PROCESS FOR MANUFACTURING A PELLET OF AT LEAST ONE METAL OXIDE

Abstract

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.

Claims

1. A process for sintering a compacted powder of at least one oxide of a metal M.sub.1 selected from an actinide and a lanthanide, this process comprising the following successive steps (a) to (c), carried out in a furnace and under an atmosphere comprising an inert gas, dihydrogen and water, of: (a) increasing the temperature from an initial temperature T.sub.I to a hold temperature T.sub.P, T.sub.P being between 1 400° C. and 1 800° C., (b) maintaining the temperature at the hold temperature T.sub.P for a duration between 1 h and 10 h, and (c) decreasing the temperature from the hold temperature T.sub.P to a final temperature T.sub.F, wherein the ratio of the partial pressures P(H.sub.2)/P(H.sub.2O) in said atmosphere 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 1 000° 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 1 000° C., until T.sub.F is reached.

2. The process according to claim 1, wherein the first intermediate temperature T.sub.i1 is between 1 300° C. and T.sub.P.

3. The process according to claim 1, wherein the second intermediate temperature T.sub.i2 is between T.sub.P and 1 300° C.

4. The process according to claim 1, wherein the ratio P(H.sub.2)/P(H.sub.2O)≦500 is implemented as soon as the first intermediate temperature T.sub.i1 is reached and maintained until T.sub.F is reached.

5. The process according to claim 1, which further comprises, between steps (a) and (b), the following successive steps (a.sub.1) to (a.sub.3) of: (a.sub.1) maintaining, for a duration between 1 h and 10 h, the temperature at the hold temperature T.sub.P, (a.sub.2) decreasing the temperature from the hold temperature T.sub.P to a third intermediate temperature T.sub.i3, (a.sub.3) increasing the temperature from the third intermediate temperature T.sub.i3 to the hold temperature T.sub.P, and wherein the ratio P(H.sub.2)/P(H.sub.2O) in said atmosphere is such that: 500<P(H.sub.2)/P(H.sub.2O)≦50 000, during steps (a), (a.sub.1) and (a.sub.2), and P(H.sub.2)/P(H.sub.2O)≦500, during steps (a.sub.3), (b) and (c).

6. The process according to claim 5, wherein the third intermediate temperature T.sub.i3 is between 20° C. and 500° C.

7. The process according to claim 1, wherein T.sub.P is 1 700° C.

8. The process according to claim 1, wherein at least one from T.sub.I and T.sub.F is room temperature.

9. The process according to claim 1, wherein increasing the temperature in step (a) is performed at a rate between 30° C./h and 400° C./h.

10. The process according to claim 1, wherein decreasing the temperature in step (c) is performed at a rate between 100° C./h and 900° C./h.

11. The process according to claim 1, further comprising, after step (c), an oxidation heat treatment.

12. The process according to claim 1, wherein the inert gas is chosen from the group consisting of argon, helium and nitrogen.

13. The process according to claim 1, wherein the volume ratio between the inert gas and dihydrogen is between 90/10 and 98/2.

14. The process according claim 1, wherein M.sub.1 is an actinide.

15. The process according to claim 1, wherein M.sub.1 is a lanthanide.

16. The process according to claim 1, wherein the compacted powder further comprises at least one oxide of a metal M.sub.2 selected from Sc, Y, an actinide and a lanthanide, M.sub.2 being different from M.sub.1, whereby, at the end of step (c), a mixed oxide comprising M.sub.1 and M.sub.2 is obtained.

17. The process according to claim 16, wherein M.sub.2 is an actinide.

18. A process for manufacturing a pellet of at least one oxide of a metal M.sub.1 chosen from an actinide and a lanthanide, said process comprising the following successive steps of: preparing a powder of said at least one oxide of metal M.sub.1, compacting the powder prepared as a pellet, and heat treating the pellet of compacted powder by the sintering process according to claim 1.

19. The process according to claim 14, wherein M.sub.1 is selected from the group consisting of U, Pu and Th.

20. The process according to claim 15, wherein M.sub.1 is Ce.

21. The process according to claim 17, wherein M.sub.2 is selected from the group consisting of U, Pu and Th.

22. The process according to claim 18, wherein the pellet is a nuclear fuel pellet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0131] FIGS. 1A and 1B schematically illustrate the steps of the sintering process according to the invention implementing a single thermal cycle.

[0132] FIG. 2 schematically illustrates the steps of the sintering process according to the invention implementing two consecutive thermal cycles.

[0133] FIGS. 3A and 3B correspond to photographs taken with an optical microscope and showing the microstructure of pellets from the sintering processes F.sub.1.1 (FIG. 3A) and F.sub.1.3 (FIG. 3B) after attacking UO.sub.2.

[0134] FIGS. 4A and 4B correspond to photographs taken with an optical microscope and showing the microstructure of pellets from the sintering processes F.sub.2 (FIG. 4A) and F.sub.2.2 (FIG. 4B) after attacking UO.sub.2.

[0135] FIGS. 1A, 1B and 2 each have been commented in the previous chapter.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

Example 1

[0136] Manufacturing Pellets of Uranium and Plutonium Mixed Oxide Having the Formula (U.sub.0.9023,Pu.sub.0.0977)O.sub.2

Preparing Pellets of Compacted Powders

[0137] The present example 1 is carried out from a mixture of a powder of uranium oxide UO.sub.2 and a powder of plutonium oxide PuO.sub.2. This mixture has a Pu atomic content of 44% with respect to the total Pu and U atomic content as well as a carbon content of 1 850 ppm+/−100 ppm.

[0138] To this mixture, chamotte (that is a powder of uranium and plutonium mixed oxide (U,Pu)O.sub.2 from recycled manufacturing rejects) is added for forming a primary mixture. This chamotte is introduced in a quantity enabling the Pu atomic content to be decreased from 44% to 28% with respect to the total Pu and U atomic content in this primary mixture.

[0139] This primary mixture is then ground and then sieved so as to obtain a so-called “master-batch”.

[0140] Then, this master-batch is Pu-diluted by adding a powder of uranium oxide UO.sub.2, to obtain a final mixture of powders in which the U/Pu atomic ratio is 90.23/9.77. This final mixture is then compacted into pellets having the form of cylindrical tablets.

[0141] Three distinct sintering processes, noted F.sub.1.1, F.sub.1.2 and F.sub.1.3, have been conducted on these pellets to obtain sintered pellets of uranium and plutonium mixed oxide having the formula (U.sub.0.9023,Pu.sub.0.0977)O.sub.2.

Sintering Process F.SUB.1.1 .(Reference)

[0142] A first series of pellets is placed in a sintering furnace in which they are subjected to the following thermal cycle of: [0143] (a.sub.1.1) increasing the temperature, at a rate of 120° C./h, from room temperature to a hold temperature of 1 700° C., [0144] (b.sub.1.1) maintaining, for 4 h, at this hold temperature of 1 700° C., and then [0145] (c.sub.1.1) decreasing the temperature, at a rate of 300° C./h, from the hold temperature of 1 700° C. to the room temperature of 20° C.

[0146] The three steps (a.sub.1), (b.sub.1) and (c.sub.1) of the thermal cycle detailed above have been conducted in an atmosphere comprising argon, dihydrogen in a Ar/H.sub.2 volume ratio of 95/5, and water in a ratio P(H.sub.2)/P(H.sub.2O)=42. This ratio of 42 corresponds to an amount of 1 200 ppm of water in the atmosphere prevailing in the furnace.

Sintering Process F.SUB.1.2 .(According to the Invention)

[0147] A second series of pellets is placed in a sintering furnace in which they are subjected to the following thermal cycle of: [0148] (a.sub.1.2) increasing the temperature, at a rate of 120° C./h, from room temperature to a first intermediate temperature of 1 500° C., [0149] (a′.sub.1.2) increasing the temperature, at a rate of 120° C./h, of this first intermediate temperature of 1 500° C. to a hold temperature of 1 700° C., [0150] (b.sub.1.2) maintaining, for 4 h, at this hold temperature of 1 700° C., and then [0151] (c.sub.1.2) decreasing the temperature, at a rate of 300° C./h, from the hold temperature of 1 700° C. to the room temperature of 20° C.

[0152] The four steps (a.sub.1.2), (a′.sub.1.2), (b.sub.1.2) and (c.sub.1.2) of the thermal cycle detailed above have been conducted in an atmosphere comprising argon, dihydrogen in an Ar/H.sub.2 volume ratio of 95/5, and water: [0153] in a P(H.sub.2)/P(H.sub.2O) ratio between 1 000 and 5 000, that is an amount of water between 50 ppm and 10 ppm in the atmosphere, during step (a.sub.1.2), and then [0154] in a ratio P(H.sub.2)/P(H.sub.2O)=42, that is 1 200 ppm of water in the atmosphere, during steps (a′.sub.1,2), (b.sub.1.2) and (c.sub.1.2).

[0155] The amount of water implemented in the atmosphere during step (a.sub.1.2) is given by a range, in view of the low amounts involved,

Sintering Process F.SUB.1.3 .(According to the Invention)

[0156] A third series of pellets is placed in a sintering furnace in which they are subjected to the following thermal cycle: [0157] (a.sub.1.3) increasing the temperature, at a rate of 120° C./h, from room temperature to a hold temperature of 1 700° C., [0158] (b.sub.1.3) maintaining, for 4 h, at this hold temperature of 1 700° C., and then [0159] (c.sub.1.3) decreasing the temperature, at a rate of 300° C./h, from the hold temperature of 1 700° C. to the room temperature of 20° C.

[0160] The three steps (a.sub.1.3) to (c.sub.1.3) of the thermal cycle detailed above have been conducted in an atmosphere comprising argon, dihydrogen in an Ar/H.sub.2 volume ratio of 95/5, and water: [0161] in a P(H.sub.2)/P(H.sub.2O) ratio between 1 000 and 5 000, that is an amount of water between 50 ppm and 10 ppm in the atmosphere, during step (a.sub.1.3), and then [0162] in a ratio P(H.sub.2)/P(H.sub.2O)=42, that is 1 200 ppm of water in the atmosphere, during steps (b.sub.1.3) and (c.sub.1.3).
Characterisation of the Pellets of Mixed Oxide of the Formula (U.sub.0.9023,Pu.sub.0.0977)O.sub.2

[0163] The three series of pellets of mixed oxide (U.sub.0.9023,Pu.sub.0.0977)O.sub.2 obtained, after the implementation of the sintering processes F.sub.1.1, F.sub.1.2 and F.sub.1.3 are all flawless.

[0164] These three series of sintered pellets have been characterised, according to the protocols described hereinafter, to determine their respective O/M ratio and apparent specific gravity values (the open porosity and closed porosity values are also deduced therefrom), their microstructure as well as their thermal stability.

Characterisation Protocols

[0165] The O/M ratio is determined by thermogravimetry, under water saturated dihydrogen and at a temperature in the order of 950° C. The value obtained has a very low uncertainty, typically lower than 0.003.

[0166] The apparent specific gravity, noted Dh and expressed as a % of the theoretical specific gravity, is measured by hydrostatic weighing, in bromobenzene, by means of a hydrostatic specific gravity device, by applying the Archimedes' principle.

[0167] The open porosity, noted Po, as well as the closed porosity, noted Pf, are deduced from the value of the apparent specific gravity Dh by the formulae below:

[00001]$\mathrm{Po}=\frac{\mathrm{Dh}-\mathrm{Dg}}{\mathrm{Dth}}\times 100$$\mathrm{Pf}=\frac{\mathrm{Dth}-\mathrm{Dh}}{\mathrm{Dth}}\times 100$

Dth and Dg respectively corresponding to the theoretical specific gravity and the geometrical specific gravity.

[0168] The microstructure is analysed by observing, with an optical microscope, sintered pellets in longitudinal cross-section and radial cross-section views. The presence of UO.sub.2 and UPuO.sub.2 islands is revealed by UO.sub.2 chemical attack. A prior calibration carried out with COFRAC No 74541B, or 74541A, certified object micrometer, indicates that the uncertainty on the dimensional measurements is typically lower than 10% on a relative basis.

[0169] To determine the thermal stability of the apparent specific gravity of the three series of sintered pellets, a thermal treatment of these pellets is operated by placing them into a furnace for 24 h at 1 700° C. under an atmosphere formed by an argon and dihydrogen mixture (in an Ar/H.sub.2 volume ratio of 95/5). At the outlet of the furnace, and at the end of these 24 h, the apparent specific gravity of said pellets is measured by hydrostatic weighing. The sintered pellets are considered as “satisfactory”, that is they have an apparent specific gravity which is thermally stable, when the variation in the apparent specific gravity is between 0 and 1.9% after this thermal treatment of 24 h at 1 700° C.

[0170] The carbon content is determined using a Sylab brand analyser (model CSBOX-HF) by the IR spectrophotometry method, after melting the sintered pellets.

Characterisation of the Sintered Pellets

[0171] The pellets resulting from the sintering processes F.sub.1.1, F.sub.1.2 and F.sub.1.3 all have an O/M ratio equal to 2.00, in accordance with the oxygen potential set by the ratio P(H.sub.2)/P(H.sub.2O)=42 during steps (b.sub.1.1), (b.sub.1.2) and (b.sub.1.3) of maintaining the temperature at 1 700° C. and steps (c.sub.1.1), (c.sub.1.2) and (c.sub.1.3) of decreasing the temperature from 1 700° C. to the room temperature of 20° C.

[0172] Other characterisation data of these pellets from the sintering processes F.sub.1.1, F.sub.1.2 and F.sub.1.3 (determined according to the protocols described above) are gathered in Table 1 hereinafter.

TABLE-US-00001 TABLE 1 Sintering F.sub.1.1 F.sub.1.2 F.sub.1.3 Apparent specific 93.5 96.6 97.1 gravity (% Dth) Pf (%) 6.1 3.1 1.8 Po (%) <1 <1 <1 Thermal stability of the non satisfactory satisfactory satisfactory apparent specific gravity FIG. 3A 3B

[0173] As appears from Table 1, when switching from sintering F.sub.1.1 to sintering F.sub.1.3, the apparent specific gravity of the sintered pellets increases whereas their respective closed porosity decreases.

[0174] More precisely, the porosity of the pellets from the reference sintering F.sub.1.1 is mostly closed (6.1%) and is found in master-batch aggregates. As can be seen in FIG. 3A, these master-batch aggregates have dimensions between 50 μm and 100 μm. On the other hand, pellets from sintering F.sub.1.1 do not meet the criterion of thermal stability of the apparent specific gravity, in that the variation in the apparent specific gravity is established at −1%, meaning that the pellets have been dedensified and have consequently swollen after the heat treatment of 24 h at 1 700° C.

[0175] Pellets from sintering F.sub.1,3 according to the invention have the strongest apparent specific gravity value (97.1% Dth). Further, FIG. 3B clearly highlights a better plutonium distribution homogeneity within the microstructure of these sintered pellets, which results in a higher volume ratio of the plutoniferous phase to the uraniferous phase. Further, if FIGS. 3A and 3B are compared, it is observed that the master-batch aggregates present in the pellets from sintering F.sub.1.3 include less porosities than master-batch aggregates present in the pellets from sintering F.sub.1.3.

Example 2

[0176] Manufacturing pellets of uranium and plutonium mixed oxide having the formula (U.sub.0.7,Pu.sub.0.3)O.sub.2

Preparing Pellets of Compacted Powders

[0177] The present example 2 is made from a mixture of a powder of uranium oxide UO.sub.2 and a powder of plutonium oxide PuO.sub.2. This mixture has a Pu atomic content of 30% with respect to the total Pu and U atomic content as well as a carbon content of 3 500 ppm+/−100 ppm.

[0178] This mixture is compacted into pellets having the form of cylindrical tablets.

[0179] Three distinct sintering processes, noted F.sub.2.1, F.sub.2.2 and F.sub.2.3, have been conducted, made from these pellets to obtain sintered pellets of uranium and plutonium mixed oxide having the formula (U.sub.0.7,Pu.sub.0.3)O.sub.2.

Sintering Process F.SUB.2 .(Reference)

[0180] A first series of pellets is placed into a sintering furnace in which they are subjected to the following thermal cycle of: [0181] (a.sub.2.1) increasing the temperature, at a rate of 50° C./h, from room temperature to a hold temperature of 1 700° C., [0182] (b.sub.2.1) maintaining, for 4 h, at this hold temperature of 1 700° C., and then [0183] (c.sub.2.1) decreasing the temperature, at a rate of 300° C./h, from the hold temperature of 1 700° C. to the room temperature of 20° C.

[0184] The three steps (a.sub.2.1), (b.sub.2.1) and (c.sub.2.1) of the thermal cycle detailed above have been conducted in an atmosphere comprising argon, dihydrogen in an Ar/H.sub.2 volume ratio of 95/5, and water in a ratio P(H.sub.2)/P(H.sub.2O)≦500. This ratio corresponds to an amount higher than or equal to 100 ppm of water in the atmosphere prevailing in the furnace during all the time of the thermal cycle.

Sintering Process F.SUB.2,2 .(According to the Invention)

[0185] A second series of pellets is placed into a sintering furnace in which they are subjected to the following thermal cycle: [0186] (a.sub.2.2) increasing the temperature, at a rate of 50° C./h, from room temperature to a hold temperature of 1 700° C., [0187] (b.sub.2.2) maintaining, for 4 h, at this hold temperature of 1 700° C., and then [0188] (c.sub.2.2) decreasing the temperature, at a rate of 300° C./h, from the hold temperature of 1 700° C. to the room temperature of 20° C.

[0189] The three steps (a.sub.2,2), (b.sub.2.2) and (c.sub.2.2) of the thermal cycle detailed above have been conducted in an atmosphere comprising argon, dihydrogen in an Ar/H.sub.2 volume ratio of 95/5, and water: [0190] in a P(H.sub.2)/P(H.sub.2O) ratio between 1 000 and 10 000, that is an amount of water between 50 ppm and 5 ppm in the atmosphere, during step (a.sub.2.2), and then [0191] in a ratio P(H.sub.2)/P(H.sub.2O)≦500, which corresponds to an amount of water of 150 ppm, during steps (b.sub.2.2) and (c.sub.2,2).

[0192] Under the aforementioned conditions, sintered pellets characterised by an O/M ratio of about 1.94 have been obtained.

[0193] A first part, or batch 1, of the pellets thus resulting from sintering F.sub.2.2 is set aside to be characterised. The second part, or batch 2, of the pellets thus resulting from sintering F.sub.2.2 will be subjected to a further step of controlled oxidation.

Sintering Process F.SUB.2.3 .(According to the Invention)

[0194] The third series of pellets consists of the batch 2 of sintered pellets from sintering F.sub.2.2 above.

[0195] These sintered pellets of batch 2 are replaced in the sintering furnace in which they are subjected to a controlled oxidation heat treatment which comprises the following successive steps, made after step (c.sub.2.2) above, of: [0196] (d.sub.2.3) increasing the temperature, at a rate of 50° C./h, from room temperature to a hold temperature of 950° C., [0197] (e.sub.2.3) maintaining at this hold temperature of 950° C. for 4 h, and then [0198] (f.sub.2.3) decreasing the temperature, at a rate of 300° C./h, from the hold temperature of 950° C. to the room temperature of 20° C.

[0199] Under the aforementioned conditions, sintered pellets characterised by an O/M ratio of about 1.97 are obtained.

Characterisation of Pellets of Mixed Oxide of the Formula (U.sub.0.7,Pu.sub.0.3)O.sub.2

[0200] Characterisation data of pellets from sintering processes F.sub.2.1, F.sub.2.2 and F.sub.2.3 (determined according to the protocols described above) are gathered in Table 2 hereinafter.

TABLE-US-00002 TABLE 2 Sintering F.sub.2.1 F.sub.2.2 F.sub.2.3 Apparent specific 90.0 97.9 97.9 gravity (% Dth) Pf (%) >9 1.7 1.7 Po (%) <1 <1 <1 O/M ratio not measured 1.94 1.97 C content (ppm) 100 200 200 Thermal stability of the non satisfactory satisfactory satisfactory apparent specific gravity FIG. 4A 4B

[0201] As previously, Table 2 shows that, when switching from sintering F.sub.2.1 to sintering F.sub.2.3, the apparent specific gravity of the sintered pellets increases whereas their respective closed porosity decreases.

[0202] The porosity of pellets from sintering F.sub.201 (reference) is mostly a closed porosity (9%). As can be seen in FIG. 4A, the microstructure of pellets from this sintering F.sub.2.1 have a few macropores having a diameter of several hundreds of μm as well as a very significant microporosity. On the other hand, pellets from sintering F.sub.2.1 do not meet the criterion of thermal stability of the apparent specific gravity, because of a too high closed porosity of these sintered pellets.

[0203] Pellets from sintering F.sub.2.2 in accordance with the invention have an apparent specific gravity of 97.9% Dth and meet the criterion of thermal stability of the apparent specific gravity. Their O/M ratio is 1.94. It is to be noted that in the absence of an intentional humidification introduced from step (b.sub.2.2), an O/M ratio in the order of 1.92 would have be obtained. In reference to FIG. 4B, it is observed that pellets from sintering F.sub.2.2 have a microstructure characterised by an improved homogeneity, a decreased porosity and, hence, an apparent specific gravity increased with respect to the microstructure of pellets from sintering F.sub.2.1.

[0204] Pellets from sintering F.sub.2.3, which correspond to pellets from sintering F.sub.2.2 and then subjected to a further heat treatment step, under an atmosphere comprising water in an amount similar to that implemented in steps (b.sub.2.2) and (c.sub.2.2), are characterised by an O/M ratio higher than that of pellets from sintering F.sub.2.2. It is set out that, to obtain such an increase from 1.94 to 1.97 of the O/M ratio, it would also have been contemplatable to increase the amount of water in the argon and dihydrogen atmosphere (Ar/5% H.sub.2) during steps (b.sub.2.2) and/or (c.sub.2.2) of the sintering process F.sub.2.2, in particular by passing it from 100 ppm to 300 ppm of water.