Abstract:
A substance in a condensed state, for example a powdered solid, is in continuous movement in the longitudinal direction ( 6 ) of a furnace ( 4, 5 ). A reactive gas mixture is brought into contact with the substance in the condensed state. A plurality of samples of the gaseous mixture are removed at a plurality of reference points ( 14 ) spaced apart from one another along the longitudinal direction ( 6 ) of the furnace ( 4, 5 ); each of the gas samples is analyzed outside the furnace to determine the composition of the gas mixture and for each point ( 14 ), the extent of a chemical reaction between the condensed substance and the reactive gas mixture is deduced from the composition of the gas mixture at each of the reference points ( 14 ). In particular, the apparatus comprises a sampling and injection rod ( 10 ) introduced into the furnace ( 4, 5 ) and disposed in its longitudinal direction ( 6 ). The invention is of particular application to modeling a rotary furnace ( 4, 5 ) for converting uranium oxyfluoride into uranium oxides and for controlling the conversion reaction in the furnace ( 4, 5 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for determining the progress of a chemical reaction in a furnace and for controlling the reaction. In particular, the invention is applicable to the production of uranium dioxide powder used to manufacture nuclear fuel pellets. 
     BACKGROUND INFORMATION 
     In nuclear reactors, for example in pressurized water nuclear reactors, a fuel is used that can be constituted mainly of uranium oxide or of a mixture of uranium oxide and plutonium. 
     Nuclear fuel, which is enriched in fissile elements, for example in uranium-235 in the case of a fuel constituted by uranium oxide, is generally obtained by a process in which the final enrichment product is constituted by gaseous uranium hexafluoride UF 6 . 
     The UF 6  is then converted into uranium oxide by oxidation using steam, for example. 
     Current processes producing the best results for converting uranium hexafluoride into uranium oxide are dry direct conversion processes which are conducted in apparatus comprising, in succession, a reactor provided with means for introducing UF 6  and steam into a chamber of the reactor in which uranium oxyfluoride UO 2 F 2  is formed from the UF 6 , and a rotary tube furnace in which the solid powdered uranium oxyfluoride UO 2 F 2  is transformed into uranium oxide, the tube furnace being provided with heater means and means for introducing steam and hydrogen via the outlet portion of the rotary furnace as a counter-current to the powdered solid moving in the longitudinal direction of the furnace. 
     Uranium oxide powder principally constituted by the dioxide UO 2  is recovered from the outlet of the rotary furnace, that powder then being conditioned in a conditioning unit before being used to produce sintered nuclear fuel pellets. 
     The process for transforming uranium oxyfluoride UO 2 F 2  into uranium oxide is carried out by bringing a powdered solid into contact with a reactive gas mixture containing steam and hydrogen in particular. 
     Hydrofluoric acid HF is formed in the rotary furnace by oxidizing the sulfur oxyfluoride with steam. 
     The solid material circulating in the rotary furnace coming into contact with the reactive gas mixture is the seat of various chemical reactions that result in the formation of uranium oxide, and principally of the dioxide UO 2 ; in particular, the chemical reactions indicated below occur:
 
UO 2 F 2 +H 2 O→UO 3 +2HF
 
3UO 3 →U 3 O 8 +½O 2  
 
U 3 O 8 +2H 2 →3UO 2 +2H 2 O
 
     Thus, the composition of the continuously moving solid material in the furnace and the composition of the reactive gas mixture vary essentially in the longitudinal direction of the furnace along which the powdered solid material moves, with the gas mixture moving as a counter-current. 
     In order to control the chemical reactions in the furnace to the best possible extent, the heater elements of the furnace disposed at the periphery of the jacket of the rotary furnace are adjusted to obtain a regular temperature distribution in the longitudinal direction of the furnace. 
     However, that method of adjusting the temperature in the longitudinal axial direction of the furnace cannot effectively control the composition of the uranium oxides at the furnace outlet. 
     Adjusting the flow rates of the hydrogen and steam introduced via the outlet end portion of the rotary furnace cannot improve control of the conversion reactions in the furnace because the reactive gases are diluted in the furnace and because of the random nature of the distribution of the reactive gases obtained inside the furnace chamber. 
     Further, uranium dioxide UO 2  can be produced from the oxide U 3 O 8  via intermediate reactions during which different uranium oxides are obtained in accordance with the transformation sequence U 3 O 8 →U 3 O 7 →U 4 O 9 →UO 2 . 
     In general, no method is known for determining how the reactions between the reactive gas mixture and the oxyfluoride or uranium oxides moving along the rotary furnace are progressing in the longitudinal direction of movement of the substances inside the rotary furnace. Access to a graph of the progress of the reactions inside the rotary furnace would mean that the reactions could be manipulated to optimize the conversion process to obtain oxides with the desired composition at the furnace outlet. 
     In particular, in order to obtain green pellets with very high mechanical strength as measured by crush, microhardness or wear tests in which the uranium oxide powder is compressed, it has been observed that it is necessary to use oxide powders of a composition such that the atomic ratio of the oxygen over the uranium (O/U) is substantially higher than the ratios normally obtained with oxides from the outlet of a uranium oxyfluoride converting furnace, which oxides are constituted principally by uranium dioxide UO 2 . 
     To increase the O/U ratio, mixing certain proportions of particles of uranium dioxide UO 2  obtained by dry conversion with particles of an oxide such as U 3 O 8  has been proposed, for example. That method, which can increase the O/U ratio of the oxides used to produce fuel pellets, necessitates oxidizing the uranium oxide UO 2  under perfectly controlled conditions in order to obtain the oxide U 3 O 8 , and then forming a homogeneous mixture of UO 2  and U 3 O 8  particles. Thus, that method of producing uranium oxide powders is complex. 
     Currently, no method is known that can control a reaction from an accurate determination of the progress of a chemical conversion reaction in a furnace to obtain uranium oxides at the furnace outlet with the desired composition, and in particular uranium oxides with a high O/U ratio, i.e., oxides with a mean formula of the type UO 2+x , where x is relatively high (x in the range 0.03 to 0.7, preferably in the range 0.05 to 0.25). 
     More generally, when a chemical reaction is carried out between a substance in a condensed state, for example a solid powdered substance moving continuously in the longitudinal direction of a furnace, and a reactive gas mixture, no method is known for accurately determining the extent of the reaction at different points along the longitudinal direction of the furnace, and no method is known for controlling the reaction in the furnace from any such accurate determination. 
     An accurate determination of the extent of the reaction in the furnace must be carried out without opening the furnace and without risking the introduction of air into the furnace interior, since that would both completely falsify the measurements and analyses carried out, and would also modify the product being produced in the furnace. Thus, it is not possible to monitor the extent of the reactions by removing samples of the moving powdered substance at different points along the axis and inside the furnace chamber. 
     SUMMARY 
     The aim of the invention is to provide a method of determining the progress of at least one chemical reaction along the longitudinal direction of a furnace, the reaction taking place inside a chamber of the furnace between a reactive gas mixture and a substance in a condensed state, for example a powdered solid, in continuous movement in the longitudinal direction of the furnace, said method allowing the extent of the reaction and the progress of the chemical reaction in the longitudinal direction of the furnace to be determined in a precise and accurate manner, without needing to open the furnace and without modifying the progress of the chemical reaction while implementing the method. 
     To this end, a plurality of samples of the gas mixture are removed at a plurality of reference points spaced apart from one another along the longitudinal direction of the furnace, each of the gas samples is examined outside the furnace to determine the composition of the gas mixture, and the extent of the at least one chemical reaction is deduced from the composition of the gas mixture at each point. 
     The invention also provides a method of controlling at least one chemical reaction in a furnace to obtain a final product with a predetermined composition by using a prior determination of the progress of the chemical reaction in the longitudinal direction of the furnace to model the operation of the furnace. The chemical reaction is then controlled by injecting gases with a carefully selected composition and with a predetermined flow rate at least one point into the interior of the furnace chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention, reference is now made to the accompanying figures to illustrate carrying out the determination method and the control method of the invention, in the case of a rotary furnace for converting uranium oxyfluoride into uranium oxide. 
         FIG. 1  is a longitudinal section through a unit for converting uranium hexafluoride into uranium oxide, comprising a rotary furnace provided with a sampling and injection apparatus that enables the methods of the invention to be carried out. 
         FIG. 2  is a perspective view of the sampling and injection apparatus of the invention. 
         FIG. 3  is a view of a central support portion of a sampling rod of the apparatus shown in  FIG. 2 . 
         FIG. 4  is a front view of a flange of the sampling and injection apparatus shown in  FIG. 2 . 
         FIG. 5  is a cross-section on  5 - 5  of  FIG. 4 . 
         FIG. 6  is an axial section of a sampling and injection assembly of the apparatus shown in  FIG. 2 . 
         FIGS. 7A ,  7 B and  7 C are diagrammatic views in three different functional positions of an analysis and purge circuit disposed outside the rotary furnace and connected to the sampling and injection apparatus of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a unit for converting uranium hexafluoride into uranium oxide, the unit being generally designated by the reference numeral  1  and comprising a reactor  2  for converting uranium hexafluoride UF 6  into uranium oxyfluoride UO 2 F 2  by injecting gaseous hexafluoride UF 6 , steam, and nitrogen into the reactor  2 . 
     Uranium hexafluoride UF 6  is transformed into the oxyfluoride UO 2 F 2  by oxidation in steam in accordance with the reaction:
 
UF 6 +2H 2 O→UO 2 F 2 +4HF
 
     The uranium oxyfluoride UO 2 F 2  produced by the reaction is in the form of a powder that falls into the bottom of the reactor where this powder is taken up by a screw conveyor  3  for introduction into the inlet portion of a rotary furnace  4  inside which the uranium oxyfluoride powder is converted into uranium oxide. 
     The furnace  4  comprises a rotatably mounted tubular jacket  5  driven in rotation about a longitudinal axis  6 , which is inclined at an angle α to the horizontal plane so that the inlet end  5   a  of the rotary jacket of the furnace is disposed at a higher level than the outlet end  5   b.    
     Around the rotary furnace jacket inside an insulated chamber  7 , heater elements  8  are disposed that heat the internal volume of the rotary jacket  5  in which the uranium oxyfluoride is transformed into uranium oxide at high temperature in accordance with the following reactions:
 
UO 2 F 2 +H 2 O→UO 3 +2HF  (1)
 
3UO 3 →U 3 O 8 +½O 2   (2)
 
U 3 O 8 +2H 2 →3UO 2 +2H 2 O  (3)
 
½O 2 +H 2 →H 2 O  (4)
 
U 3 O 8 →U 3 O 7 →U 4 O 9 →UO 2   (5)
 
     The steam and hydrogen required to carry out the conversion inside the rotary furnace are introduced into the internal volume of the rotary jacket  5 , generally by an injection arrangement introduced via the outlet end  5   b  of the rotary jacket  5  of the furnace. 
     The uranium oxyfluoride powder UO 2 F 2  introduced via the screw conveyor  3  into the inlet end  5   a  of the rotary jacket  5  is transported towards the outlet end  5   b  during rotation of the rotary furnace by dint of the slope α of the longitudinal axis of said jacket  5 . Further, rotation of the furnace produces agitation and lifts the powder which then comes into intimate contact with the steam and hydrogen injected into the furnace jacket and which generally move as a counter-current to the movement of the powder inside the rotary jacket. 
     The end  5   b  of the rotary jacket  5  opens into a chamber of a unit  11  for recovering and conditioning the uranium oxide powder formed in the furnace by conversion of the uranium oxyfluoride powder. 
     The different gases formed inside the furnace chamber by the reactions transforming the oxyfluoride into uranium oxides and in particular hydrofluoric acid HF are recovered with excess hydrogen and steam which have not been used up in the chemical reactions occurring in the furnace. 
     As explained above, one of the inherent problems with carrying out a dry type conversion process in a unit such as unit  1  arises from the fact that in general, no information is available regarding the progress of the conversion inside the furnace jacket and on the extent of the different reactions occurring in the axial direction  6  of the rotary furnace. 
     As is explained below, accurate knowledge of the progress of the chemical reactions inside the rotary furnace enables the reactions to be controlled by injecting reactive gases and in particular hydrogen inside the rotary furnace at predetermined locations to obtain a uranium oxide mixture with general composition UO 2+x , where x is fixed at a pre-determined value, i.e., with an O/U ratio that is much higher than 2 and fixed at a well determined value. 
     Up to now, different means have been used to influence the chemical reactions inside the furnace, in particular by controlling the heater elements to obtain an optimal temperature distribution inside the furnace jacket along a longitudinal direction  6 , and by the use of baffles  13  fixed inside the furnace to encourage contact between the reactive gas mixture moving in the furnace and the powder during conversion. 
     Chemical reactions inside the rotary furnace may also be controlled by adjusting the flow rates of hydrogen and steam introduced via the outlet end  5   b  of the rotary jacket, generally via a fixed chamber of the uranium oxide powder recovery and conditioning unit. However, satisfactory control of the uranium oxides produced has never been achieved by adjusting the flow rates of the steam and hydrogen introduced via the outlet end of the furnace. 
     In accordance with the invention, the progress of the chemical reactions occurring inside the furnace is accurately determined using a rod  10  for sampling gas at different points distributed along the longitudinal direction  6  of the furnace, connected to a gas analyzer  12  outside the furnace. 
     The extent of the reactions listed above occurring inside the rotary furnace may be determined at each of the points in the furnace by measuring the proportion or partial pressure of the gases produced by the various reactions or involved in said reactions, in particular HF, H 2 , or H 2 O. In particular, the presence or absence of a gas at a reference point along the longitudinal direction  6  of the furnace shows whether a chemical conversion reaction is in progress or has been completed. 
     Before start-up of a conversion unit such as unit  1 , a first modeling operation is carried out that consists in causing the unit to function under its normal production conditions and in removing the gases present in the furnace from a plurality of sampling points distributed along the longitudinal direction  6  of the rotary furnace, then analyzing these gases outside the furnace. 
     The results of the gas analyses at different points in the furnace are used to construct a model representing the progress of the different chemical reactions occurring along the length of the furnace. 
     After this first stage of modeling the operation of the rotary furnace, the results of the modeling are used to determine the injections required at different points in the furnace, in particular hydrogen injection to obtain a powder UO 2+x  with an optimum O/U ratio from the furnace outlet. In general, the O/U ratio is intended to be in the range 2 to 2.7, preferably in the range 2.10 to 2.25. 
     It is possible to envisage producing oxides with an O/U ratio that is as high as 2.66, which has only been possible until now by mixing UO 2  and UO 3 O 8  oxides. 
     To remove and analyze the gases inside the furnace (and optionally to inject gas into the furnace production phase), a long rod  10  is used with a length that is at least equal to the axial length of the jacket  5  of the rotary furnace and which is introduced into the internal volume of the rotary jacket  5  via its outlet end  5   b  and is disposed along its longitudinal axial direction  6 . At one end of the furnace, the rod  10  is fixed to the rotary jacket of the furnace via an outer end portion to a fixed portion such as the wall of the recovery chamber of the oxide powder recovery and conditioning unit  11 . 
     The sampling rod is connected to one or more gas analyzers  12  at its end located outside the furnace and the chamber of unit  11 . 
     The rod  10  disposed in the axial longitudinal direction  6  of the rotary furnace  5  is completely free with respect to the rotary portions of the jacket  5  and in particular with respect to the baffles  13  occupying a portion of the cross section of the rotary jacket  5  of the furnace. 
     In the embodiment illustrated, the sampling rod  10  has ten sampling points  14  distributed along its length, to remove samples at ten reference points of the internal volume of the rotary jacket  5 , with a substantially constant spacing between two successive reference points. 
     Reference is now made to  FIG. 2  to describe the sampling rod  10  assembly of the invention. 
     The sampling rod  10  comprises a long central tube  15  on which there are fixed, in a coaxial manner with predetermined spacing in the axial direction of the central support tube  5 , support and guiding elements  16  and  16 ′ which are primarily cylindrical in shape, each comprising, on its outer surface, a set of recesses each intended to receive one sampling tube  18  of a sampling tube assembly disposed parallel to the longitudinal axis of the central tube  15 . 
     In the embodiment illustrated, each of the guiding and support elements  16  and  16 ′ comprises ten recesses around its periphery in the shape of rectilinear channels of cross section having a semi-circular portion, machined into the support element  16  or  16 ′. 
     Fixed on the central tube  15 , between two support elements  16  and equidistant from the two support elements  16 , there is an intermediate support element  16 ′ which is shorter in the axial direction than are the support elements  16 . 
     The support elements  16 ′ comprise a plurality of recesses of number and cross-section that are identical to those of the recesses in the longer support elements  16 . 
     One axial end of the central tube  15  of the rod  10  is fixed to a flange  17  for connecting the rod  10  to the fixed wall of the unit  11 , at the outlet from the rotary furnace. 
     The flange  17  has tapped openings for fixing the flange  17  and the rod  10  to the wall of the unit  11 , so that the rod  10  is in a position that is coaxial with the jacket  5  of the rotary furnace, i.e., with the axis of its central tube  15  along the longitudinal axis  6  of the jacket  5  of the rotary furnace. 
     The end of the central tube  15  opposite from the end connected to the flange  17  is fixed to a pad  19  for closing the end of the central tube  15  which is inside the furnace, in a position close to the inlet end  5   a  of the rotary jacket  5 . 
     The face of flange  17  that is opposite in the axial direction to the face thereof connected via an extension  21  to the central tube  15  of the rod is integral with a sampling assembly  22 . 
     Each small diameter sampling tube  18  passes through a portion of the flange  17  in the axial direction, then is bent at 90° in a radial direction for connection, at the outer periphery of the flange  17 , to an element  23  for connecting the tube  18  to an extension piece  18 ′ that places the tube  18  in communication with a valve housing  24 ′ of a valve  24  of the sampling apparatus  22 . 
     Each valve housing  24 ′ of a valve  24  of the sampling apparatus  22  is connected, via a curved connecting tube  18 ″, to a sampling chamber machined in the housing of the sampling apparatus  22 . 
     Each tube  18  may be placed in communication with the chamber of the sampling apparatus  22  via tubes  18 ′ and  18 ″ and a valve  24 ,  24 ′. 
     Each tube  18  extends in the axial direction of the rod  10  between the flange  17  and a sampling point  14  corresponding to a reference point inside the rotary furnace  4 , 5  at or near a short support and guiding element  16 ′. 
       FIG. 3A  illustrates the central tube  15  of the sampling rod  10  on which longer tube support and guiding elements  16  are fixed at regular intervals with shorter support and guiding elements  16 ′ interspersed between the longer support and guiding elements  16 . 
     A shorter guide element  16 ′ intended to receive the last tube  18  that carries out the sampling at the last sampling point  14  located near the inlet end  5   b  of the rotary jacket of the furnace is fixed after the last longer guide element  16  on the end portion of the central tube  15  of the sampling rod  10 , which is introduced into the rotary jacket of the furnace until it is close to the inlet end of the rotary jacket. 
     The end portion of the central tube  15  opposite to the end located near the inlet portion of the rotary jacket of the furnace has no support and guiding elements  16  or  16 ′ along a section having an end which is intended to receive the flange  17  for fixing the sampling rod on the fixed structure of the unit. 
     The end portion of the rod comprising the end section of the central tube  15  that is free of guiding elements  16  and  16 ′ is engaged through the chamber of the powder recovery and conditioning unit  11  and in the outlet portion of the rotary jacket  5  when the sampling rod  10  is fixed in its working position. In this portion of the sampling rod, the sampling tubes  18  located at the periphery of the central tube  15  are not fixed to the outer wall of the central tube  15 . 
       FIG. 3B  is a cross-section through a support and guiding element  16  comprising ten recesses  26  each for receiving one sampling tube  18  and formed in the shape of channels with semi-circular bases extending in the axial direction over the peripheral surface of the support and guiding element  16  with a primarily cylindrical shape. 
     The section of the short support and guiding elements  16 ′ is identical to the section of the longer support and guiding element  16  shown in  FIG. 3B . 
       FIGS. 4 and 5  illustrate the flange  17  on which one end of the central tube  15  of the rod  10  is fixed and which forms the connection between the second end portions of the sampling tubes  18  and the extensions  18 ′ of each of said sampling tubes. 
     One face of flange  17  in the axial direction comprises a projecting portion  17   a  on which the end of the central tube  15  is engaged and fixed by welding. 
     The sampling tubes  18  disposed about the central tube are each engaged in an opening passing through the axial direction of the flange  17  opening into a cylindrical cavity on the second face of the flange  17 . 
     The openings for the tubes  18  through the flange  17  are disposed in the form of a circular array in the central portion of the flange  17  around the projecting portion  17   a.    
     Tapped openings  25  for fixing the flange  17  onto a fixed portion of the conversion unit, for example on the wall of the chamber of the powder recovery and conditioning apparatus, are formed in the outer peripheral portion of the flange  17 . 
     The end portion of the sampling tubes  18  is bent at 90° to place it in a radial direction with respect to the flange  17  and is engaged in an opening in the flange  17  opening into a chamber  27  for connecting the end of the sampling tube  18  with an S-shaped extension  18 ′. 
     A connecting chamber  27  is provided for each of the ten sampling tubes  18 , around the outer periphery of the flange  17 , each of the chambers  27  being closed by a plug. 
     The radially bent portion of the tube  18  and S-shaped extension  18 ′ connect the sampling tubes surrounding the central tube  15  in a circular line of small diameter to the housings  24 ′ of valves  24  of the sampling apparatus  22  disposed in a circular zone of diameter greater than the diameter of the flange  17 . 
       FIG. 6  illustrates that the sampling apparatus  22  comprises a housing of right prismatic shape  28  on which is fixed, in a coaxial disposition, the extension  21  that is integral with a plug  29  intended to be engaged in and fixed into the cavity machined on the second face of the flange  17 . This provides the connection between the sampling apparatus  22  and the flange  17 . 
     The sampling apparatus  22  comprises ten valves  24  having housings  24 ′ which are fixed one after another in the circumferential direction around the housing  28 , which preferably has a prismatic shape and a decagonal cross-section. 
     On the face opposite to the face connected to the connecting extension  21  of the flange  17 , the housing  28  of the sampling apparatus  22  comprises a cavity  30  that is partially closed on the outer face of the housing  28  by an annular cover  31  with an internal bore that sealingly receives a tube  32  communicating with a connector  33  of the sampling apparatus. The cavity  30  has the smallest possible volume to reduce the inertia of the sampling system and to limit the risk of dilution of the mixture, and it constitutes a sampling chamber  34  with the internal space of the tube  32 . 
     Each of the valve housings  24 ′ of the valves  24  is connected to the sampling chamber  34  via a respective connection tube  18 ″. One end of each bent connection tube  18 ″ is connected to the housing  24 ′ of a valve  24  and its second end is connected to an opening in the cover  31  closing the cavity  30 . Each of the valve housings  24 ′ of a valve  24  comprises a first chamber to which an extension tube  18 ′ is connected and a second chamber to which a connection tube  18 ″ is connected. When the valve  24  is closed, the two chambers are separated by the valve obturator, which may be a bellows-sealed valve. On opening valve  24 , the two chambers of the valve housing and the sampling tube  18 ″ are placed in communication with the sampling chamber  34  via the connection tubes  18 ′ and  18 ″. 
     As may be seen in  FIGS. 7A ,  7 B and  7 C, the sampling chamber  34  is connected via the connector  33  to a gas purge, analysis, and injection circuit  35 . 
     The circuit  35  comprises at least one line  36  connecting the connector  33  of the sampling chamber  34  and at least one gas analyzer  37 , via a shut-off valve  38  and a three-port valve  39 . 
     The three-port valve  39  has one port connected to the sampling chamber  34  via the connector  33  and a second port connected to the analyzer  37  via the line  36 , and in a first position shown in  FIG. 7A , it may place the sampling chamber in communication with the gas analyzer  37  when the shut-off valve  38  is open. 
     The principal sampling line  36  is connected via the third port of the three-port valve  39  to a side line  41  connected to a reservoir  40  that may contain an inert purge gas such as nitrogen, argon, or helium, or a reactive gas such as hydrogen. 
     As illustrated in  FIG. 7B , when three-port valve  39  is placed in the position illustrated and the valve  38  is closed, analyzer  37  may be purged with purge gas from reservoir  40 . 
     When three-port valve  39  is placed in its position shown in  FIG. 7C  and stop valve  38  is open, a purge gas may be sent into at least one of sampling tubes  18  via sampling chamber  34 . 
     When reservoir  40  is a reservoir for a reactive gas such as hydrogen, the reactive gas may be sent into at least one sampling tube  18  via the sampling chamber  34 , the three-port valve  39  being in its position shown in  FIG. 7C  and the shut-off valve  38  being open. 
     By actuating both the valves  24  of the sampling and distribution apparatus  22 , which valves are preferably solenoid valves, and also the circuit valves  35 , it is possible to purge the gas analyzer, purge any sampling tube  18  or a plurality of sampling tubes, or inject the reactive gases such as hydrogen into the rotary furnace of the conversion unit, at any reference point  14  or at a plurality of reference points in the furnace chamber. 
     As explained below, the apparatus described may make it possible to remove samples of gas at a plurality of points distributed along the longitudinal direction of the furnace and to analyze gas samples under very good conditions, enabling a graph of the progress of the chemical reactions in the furnace to be produced and thus providing a model of the furnace for the production of uranium oxide. 
     From a model of the oxide production furnace, it is possible to determine the injections of reactive gases, in particular hydrogen, required to obtain uranium oxide with a mean composition UO 2+x  with an O/U ratio of a desired value at the furnace outlet. 
     The reactive gases may be injected at one or more reference points in the furnace chamber each corresponding to a sampling point  14  by opening one or more valves  24  of the sampling and injection apparatus  22 . 
     It is also possible to inject reactive gas at each reference point in the rotary furnace by opening all of the valves  24 . 
     To carry out the gas sampling and analysis phase in the rotary furnace, the analyzer  37  is first purged with an inert gas then a first sampling line is purged. The reactive gas at the corresponding reference point  14  in the furnace is then sampled using the sampling tube, which has previously been purged. The gas samples are sent to the gas analyzer  37  which provides the composition of the sampled reactive gas mixture, i.e., the concentration or partial pressure of gases such as hydrofluoric acid HF and/or steam in the mixture and/or hydrogen and/or nitrogen. 
     To carry out an analysis of a sample that is perfectly representative of the atmosphere in the furnace in the sampling zone under consideration, it is necessary to ensure that the gas mixture is not modified by an internal reaction by deposition of substances or by condensation of the sampled gas. 
     To prevent further chemical reaction in a gas sample between the sampling point and the analyzer, the gas mixture is filtered at the inlet to the sampling tube by passing it through a metal filter made of a material that is resistant to the atmosphere of the furnace, for example a nickel alloy, to stop any solid material particles that may be in suspension in the gas sample. This prevents the chemical reaction from continuing by removing one component. 
     To avoid any condensation or deposition in the removed gas sample, the portions of the sampling tubes that are located outside the furnace and connected to the analyzer are heated. The valves  24  of the sampling apparatus  22  are also heated. 
     It is possible to use a single analyzer and thus a single line for moving the gas samples between the sampling apparatus  22  and the analyzer  37  or, in contrast, a plurality of analyzers connected to the sampling apparatus  22 , each of the analyzers, for example, assaying one of the gases in the reactive gas mixture removed from the furnace. 
     Thus, in a first furnace modeling phase, the method and apparatus of the invention may remove gas samples from the furnace and analyze said gas samples, so that the result of the analyses is completely representative of the composition of the reactive gas mixture removed at the sampling points inside the furnace. 
     It is therefore possible to obtain a very accurate model of the rotary conversion furnace by determining the extent of the chemical conversion reactions at each of the furnace sampling points, more particularly the degree of completion of said chemical reactions. The model is in the form of graphs of the progress of the chemical reactions for forming oxides along the longitudinal direction of the furnace. 
     As an example, it is possible to monitor the concentration of HF or H 2  in the furnace in its longitudinal direction and to deduce the extent of the uranium oxyfluoride transformation reactions. 
     From the modeling of the furnace, it is possible to accurately determine the reference points in the furnace chamber where reactive gas such as hydrogen must be injected along with the rate at which it should be injected, and thus to steer the chemical reactions so as to obtain a uranium oxide with the desired mean composition at the furnace outlet. 
     This mean composition is then obtained without mixing oxide powders with different compositions, the powder leaving the furnace having the desired composition. 
     Further, injecting reactive gases directly into a zone of the furnace where said reactive gases are required may prevent a deficiency in the reactive gases, a disadvantage that arises when injecting reactive gases via the end of the rotary furnace. The reactive gases are not diluted by the furnace atmosphere and are introduced into the precise location where they have to be used, which limits the quantities of reactive gases used. Thus, the cost of producing uranium oxide is reduced. 
     The gases are removed from the furnace without using a suction or pumping arrangement because the pressure of the furnace exceeds that of the outer atmosphere. When a valve in the sampling apparatus is opened, the reactive gas then flows at high speed into the sampling tube  18  which is of section that is sufficiently small for the flow of the removed gas to remain very limited. In this way, samples are obtained of a composition that is entirely representative of the composition of the atmosphere in the furnace at the sampling points. 
     When converting uranium oxyfluoride into uranium oxide, assaying hydrogen in the samples may in particular allow determination of where and by how much the oxide UO 2  is being formed by reduction. It is then possible to influence the reactions to displace the point for total transformation into UO 2  to modify the composition of the oxides and the O/U ratio. 
     Assaying the hydrofluoric acid HF in the samples allows the uranium oxyfluoride to oxide transformation to be monitored. 
     The invention is not limited to the embodiment described. 
     Thus, it is possible to sample and inject gas in the furnace using arrangements other than those described. 
     It is possible to remove samples of the furnace atmosphere and to inject gas at any number of points. As an example, it is possible to implement the invention in a uranium oxyfluoride conversion furnace by injecting at five points of the ten sampling points. 
     The nature and flow rates of the injected gases depend on the nature of the chemical reactions occurring in the furnace and on the flow rates of the moving substances. 
     The invention is not limited to furnaces for converting uranium oxyfluoride into uranium oxide but may encompass applications in a plurality of furnaces in which a substance moves in its dense form, for example a powder, a paste or even a liquid form, which comes into contact with a reactive gas mixture and which therefore undergoes transformations in said furnace by chemical reactions. 
     The invention is also applicable outside the field of powder production for the manufacture of nuclear fuel.