Method for chemically stabilizing uranium carbide compounds, and device implementing the method

Abstract

A process for chemical stabilization of a uranium carbide composite material: UC.sub.x+yC with x≧1 and y>0, placed in a stabilization chamber, comprises: a rise in chamber internal temperature for oxidation of the compound based on uranium carbide between approximately 380° C. and 550° C., the chamber being fed with a neutral gas; isothermal oxidative treatment at the oxidation temperature, the chamber being placed under O.sub.2 partial pressure; controlling completion of stabilization of the compound, comprising monitoring the amount of molecular oxygen consumed and/or carbon dioxide or carbon dioxide and carbon monoxide given off, until achievement of an input set-point value for the amount of molecular oxygen, of a minimum threshold value for the amount of carbon dioxide or minimum threshold values for the carbon dioxide and carbon monoxide. A device implements the process.

Claims

1. A process for the chemical stabilization of a uranium carbide composite material corresponding to: UC.sub.x+yC with x≧1 and y>0, placed in a stabilization chamber, comprising the following stages: a stage of rise in temperature of the internal temperature of said chamber to a temperature of between approximately 380° C. and 550° C., said chamber being fed with an inert gas consisting of one or both of argon and nitrogen; a stage of isothermal oxidative treatment at said temperature of between approximately 380° C. and 550° C., said chamber being placed under O.sub.2 partial pressure; and a stage of controlling the completion of the stabilization of said composite material which comprises monitoring of the amount of molecular oxygen consumed and/or of carbon dioxide given off or of carbon dioxide and carbon monoxide given off, until at least the achievement of a value of an input set point for the molecular oxygen, of a minimum threshold value for said amount of carbon dioxide or of threshold values for the carbon dioxide and carbon monoxide.

2. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 1 wherein the stage of controlling the completion of the stabilization additionally comprises monitoring of variation in weight of the composite material based on carbon and uranium in the chamber, an increase in weight being correlated with the oxidation of uranium carbide in progress.

3. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 1, wherein the stage of controlling the completion of the stabilization is carried out with the application of a rise in temperature of the internal temperature of said chamber and the monitoring of CO.sub.2 given off.

4. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 1, comprising the introduction of a water vapor partial pressure into said chamber before and/or during and/or after the isothermal oxidative treatment stage.

5. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 4, wherein the stage of controlling the completion of the stabilization further comprises detection of H.sub.2 as marker for monitoring an end of oxidation in said chamber.

6. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 2, wherein the stage of controlling the completion of the stabilization comprises an operation of overpressurizing a plurality of reaction gases present in said chamber so as to accelerate the end of the oxidation of said composite material.

7. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 6, wherein the stage of controlling the completion of the stabilization additionally comprises a cycle of an operation of overpressurizing and an operation of underpressurizing the reaction gases present in said chamber.

8. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 2, in which said composite material exhibits a morphology of powder or of porous or dense pellet.

9. The process for the chemical stabilization of a uranium carbide composite material as claimed in claim 2, comprising a preliminary stage of determination of an optimum oxidation temperature by thermogravimetric analysis of a sample of UC.sub.x+yC composite material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A better understanding of the invention will be obtained and other advantages will become more apparent on reading the description which will follow, given without implied limitation and by virtue of the appended figures, among which:

(2) FIG. 1 illustrates an example of thermal runaway characterized by uncontrolled pseudoperiodical overheating during the oxidation of a sample of uranium metal at 390° C.;

(3) FIG. 2 illustrates a block diagram showing the various means employed to carry out the process of the present invention;

(4) FIG. 3 illustrates the various phases of operations according to the process of the invention;

(5) FIG. 4 illustrates the change in the weight of a UC.sub.x compound as a function of the time, for different isothermal oxidation temperatures;

(6) FIG. 5 illustrates the variations in release of CO.sub.2 and in local overheating events detected during the application of the process respectively for two different oxidation temperatures (T.sub.oxidation=400 and then 700° C.);

(7) FIG. 6 illustrates the variation in weight as percentage and the heat flow given off, during the oxidation under isothermal conditions of the UC.sub.x for three different concentrations of molecular oxygen;

(8) FIG. 7 illustrates the profiles for variations in weight obtained during the oxidation of the UC.sub.x under an oxidizing atmosphere under isothermal conditions for different oxidation temperatures;

(9) FIG. 8 illustrates the thermogravimetric curves showing the influence of the geometric nature on the process for the stabilization of the UC.sub.x material at moderate temperature T.sub.oxidation=400° C.

DETAILED DESCRIPTION

(10) Generally, the process of the present invention comprises:

(11) bringing the material to temperature under a neutral atmosphere in order to be under the future oxidation conditions;

(12) an operation for controlled stabilization of the UC.sub.x+yC phase by an isothermal oxidative treatment within an optimum temperature range [380° C.; 550° C.] (notably as a function of the nature, of the amount, of the morphology and of the composition of the x and y values of the input material) under an O.sub.2 partial pressure (from 5% to 25% of O.sub.2) (preferably 10% O.sub.2). During this stage, the treatment conditions are chosen in particular in order to make sure that the products are reactive and that this reactivity is controlled solely by the supply of oxygen. Confirmation of the satisfactory progression of the oxidative treatment process is carried out by monitoring, in real time, the molecular oxygen O.sub.2 consumed and the carbon dioxide CO.sub.2/carbon monoxide CO given off;

(13) an operation for confirming the completion of the stabilization of the composite material. This final stage can notably be carried out by the simultaneous application of a pronounced but controlled increase in the oxidation temperature or the sequential insertion of a water vapor partial pressure having the aim of promoting the oxidation of final UC.sub.2 fragments possibly not oxidized during the first oxidation phase or the variation in the pressure of the reaction gases in the process (positive variation (limited to 1 bar max) or negative variation (limited to 1 mbar min)) or else by a combination of two or three alternative forms.

(14) The detection of the reactivity with regard to the contents of the reaction gases (CO, CO.sub.2, H.sub.2) from this change in conditions makes it possible to reveal the completion of the stabilization reaction without fear of a high reactivity of a portion of the waste which may potentially not yet be stabilized during the preceding stage. In the absence of reactivity of these gases, the halting of treatment is ordered.

(15) The detailed description below has the aim of revealing that, from the viewpoint of the chemical nature of the material to be treated UC.sub.x+yC, amounts and volumes which can be involved for notably applications targeted in the present invention, i.e. large amounts of waste to be reprocessed greater typically than several kilograms, the process of the present invention makes it possible to provide complete stabilization of the material in an oxidized form which is stable to air at ambient temperature and pressure by imposing an appropriate treatment temperature with an optimum gas flow rate and an optimum O.sub.2 concentration.

(16) In point of fact, the structural specificity of said composite material (two-phase compound notably of UC.sub.2, for example, and of free carbon in the graphitic form, structural heterogeneity, high porosity) brings about contradictions in terms of objective, indeed even of physical constraints, which render particularly advantageous different optimizations of the process of the present invention specified in the continuation of the description.

(17) These difficulties are based notably on the following contradictions: the need to guarantee the stabilization of the UC.sub.x+yC waste without, however, converting all of the carbon (yC) initially present in the UC.sub.x+yC material or present in the form of reaction intermediates resulting from the various oxidation reactions. This is because the complete conversion of these carbon-comprising forms gives off a large amount of gas (CO.sub.2, CO mainly) which is highly damaging and thus prejudicial in terms of reprocessing of gas (significant discharge) and of duration of application of a process on a semi-industrial scale. In addition, the choice of complete stabilization of all of the constituents of the UC.sub.x+yC material (UC.sub.2, UC and carbon) involves operating at higher oxidation temperatures, which significantly promotes the release of radioactive elements at the departure of effluent gases; a stabilization specifically adapted to a portion of the constituents of the UC.sub.x+yC material (the UC.sub.2, UC carbide phase) is rendered all the more problematic as the reaction for stabilization by oxidation is highly exothermic (difficulty of controlling the reactivity), which conflicts with the targeted objective; the control of the reactivity is rendered all the more difficult, beyond the phenomena of exothermicity, as it is conditioned by the accessibility of the oxidant to the reaction sites and depends on the byproducts formed (UO.sub.x), which can create reaction-limiting barriers which can break more or less suddenly during the treatment.

(18) The process of the present invention thus has to make it possible to control the physical constraints listed above by making use of an optimum operating range in order:

(19) to completely but solely oxidize the UC.sub.x phase without completely incinerating the excess graphite present in the initial material (yC) but also optionally in the target container which can be employed, also composed of graphite and conventionally estimated at more than 1 kg by weight;

(20) to limit the treatment time for stabilization/conversion of the UC.sub.x material by a range of oxidation temperatures which are studied which makes it possible to result in rapid kinetics of oxidation of the UC.sub.x to give UO.sub.x;

(21) to limit only the production of CO.sub.2 resulting solely from the oxidation of the UC.sub.x to give U.sub.3O.sub.8/UO.sub.2 by inhibiting the strong release of CO.sub.2 produced by the oxidation of the excess carbon/graphite, the volumes of which, introduced by the UC.sub.x+yC material and the graphite container of the UC.sub.x targets, involve a treatment process which is lengthy to carry out;

(22) to limit the volatility and the propagation of potential fission or activation products by confining them as much as possible within the UC.sub.x targets to be treated by the use of a suitable and moderate oxidation temperature;

(23) to provide a system for running the process which makes it possible to control the chemical reactivity and to confirm good stabilization of the material once the latter has been oxidized by the process;

(24) to prevent any unstable form of oxidation of the UC.sub.x material notably with regard to the variability in the geometry (pellets, powder, spherical beads) and to the nature of the input material based on uranium carbides.

Example of a Device which Makes it Possible to Carry Out the Process for the Stabilization of UCx+yC

(25) FIG. 2 gives a diagrammatic representation of an example of a device which makes it possible to carry out the isothermal oxidative treatment of the compound under O.sub.2 partial pressure in an oxidation furnace: a first module B.sub.1 is used to feed with gas and makes it possible to generate neutral atmospheres of argon or nitrogen or else partially oxidizing atmospheres of O.sub.2 and/or H.sub.2O using an external feed circuit. These atmospheres are continually adjusted by pressure and flow gauges and then injected into the oxidation furnace in order to stabilize the composite material made of UC.sub.x+yC. More specifically, this module B.sub.1 can comprise notably a circuit which generates water vapor B.sub.11, coupled to a regulator of water vapor pressure B.sub.14, an argon/nitrogen feed B.sub.12, an argon/molecular oxygen feed B.sub.13, coupled to a regulator of molecular oxygen pressure B.sub.15, the two regulators feeding a mixer B.sub.16 of O.sub.2 and/or H.sub.2O in the direction of a regulator of input pressure B.sub.17 connected to a regulator of gas output flow rate B.sub.18 in order to feed, via a flow F.sub.1-3, a chamber corresponding to a third module B.sub.3 for stabilization heat treatment comprising an oxidation furnace in which the stabilization of the compound takes place; a second module B.sub.2 for feeding with electricity is provided in order to feed the block B.sub.3 via a set-point flow F.sub.2-3 and comprises a module for feeding with electricity B.sub.21 and a module for programming B.sub.22 the stabilization heat cycle suited to the variability in the input composite material; the third module B.sub.3 comprises an oxidation furnace having a regulated atmosphere; it also makes it possible to charge the input material distributed over a boat optimized with respect to the variability in the nature and in the morphology of the input material and then to discharge the stabilized waste for the purpose of potential analyses (in particular the weighing of the final residue and the withdrawal of a sample from the residue in order to carry out characterizations) before being subsequently packaged and stored; a fourth module B.sub.4 is provided in order to provide the functions of regulation and automatic control; it comprises a module B.sub.41 which makes possible measurements of temperature and thermal power and a module for analyses B.sub.42 of concentrations of different gases, such as O.sub.2, CO.sub.2, CO, H.sub.2O or H.sub.2. This fourth module makes it possible to carry out a continuous feedback adjustment of the parameters for running the process, notably: the oxidant partial pressure, the stabilization temperature, by monitoring, in real time, the temperature and the thermal power of the oxidation furnace, the consumption of gas (O.sub.2, N.sub.2, Ar, H.sub.2O) and the production of gaseous reactants (CO.sub.2, CO, H.sub.2, CH.sub.4, C.sub.2H.sub.6). Optionally, the change in the weight of UC.sub.x during its oxidation is also recorded in order to identify the different oxidation reactions, to distinguish the opposing phases and to monitor the degree of conversion of the charge to be stabilized.

(26) The gas flows exiting from the chamber F.sub.3-S are, on the one hand, filtered before discharge via a pump P.sub.1 and a filter fi.sub.G to produce gas sample G′.sub.S and, on the other hand, analyzed via a withdrawn gas sample G.sub.S of said gas flow F.sub.3-4.

(27) Detailed Description of the Different Stages of Implementation in the Process of the Invention in the Context of an Example:

(28) 1) The stage of rise in temperature to an oxidation temperature can advantageously be between approximately 380° C. and 550° C. and be carried out in a chamber under an inert atmosphere.

(29) In order to arrive at conditions of oxidation under isothermal conditions, the UC.sub.x+yC material is gradually heated under an inert gas up to the oxidation temperature for the application of the process. The choice of this oxidation temperature depends in particular on the type of furnace and on its performance, on the nature and on the morphology of the input material, on the geometry of the charging boat and on the arrangement of the material to be oxidized inside this boat. Preliminary tests on reduced amounts are potentially necessary to best adjust the treatment temperature (and will be described subsequently in the present description). The duration of this first stage can typically be of the order of approximately sixty minutes.

(30) 2) After a period of stabilization under an inert atmosphere (mean duration 60 min), a gas composed of an O.sub.2 partial pressure is introduced into the oxidation furnace. Generally, after application of the process at temperatures T.sub.oxidation varying from 380 to 550° C., the UC.sub.x material, with the initial chemical composition UC.sub.2+graphitic carbon C.sub.F and with geometry of pellets type, is oxidized and forms a homogeneous profuse powder, with the chemical composition U.sub.3O.sub.8+graphitic carbon C.sub.F. The expansion by volume of the UC.sub.x material after treatment of the process is of the order of 50%. The oxidation of the UC.sub.x material is monitored in real time with a gas analyzer at the outlet of the oxidation furnace. The oxidation treatment is halted when the O.sub.2 concentration reaches the imposed inlet value and when the CO.sub.2 concentration given off during the oxidation of the UC.sub.x targets is less than a threshold value which can typically be of the order of 100 ppm.

(31) 3) The oxidation of the UC.sub.x+yC can advantageously be monitored by the analysis of the change in weight (if the measurement device allows it) and by the measurement in real time of the output gases of the process, in particular: the monitored molecular oxygen O.sub.2 of the consumption, the CO.sub.2 produced by the oxidation of the UC.sub.x to give the oxide form UO.sub.x, optionally the carbon monoxide CO and the molecular hydrogen H.sub.2 given off during sequential programmed addition of water vapor during reaction. This is because it can be advantageous to use water vapor also for milder stabilization via a controlled oxidation of the oxygen.

(32) 4) The stabilization of the UC.sub.x material is regarded as complete when: the initial weight of the material to be treated reaches a stabilized weight gain Δm compatible with the formation of UO.sub.x, mainly U.sub.3O.sub.8 (it being possible for the variation in weight Δm typically to be between 6% and 10%); the outlet O.sub.2 concentration reaches the imposed inlet value of the process (preferably 10% concentration by volume); the gases produced, CO, CO.sub.2, H.sub.2, reach a value lower than a threshold value (typically less than 100 ppm); the oxidized UC.sub.x material no longer reacts after stresses (absence of thermal reactivity) by a difference in temperature ΔT, in concentration (Δ[O.sub.2], for example), in humid atmosphere (Δ[H.sub.2O]) or in pressure ΔP.
It should be noted that the stresses can be as follows: the rapid but controlled increase ΔT in the oxidative treatment temperature such that T.sub.oxidation+ΔT<T.sub.max, T.sub.oxidation being the temperature of application of the oxidative treatment (T.sub.oxidation of between 300 and 550° C.) and T.sub.max being the maximum temperature admissible before the oxidation of the excess free carbon (T.sub.max in the vicinity of 560° C.), the absence of O.sub.2 consumption and of CO.sub.2 release during this stress marking the halting of the process; the variation in the pressure in the furnace. A variation in pressure facilitates the penetration of the gases to the core of the body to be oxidized and promotes the reaction kinetics. To do this, a reduction in pressure (P.sub.min in the vicinity of 1 mbar)-compression (P.sub.max in the vicinity of 1 bar) cycle can be carried out by virtue of a pumping and solenoid valve system connected to the oxidation furnace; the addition of a residual content of water vapor either before, during or after the treatment in order to facilitate the preferred oxidation of UC.sub.x materials, in particular having a high specific density, with the preferred oxidation of UC.sub.2 beads under a water-comprising oxidizing atmosphere). The addition of water vapor is limited to a maximum of 5% by volume in order to exclude the presence of an atmosphere excessively charged with H.sub.2 (maximum admissible safe value 5% H.sub.2 as concentration by volume), the gas H.sub.2 being generated during the oxidation of the UC.sub.x with the water vapor. The introduction of H.sub.2O at the end of the cycle represents an advantage insofar as this makes it possible to use the H.sub.2 as new gas tracer for a specific oxidation of the UC.sub.x and in complete safety in the case of a recovery in the reactivity of the UC.sub.x material, as the amounts produced are then significantly lower owing to the fact that the UC.sub.x material is already stabilized for the most part in the oxidized form and as a result of the limitation of the temperature (gasification reaction impossible for T.sub.oxidation<T.sub.max and as a result of the limitation of the [H.sub.2O] concentration); it is also possible to carry out a simultaneous combination of the different stresses mentioned above.

(33) FIG. 3 illustrates all of these stages, diagrammatically represented as phase Ph.sub.1, Ph.sub.2 and Ph.sub.3. The curve C.sub.3a relates to the change in the temperature as a function of the time, the curve C.sub.3b relates to the amount of CO.sub.2 given off, the curve C.sub.3c relates to the change in the weight of the solid compounds, the curve C.sub.3d relates to the amount of O.sub.2 and the curve C.sub.3e relates to the amount of H.sub.2 present in the water vapor.

(34) Typically, it is possible to have an imposed oxidant partial pressure of 10%.

(35) In order to achieve these criteria for satisfactory progression of the process, the applicant has demonstrated that it is advantageously possible to define beforehand optimum stabilization temperatures of between 300 and 550° C. These temperatures are carefully chosen in order to promote only the oxidation of the UC.sub.2 phase to give UO.sub.x, without detrimentally affecting the excess graphite present in the initial UC.sub.x material, the objective being to oxidize as little as possible of the graphite of the material and its container.

(36) This stage of optimization of the oxidation temperature is illustrated below more specifically in the case of a material with the composition UC.sub.2+2C. As this UC.sub.x material is multiphase and heterogeneous, its oxidation under isothermal conditions has formed the subject of an in-depth analysis by the applicant. In order to show that the desired response of the material subsequent to the application of a stabilization treatment depends on many parameters and in particular on an optimum range of oxidation temperatures, an example of isothermal networks, obtained by thermogravimetric and differential thermal analyses at an O.sub.2 partial pressure of 10%, is represented as a specific example in FIG. 4. Each curve represents the change in the variation in weight of the UC.sub.x material as a function of the time for different oxidation temperatures, denoted T.sub.oxidation. An increase in weight detected reflects the fact that the UC.sub.x material, with the initial chemical formula UC.sub.2+yC.sub.F (C.sub.F symbolizing the excess graphite present in the initial UC.sub.x material), is oxidized to form a solid chemical compound of UO.sub.z+yC.sub.F and/or UO.sub.z type. When a loss in weight is measured, it reflects the fact that the oxidation of a reactive solid to give a gas takes place, which corresponds, in the present case, to the oxidation of a carbon-comprising form to give CO/CO.sub.2.

(37) It is thus apparent that, for a temperature T.sub.oxidation in the vicinity of 300° C., the kinetics of oxidation of the UC.sub.x material to give the UO.sub.z phase (in this instance, to give U.sub.3O.sub.8, by way of example) are gradual and fairly slow.

(38) It should be remembered that the main reaction during the oxidation process is as follows:
UC.sub.2+2C.sub.F+4/3O.sub.2.fwdarw.⅓U.sub.3O.sub.8+2C.sub.UCx+2C.sub.F
and results in a theoretical increase in weight Δm.sub.theoretical=15%. The weight gains obtained should thus be compared with the theoretical weight gains.

(39) At this temperature, no gaseous discharge of CO.sub.2 should take place, which was confirmed using a coupled gas analyzer at the outlet of the thermogravimetric device.

(40) For a temperature T.sub.oxidation in the vicinity of 400° C., the increase in weight is faster and results in a well-defined stationary state being obtained, showing that the oxidized UC.sub.x material is no longer changing, although the latter is still under an oxidizing atmosphere. This optimum oxidation temperature thus makes possible a rapid and stable conversion of the UC.sub.x material to give the oxide phase (very particularly U.sub.3O.sub.8) which is defined in this example by the following reaction:
UC.sub.2+2C.sub.F+10/3O.sub.2.fwdarw.⅓U.sub.3O.sub.8+2C.sub.F+2CO.sub.2Δm.sub.theoretical=7.2%

(41) For a temperature T.sub.oxidation of 500° C., the profile of variation in weight during the oxidation of the UC.sub.x reveals an increase followed by a temporary loss in weight which subsequently tends toward a stabilized stationary state Δm. The increase in weight corresponds to the oxidation of the UC.sub.2 phase to give U.sub.3O.sub.8 and the loss in weight reflects the oxidation of the residual carbon resulting from the UC.sub.2 present in a small amount, which is accompanied by a slight release of CO.sub.2. At the end of the oxidation stationary state, the remaining chemical phases are U.sub.3O.sub.8 and C.sub.F, so that the overall oxidation reaction can be written in the form:
UC.sub.2+2C.sub.F+(10/3+α)O.sub.2.fwdarw.⅓U.sub.3O.sub.8+(2−α)C.sub.F+(2+α)CO.sub.2Δm.sub.theoretical=<7%

(42) For temperatures greater than or equal to 600° C., the profiles of variation in weight simultaneously reveal an increase followed by a gradual loss in weight, the amplitude of which is proportional to the oxidation temperature applied. The Δm profiles thus pass through a maximum, also known as overshoot, the amplitude and position of which for one and the same material vary as a function of the oxidation temperature applied. From this point, a strong release of CO.sub.2 accompanies this loss in weight, demonstrating the oxidation of all of the excess graphite, in addition to the oxidation of the UC.sub.x to give the U.sub.3O.sub.8 form. The rate of oxidation of the 2 phases (UC.sub.2 and C.sub.F) forming the UC.sub.x material thus depends strongly on the oxidation temperature T.sub.oxidation applied.

(43) This determination of the oxidation kinetics for the UC.sub.x material and of the influence of the chosen temperature under isothermal conditions thus makes it possible to identify a range of optimum temperatures in the vicinity of 400° C.+/−100° C. for the application of the process of the present invention. These temperatures make it possible to make sure of the complete oxidation of the UC.sub.2 phase, this being achieved, all at the same time: without completely oxidizing the residual carbon (either resulting from the oxidation of the UC.sub.x (C.sub.UCx) or initially present (C.sub.F)) contained in the targets; without requiring a treatment time completely unacceptable at the process level: the thermogravimetric curves presented in FIG. 4 show that the final stabilization of the UC.sub.x material (that is to say, a variation in weight which no longer changes during the oxidation) at a temperature T.sub.oxidation=400° C. is four times faster than for an oxidation temperature of 700° C., while preventing the oxidation of the residual graphite; without excessive overheating of the charge to be stabilized in order to prevent any runaway and also oxidation of other elements not requiring it and which can even be damaging for the treatment of the gases.

(44) By way of example, FIG. 5 demonstrates the variations in release of CO.sub.2 (C.sub.5a 400° C. and C.sub.5a 700° C.) and in overheating events corresponding to local excess temperatures (C.sub.5b 400° C. and C.sub.5b 700° C.) detected locally during the application of the process. The data obtained show notably phenomena of recovery of reactivity very particularly with a temperature of 700° C. (identified in FIG. 5 by Zone A and Zone B) which testify to the exothermicity of the reactions involved. Furthermore, still at this oxidation temperature of 700° C., the release still present of CO.sub.2 after an oxidation treatment of 280 minutes shows that the stabilization process still remains incomplete. On the other hand, for more moderate temperatures in the vicinity of 400° C., the release of CO.sub.2 becomes less than the threshold value (100 ppm) after an oxidation treatment of only 200 min, which means the conversion of virtually all the UC.sub.x material to UO.sub.x. Likewise, the phenomena of recovery of thermal reactivity at these “mild” temperatures are much weaker, indeed even nonexistent.

(45) The reactions taking place during the process are schematically as follows (with a priority with regard to the reaction (1)):
UC.sub.x+yC.sub.F+(x+4/)3O.sub.2.fwdarw.⅓U.sub.3O.sub.8+yC.sub.F+xCO.sub.2 x=1 to 2,y=1 to n  (1)
UC.sub.x+yC.sub.F+4/3O.sub.2.fwdarw.⅓U.sub.3O.sub.8+xC.sub.UCx+yC.sub.F x=1 to 2,y=1 to n  (2)
UC.sub.x+yC.sub.F+(x+z/2)O.sub.2.fwdarw.1UO.sub.z+yC.sub.F+xCO.sub.2 x=1 to 2,y=1 to n,z=2 to 3  (3)
In contrast, the reactions which are undesirable for the UC.sub.x material are those which involve the oxidation of the carbon at the same time as the oxidation of the UC.sub.2 phase and more particularly the free carbon, denoted C.sub.F, present in the graphitic form in large amounts in the initial UC.sub.x material (70% by volume). By way of example, a few undesirable reactions are presented below which no longer demonstrate the presence of C.sub.F and/or C.sub.UCx carbon in the product of the oxidation reaction.
UC.sub.x+yC.sub.F+(4/3+x+y)O.sub.2.fwdarw.⅓U.sub.3O.sub.8+(x+y)CO.sub.2 x=1 to 2,y=1 to n  (4)
UC.sub.x+yC.sub.F+(z/2+x+y)O.sub.2.fwdarw.1UO.sub.z+(x+y)CO.sub.2 x=1 to 2,y=1 to n,z=2 to 3  (5)

(46) Optimization of the Oxidant Partial Pressure and of the Heat Given Off:

(47) The applicant has also demonstrated that the oxidant partial pressure and heat given off as a function of time can be optimized. For this, the effect of the O.sub.2 partial pressure was studied with regard to the behavior toward the oxidation of the UC.sub.x. A specific example is illustrated in FIG. 6, which represents the variation in weight as % (solid lines) and the heat flow given off (dotted lines) during the oxidation under isothermal conditions of the UC.sub.x for an application temperature of the process of 400° C. at 3 different [O.sub.2] partial concentrations ([O.sub.2]=6.7%, 10% and 21%).

(48) The results obtained show that the O.sub.2 partial pressure does not influence the range of application of the process: the variations in weight gain are identical and settle down around a mean final value Δm=+8%, whatever the O.sub.2 partial pressure applied. The result of this is that only the UC.sub.2 phase is oxidized to give the oxide form of U.sub.3O.sub.8 type. The excess graphite C.sub.F, for its part, is still present in the oxidized material, thus limiting the generation of carbon dioxide CO.sub.2 damaging for the post-treatment management of the gases of the process.

(49) The partial pressure simply plays a role in the kinetics of oxidation of the UC.sub.x and consequently for the treatment time of the process: at high concentration ([O.sub.2]=21%, this O.sub.2 partial pressure makes it possible to stabilize the UC.sub.2 phase of the UC.sub.x only after application of the process for 40 min whereas, at low concentration ([O.sub.2]=10%), the stabilization of the UC.sub.x reaches the threshold value Δm=+8% after 70 min.

(50) The O.sub.2 partial pressure also plays a role in the values measured for heat flow, which quantities are characteristic of the exothermicity given off during the reaction for the oxidation of the UC.sub.x to give U.sub.3O.sub.8; the maximum amount of heat given off is twice as great when the process for the stabilization of the UC.sub.x is carried out with an O.sub.2 partial concentration varying from 6.7% to 21%. As it is possible for this amount of instantaneous heat given off to negatively impact the process in the case where the increase in the local excess temperature might result in an increase in the overall oxidation temperature greater notably than the value T.sub.max (defined as being the temperature at which the oxidation of the excess carbon begins), it is essential to establish optimum experimental conditions which make it possible to find a compromise between rate of conversion and control of release of heat which may bring about a modification to the reactivity.

(51) Consequently, an O.sub.2 partial concentration in the vicinity of 10% thus makes it possible to optimize the time for conversion of the UC.sub.x into the oxide form while limiting the exothermicity given off related to this oxidation reaction.

(52) Optimization of the Temperature of the Stabilization Heat Treatment:

(53) The weight gains obtained at oxidation temperatures varying from 380 to 550° C. and the stabilization of these quantities around a threshold value Δm=[6,8]% define the robustness of the process with respect to the temperature for application of the treatment of the process, making possible the sole and controlled conversion of the UC.sub.2 phase of the UC.sub.x material into the oxide form of U.sub.3O.sub.8 type (with possible traces of UO.sub.2).

(54) FIG. 7 presents an isothermal network obtained around an optimum application temperature of the process of 400° C. The profiles obtained (produced under similar isothermal conditions to those obtained in FIG. 3) make it possible to test the robustness of the process by determining the maximum temperature which will result in the oxidation of the excess carbon in the thermogravimetric curves presented.

(55) It should be noted that the thermogravimetric curves obtained from oxidation temperatures greater than 550° C. (2 thermogravimetric curves obtained at T.sub.oxidation=575° C. and then 700° C. represented, for example, in FIG. 7) demonstrate a loss in weight which is increasingly great and decreasingly linear: they emphasize the gradual oxidation of the excess carbon C.sub.F, which becomes increasingly pronounced as a function of the increase in oxidation temperature.

(56) Optimization of the Process of the Invention by Addition of Water Vapor:

(57) The applicant has also studied the addition of water vapor before and during the isothermal cycle of the treatment of the process and has been able to demonstrate the following conclusions: an effect of the water vapor on the rate of conversion of the UC.sub.x material into UO.sub.x under an oxidizing atmosphere, whatever the time of the addition of water vapor (before or during the oxidative treatment); the possibility of using a new gaseous tracer H.sub.2 related to the reaction between the UC.sub.x and the H.sub.2O according to the reaction:
UC.sub.2+yC.sub.F+xH.sub.2O.fwdarw.UO.sub.x+xH.sub.2+yC.sub.F  (6) The presence of H.sub.2, measured at a concentration with a factor greater than 100 times lower than the CO.sub.2 given off during the oxidation of the UC.sub.x, can be used in the same way as the latter as factor of criterion for halting the satisfactory progression of the process, this criterion being achieved when the H.sub.2 release is less than a minimum threshold value; the acceleration in the chemical fragmentation of very dense materials and in the rate of oxidation of the UC.sub.x to give the oxide form (for example, a gain in time of 10 min was measured during an oxidative treatment under isothermal conditions carried out at 420° C.); the lowering in the amount of heat given off and consequently the excess temperature ΔT observed during the process and notably at the start of the exothermic oxidation reaction of the UC.sub.x to give U.sub.3O.sub.8 (decrease ET of 8% in the presence of water vapor).

(58) The applicant has also studied the effect of water vapor on the stabilization of the UC.sub.x by environmental scanning electron microscopy. The results of in situ oxidation under environmental electron microscopy at different oxidation temperatures and water vapor partial pressures have made it possible to demonstrate the appearance of localized cracks at the surface of the UC.sub.x. These cracks facilitate the interaction between O.sub.2 molecules and UC.sub.2 clusters which are not very accessible as they are present in the body within the UC.sub.x material. These cracks allow the O.sub.2 molecules to more readily reach into the body and to thus greatly improve the overall rate of conversion of the UC.sub.2 into the oxide phase. Post-mortem measurements by X-ray diffraction studies on tests of oxidation of UC.sub.x under environmental microscopy at different water vapor partial pressures P(H.sub.2O) have revealed the presence of UO.sub.2, U.sub.3O.sub.8 and excess carbon in the oxidized material.

(59) The use of a combination of O.sub.2/H.sub.2O reactant in the treatment of the process also makes it possible to involve two types of reaction (corrosion and oxidation) with change in molar volume of the products resulting from the oxidation of the UC.sub.2 phase (UO.sub.2 and U.sub.3O.sub.8 among them). The presence of these two oxides promotes the change in volume of the oxidized product and the appearance of interstitial stresses which result in the appearance of cracks which allow better accessibility of the O.sub.2 in contact with nonoxidized surfaces and a significant improvement in the kinetics of treatment.

(60) The addition of water vapor for the process is all the more relevant with regard to bulk and dense initial materials, the core of the material of which is difficult to access for molecular oxygen. The water vapor thus has an influence on the morphology of the initial material to be stabilized.

(61) Validation of the Process of the Invention for Different Types of Morphology of the Uranium Carbide Compound:

(62) The stabilization of the UC.sub.x targets was carried out at a stabilization temperature of 400° C. using two different geometrical forms: UC.sub.x powder (particle size of 150 μm) and an assembly of several UC.sub.x pellets stuck to one another (pellets φ=15 mm, t=1 mm, hydrostatic density=8, porosity>50%). The programming of the isothermal oxidation cycle and the change in weight of these two UC.sub.x geometrical forms during the oxidative treatment are represented in FIG. 8. More specifically, the curve C.sub.8a relates to the variation in weight in the case of pellets, the curve C.sub.8b relates to the variation in weight in the case of powder, the curve C.sub.8c relates to the change in the temperature with pellets and the curve C.sub.8d relates to the change in the temperature with powder.

(63) During these tests, an oxidation cycle under anisothermal conditions (rise to T.sub.oxidation=800° C. with gradient of 10° C./min) was also programmed after applying the process for 300 min in order: to determine the maximum temperature T.sub.max corresponding to the initiation of the oxidation of the excess carbon of the UC.sub.x material; to analyze the differences in weight loss of the excess carbon as a function of the morphological nature of the initial material.

(64) The results obtained thus show that the process of the present invention is: applicable for variable UC.sub.x materials of powder or pellet type as the weight gain of the UC.sub.x material (form or powder) during the oxidation tends toward a stationary state equal to Δm=7.6% in conformity with the achievement of a stabilized final product defined by U.sub.3O.sub.8+C.sub.F and confirmed by X-ray diffraction, XRD; optimum for an initial UC.sub.x material of “pellet” geometry as the reaction kinetics relating to the reaction for the oxidation of the UC.sub.x to give U.sub.3O.sub.8 are faster (stationary state Δm reached sooner) and less exothermic than in the case of a geometry of “powder” type (stationary state Δm reached more rapidly and local excess temperature ΔT with a lower and shorter amplitude); adjustable with regard to the treatment temperature of the process, whatever the geometrical nature of the UC.sub.x. This is because, for both scenarios, the temperature T.sub.max corresponding to the initiation of the oxidation of the excess carbon is identical and measured as being equal to 565° C. Experience thus shows that a maximum difference ΔT=T.sub.max−T.sub.oxidation is applicable in order to test, at the end of the reaction, the satisfactory progression of the process for the stabilization of the UC.sub.x to give the UO.sub.x form. The process can also be adjusted in both scenarios as the weight gain is identical during the oxidative treatment under isothermal conditions.

(65) The differences recorded for weight loss during the oxidation of the excess carbon show that, beyond a temperature T.sub.max, the application of the process does not make it possible to completely oxidize the excess carbon C.sub.F present in the initial UC.sub.x material, in particular if the latter has a geometry of “pellet” type. Nevertheless, on the basis of strictly geometrical comparison factors, if the temperature for application of the process has to be greater than the temperature T.sub.max (in particular for the test for the end of reaction), the use of a UC.sub.x material of “pellet” type at the expense of a geometry of “powder” type appears beneficial in the sense that the oxidation of the excess carbon is only partial, thus limiting the production of not insignificant amounts of CO/CO.sub.2 to be handled after application of the process.

Example of Operating Conditions for the Stabilization Process According to the Invention

(66) The initial UC.sub.x material, in the powder or centimeter-sized pellet form, is introduced inside a boat, itself placed inside an oxidizing furnace.

(67) A neutral gas, for example argon, is then introduced into the furnace and a heating cycle of 10° C./min is imposed until a set-point temperature, denoted T.sub.oxidation, in the vicinity of 400° C. is obtained.

(68) Once this temperature T.sub.oxidation has been reached, a stabilization stationary state of 30 min under argon is programmed.

(69) After this stabilization stationary state, reconstituted air, alone or diluted in argon, at an O.sub.2 content of 10% is suddenly introduced into the measurement device with a flow rate by volume of gas proportional to the initial amount of UC.sub.x.

(70) The oxidation of the UC.sub.x under isothermal conditions at a stabilization temperature T.sub.oxidation=400° C. then gets under way for a mean time of 5 h and a gas analysis system makes it possible to monitor, in real time, very particularly the consumption of O.sub.2 and the release of CO.sub.2 produced during the oxidation of the UC.sub.x to give U.sub.3O.sub.8.

(71) When the concentration of O.sub.2 reaches the set point imposed at the inlet of the process (preferably 10% as content by volume) and when the concentration of CO.sub.2 indicates a value of less than 100 ppm, a test of confirmation of recovery of reactivity is carried out. This test consists, for example, in increasing the temperature rapidly but in a controlled fashion above the set point, typically by ΔT=+50° C., and measuring the change in the O.sub.2 and the CO.sub.2 during this change in temperature. A variation in pressure and/or insertion of a water vapor partial pressure can also be envisaged as stress criteria/tests.

(72) In the absence of release of CO.sub.2 greater than a threshold value (100 ppm) and/or consumption of O.sub.2 during this test, cooling of the furnace is programmed under air (cooling of several tens of ° C./min).

(73) In the presence of release of CO.sub.2 and/or consumption of O.sub.2 during this test for the end of reaction, the stabilization of the UC.sub.x at a new temperature T.sub.oxidation+ΔT is continued as long as the amounts of CO.sub.2 are not less than the threshold value (100 ppm). An addition of water vapor to the oxidizing atmosphere can be envisaged in order to substantially accelerate the complete stabilization of the UC.sub.x in the UO.sub.x form. The presence of water vapor will also make it possible to monitor a new tracer, H.sub.2, which appears during the residual oxidation of UC.sub.x to give the oxide form. These temperature tests are carried out as long as the overall temperature imposed does not exceed a maximum value corresponding to the oxidation of the excess carbon present in the UC.sub.x material (T.sub.max in the vicinity of 560° C.). In the absence of new releases of gas, the furnace is cooled under conditions similar to those established in the case of the negative response to the test for the end of reaction.

(74) The oxidized residue, with the composition U.sub.3O.sub.8+C.sub.Free and in the powder final state, is then collected and packaged according to the standards of the outlet envisaged. A sample is also taken for analysis by X-ray diffraction, XRD.