(NO.sub.3).sub.4, 0.038 mol/L of uranium(VI) nitrate or uranyl nitrate, of formula UO.sub.2(NO.sub.3).sub.2, and 2.5 mol/L of nitric acid.

(11) The concentration of uranium(IV) nitrate in the aqueous solution A.sub.1 and the concentration of plutonium(IV) nitrate in the aqueous solution A′.sub.1 are selected so that, having regard to the volumes of the aqueous solutions involved, the initial Pu(IV)/U(IV)+Pu(IV) molar ratio is 0.45.

(12) 1. Preparation of the Powder:

(13) In the invention, the preparation of the powder of U.sub.3O.sub.8 and PuO.sub.2 successively comprises: preparing, via oxalic precipitations, an aqueous suspension S.sub.1 of particles of uranium(IV) oxalate of formula U(C.sub.2O.sub.4).sub.2.6H.sub.2O, and an aqueous suspension S.sub.2 of particles of plutonium(IV) oxalate of formula Pu(C.sub.2O.sub.4).sub.2.6H.sub.2O; mixing the aqueous suspensions S.sub.1 and S.sub.2 to obtain an aqueous suspension S.sub.1+2 comprising both particles of uranium(IV) oxalate and particles of plutonium(IV) oxalate; separating suspension S.sub.1+2 thus obtained into an aqueous phase and a solid phase that is formed of the particles of uranium(IV) oxalate and the particles of plutonium(IV) oxalate; and calcining the solid phase thus obtained to convert, on the one hand, the particles of uranium(IV) oxalate to particles of U.sub.3O.sub.8 and, on the other hand, the particles of plutonium(IV) oxalate to particles of PuO.sub.2.

(14) Preparation of the Aqueous Suspensions S.sub.1 and S.sub.2:

(15) As illustrated in FIG. 1, the aqueous suspension S.sub.1 of particles of uranium(IV) oxalate is prepared in a reactor 10, which is equipped with an agitation system 11, 12 and an overflow 13, and which initially contains an aqueous solution 14 comprising 0.05 mol/L of oxalic acid, 0.039 mol/L of hydrazinium ions (also provided in the form of hydrazinium nitrate) and 2 mol/L of nitric acid.

(16) The reactor 10, via inlets 15 and 16 respectively, is charged with the aqueous solution A.sub.1, referenced 17 in FIG. 1, and with the aqueous solution A.sub.2, referenced 18 in FIG. 1, which comprises 0.7 mol/L of oxalic acid.

(17) The addition rates of the aqueous solutions A.sub.1 and A.sub.2 to the reactor 10 are regulated by means of pumps, 19 and 20 respectively, each equipped with a flowmeter, and are 21.7 mL/min for the aqueous solution A.sub.1 and 11.7 mL/min for the aqueous solution A.sub.2, leading to an excess of oxalic acid with regard to the stoichiometric conditions for the oxalic precipitation of uranium(IV).

(18) The adding of the aqueous solutions A.sub.1 and A.sub.2 to the reactor 10 leads to the formation of a reaction medium in which uranium(IV) is precipitated in the form of particles of uranium(IV) oxalate that are discharged via the overflow 13 into a receptacle positioned below the free end of this overflow. The aqueous suspension S.sub.1 thus formed is then evacuated from the receptacle via a line 22 equipped with a pump 23.

(19) To prepare the aqueous suspension S.sub.2 of particles of plutonium(IV) oxalate, an aqueous solution A′.sub.2 is used having a composition identical to that of the aqueous solution A.sub.2 previously used, and an assembly similar to the one illustrated in FIG. 1 with the exception that: first, the aqueous solution 14 initially contained in the reactor 10 is replaced by an aqueous solution comprising 0.05 mol/L of oxalic acid, 0.02 mol/L of uranium(VI) and 2 mol/L of nitric acid; and secondly, the aqueous solution A.sub.1 is replaced by the aqueous solution A′.sub.1.

(20) The flow rate conditions are the same as those previously described for the preparation of the aqueous suspension S.sub.1 of particles of uranium(IV) oxalate.

(21) Mixing of the Aqueous Suspensions S.sub.1 and S.sub.2 and Separation of the Suspension S.sub.1+2 into Two Phases:

(22) These steps are performed using the assembly illustrated in FIG. 2.

(23) As can be seen in this Figure, each of the aqueous suspensions S.sub.1 and S.sub.2 is conveyed by means of a line, respectively 25 and 26, in one of the branches of a Y-shaped connector 27, a third branch of which—wherein these aqueous suspensions are combined and intimately mixed together to form aqueous suspension S.sub.1+2—has its end portion positioned just above a filtration system allowing this suspension to be separated into an aqueous phase (or filtrate) and a solid phase (or cake).

(24) This filtration system is composed of a Büchner funnel 28, the bottom part of which is equipped with a filter (for example, a glass microfibre filter of Whatman™ GF/B filter type) on which the oxalate particles are retained, and a vacuum flask 29 which is placed underneath the funnel and in which the aqueous phase of suspension S.sub.1+2 is collected.

(25) The inlet flow rates of the aqueous suspensions S.sub.1 and S.sub.2 in the connector 27 are regulated by means of pumps, 30 and 31 respectively, each equipped with a flowmeter, these flow rates being 48.1 mL/min for the aqueous suspension S.sub.1 and 39.9 mL/min for the aqueous suspension S.sub.2.

(26) Filtering of the aqueous suspension S.sub.1+2 is performed without placing the flask 29 under a vacuum so that the oxalate particles are homogenously distributed over the filter. Once the maximum volume of the capacity of the Büchner funnel 28 is reached, the flask 29 is placed under a vacuum by means of a vacuum pump 32 to dewater the cake formed of oxalate particles.

(27) Calcination of the Solid Phase:

(28) The cake of oxalate particles previously obtained is calcined under flushing with air.

(29) For doing that, the cake of particles is placed in an oven that is heated until its temperature reaches 700° C., with a rise of 20° C./minute. This temperature is maintained for 1 hour. Heating is then stopped and the cake of particles is left in the oven until the oven temperature returns to ambient temperature. The flow rate of the flushing gas is such that the volume of the oven is renewed 10 times with this gas throughout the calcination time.

(30) At the end of this calcination, a powder composed of a mixture of U.sub.3O.sub.8 particles and PuO.sub.2 particles is obtained.

(31) 2. Analyses:

(32) Analyses of the Filtrate:

(33) The filtrate obtained at the end of the filtering step was analysed to determine the metal cation composition thereof. Analyses showed that this filtrate comprises from 1 mg/L to 10 mg/L of uranium(IV), from 20 mg/L to 25 mg/L of plutonium(IV) and from 3 g to 4 g/L of uranium(VI).

(34) This confirms that the chemical conditions of precipitation applied above to prepare the aqueous suspensions S.sub.1 and S.sub.2 allow near-quantitative precipitation of uranium(IV) and plutonium(IV). It is therefore possible, by controlling the flow rates at the mixing step of these suspensions, to find a Pu(IV)/U(IV)+Pu(IV) molar ratio in the cake of oxalate particles that is similar to the initial Pu(IV)/U(IV)+Pu(IV) molar ratio.

(35) Also, the initial concentration of uranium(VI) in the aqueous solution A′.sub.1 leads to finding the entirety of this uranium in the filtrate.

(36) Analyses of the Oxalate Particles:

(37) The aqueous suspensions S.sub.1 and S.sub.2 and the aqueous suspension S.sub.1+2 were subjected to a laser particle size analysis (particle size analyser from MALVERN Instruments).

(38) The Table below gives the values of the volume mean diameter, denoted D[4,3] and expressed in μm, obtained for the particles of oxalate(s) of these suspensions.

(39) TABLE-US-00001 TABLE D[4,3] Particles (μm) S.sub.1: uranium(IV) oxalate 40-50 S.sub.2: plutonium(IV) oxalate 40-50 S.sub.1+2: uranium(IV) oxalate + plutonium(IV) 40-55 oxalate

(40) These particle size values, that are close to one another and centred around 45 μm, are comparable with those of particles of plutonium(IV) oxalate conventionally obtained in industrial units for the conversion of plutonium(IV) to oxalate.

(41) Also, FIG. 3 gives the particle size distribution obtained, also by laser particle size analysis, for the aqueous suspension S.sub.1+2.

(42) The aqueous suspensions S.sub.1 and S.sub.2 and the aqueous suspension S.sub.1+2 were also subjected, but after filtering and dewatering, to X-ray diffraction analyses (BRUKER AXS diffractometer of θ-2θ configuration, equipped with a copper anti-cathode having a Kα radiation at a wavelength λ of 1.5418 Å, and with a linear type BRUKER AXS detector).

(43) The X-ray diffractograms so obtained are illustrated in FIG. 4A and in FIG. 4B which corresponds to an enlargement of FIG. 4A at the peak positioned at 2θ=14° in FIG. 4A.

(44) As shown in these Figures in which diffractogram 1 corresponds to the aqueous suspension S.sub.1, diffractogram 2 corresponds to the aqueous suspension S.sub.2 whilst diffractogram 3 corresponds to the aqueous suspension S.sub.1+2, the two oxalate phases of the aqueous suspension S.sub.1+2 crystallize in one same structure of monoclinic type An(C.sub.2O.sub.4).sub.2.6H.sub.2O.

(45) This crystallization form has the advantage of only retaining a weight fraction of water of the order of 15% in the filter cake, thereby imparting a scarcely tacky nature to the mixture of uranium(IV) oxalate particles and plutonium(IV) oxalate particles forming this cake.

(46) The absence is noted in diffractograms 2 and 3 of any peaks which could correspond to uranium(VI) oxalate.

(47) As indicated in the foregoing, uranium(IV) is a powerful reductant of plutonium(IV). By preparing the aqueous suspensions of uranium(IV) oxalate particles and plutonium(IV) oxalate particles separately, it is possible to annihilate the redox effect in aqueous phase when these particles of oxalates are later mixed with each other. This is demonstrated in FIG. 5 which shows that the measurement of the concentrations of uranium(IV) and plutonium(IV) in a sample of the suspension S.sub.1+2, that was left to age for 15 hours, does not allow the detection of any phenomenon that would place in doubt the chemical stability of these particles.

(48) In addition, as shown in FIG. 6, which corresponds to a photograph of the filter cake of the aqueous suspension S.sub.1+2 taken under SEM in secondary electron mode (ZEISS field effect electron microscope associated with an EDS detector and WDS detector), the distribution of the uranium(IV) oxalate particles and plutonium(IV) oxalate particles in this cake is homogeneous.

(49) Analysis of the Oxide Particles:

(50) The powder obtained at the end of the calcination step was subjected to analyses to evaluate its BET specific surface area, its particle size distribution (by laser particle size analysis), its composition (by X-ray diffraction) and its homogeneity (by SEM).

(51) The laser particle size, X-ray diffraction and SEM analyses were conducted using the same equipments as indicated previously.

(52) These analyses showed that the powder: has a specific surface area of about 3 m.sup.2/g; has a volume mean diameter D[4,3] of about 15 μm; is exclusively composed of U.sub.3O.sub.8 and PuO.sub.2, as illustrated in FIG. 7 which shows both the X-ray diffractogram, denoted 1, of said powder and the computed diffractograms for particles of triuranium octoxide and particles of plutonium dioxide, respectively denoted 2 and 3; and exhibits a homogeneity between the U.sub.3O.sub.8 and PuO.sub.2 phases conforming to that obtained before calcinations for the phases of uranium(IV) oxalate and plutonium(IV) oxalate in the filter cake of the aqueous suspension S.sub.1+2, as illustrated by the SEM photograph in FIG. 8.

CITED REFERENCES

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