FR2969264A1 - INSTALLATION COMPRISING A ROTATING OVEN VIROLE - Google Patents

Abstract

The invention proposes an installation (10) for the treatment of U02F2 powder by transforming it into U308 powder which comprises a cylindrical shell (28) rotatably mounted with respect to a fixed structure (22, 24), in which the powder of UO2F2 flows from upstream to downstream and is converted into U308 powder, means (30) for heating at least a first section (48, 54) of the shell (28) to a first temperature, means (30) ) for heating at least a second portion (50) of the shell (28) to a second temperature greater than the first temperature characterized in that said at least a second section (50) is made from a cylinder of at least 10 mm thick.

Description

INSTALLATION COMPRISING A ROTATING OVEN VIROLE. DESCRIPTION TECHNICAL FIELD The invention proposes a ferrule for a rotary kiln intended to be traversed by a powdery product and gases. The invention relates more particularly to a ferrule of a deconversion plant of a uranium hexafluoride gas (UF6) depleted of uranium sesquioxide (U308). STATE OF THE PRIOR ART A deconversion plant for a Uranium hexafluoride (UF6) gas depleted of uranium sesquioxide (U308) comprises a first fixed structure comprising a reactor and an inlet box in which the gas UF6 is converted to UO2F2 in powder form. The inlet box opens into a shell rotatably mounted around its axis, in which the UO2F2 powder is converted into uranium sesquioxide (U308) by adding a stream of water vapor and hydrogen circulating against the current and under the action of a strong heat. The ferrule opens into a second fixed structure called output box. During its use, the shell is subjected to numerous stresses, originating from the heating means of the shell, the powdery product, the UO2F2 2 reaction reaction in U3O8, water vapor, or mechanical means for prevent the product from clogging the ferrule. These many solicitations of various natures are exerted selectively on certain sections of the ferrule. These sections are therefore likely to deteriorate more pronounced than other sections. When only one section of the ferrule is damaged, the entire ferrule must be replaced and this replacement of the ferrule is relatively expensive, especially because of the high price of the material used to make the ferrule and also because the installation must be taken out of service and immobilized while the ferrule is being replaced.

According to a known embodiment of the shell, it is made from a single cylinder of about 6 millimeters thick and about 800 mm in diameter. The life of such a shell is about 2 years, that is to say that beyond this period of 2 years, at least one section of the ferrule is too damaged to allow use of the ferrule. ferrule. The aim of the invention is to propose an installation for the treatment of UO2F2 powder by transforming it into U3O8 powder which comprises a ferrule made in such a way that its lifetime is increased compared to a ferrule of the state of the technical.

DESCRIPTION OF THE INVENTION The invention relates to an installation for the treatment of UO2F2 powder by transforming it into U308 powder which comprises a cylindrical shell rotatably mounted with respect to a fixed structure, in which the UO2F2 powder circulates upstream. downstream and is converted into U308 powder, means for heating at least a first section of the ferrule at a first temperature, means for heating at least a second section of the shell to a second temperature greater than first temperature and means for injecting a gas stream comprising water vapor and hydrogen into a third section of the shell located downstream of the first section and the second section, such that said stream gas flow countercurrent to the powder flow, characterized in that said at least one second section is made from a cylinder 20 at least 10 mm thick. This second section is the section that is subject to the greatest number of constraints. Such an increase in thickness allows it to withstand these stresses for a period of time up to about four years, thereby increasing the overall life of the ferrule. Preferably, said third section is made from a cylinder at least 10 mm thick. Preferably, said at least one first section of the ferrule is made from a cylinder approximately 6 mm thick. Preferably, the shell comprises an upstream end section and a downstream section each of which is made from a cylinder at least 6 mm thick. Preferably, the third section comprises means for connecting the ferrule to means for producing vibrations on the ferrule. Preferably, each section of the ferrule is made of a high-nickel alloy with about 68% nickel, 16% molybdenum and 16% chromium.

Preferably, the shell comprises two first sections distributed on either side of said at least one second section. Preferably, the third section is located downstream of said at least one first section and at least a second section, according to the direction of flow of the powder. Preferably, two adjacent sections are joined together by welding and the filler material used is a high nickel content metal which comprises approximately 67.2% of nickel, 15.7% of molybdenum, 16.2% of chromium. and 0.4% iron. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the invention will appear on reading the following detailed description for the understanding of which reference will be made to the appended drawings, in which: FIG. 1 is a diagrammatic representation in axial section; a deconversion plant of uranium hexafluoride gas (UF6); - Figure 2 is a detail on a larger scale of the rotary furnace shown in Figure 1, showing the ferrule in place in the installation; - Figure 3 is a schematic representation of the ferrule of the installation shown in Figure 1, showing the different sections of the ferrule. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS For the description of the invention, the longitudinal and vertical orientations according to the reference L, V indicated in FIG. 1 will be adopted in a nonlimiting manner. In the description which follows, identical elements, similar or the like will be designated by the same reference numerals. Orientation from front to back will also be adopted as the longitudinal direction and from right to left with reference to figure 1.

FIG. 1 shows a plant 10 for treating uranium hexafluoride (UF6) to transform it into uranium sesquioxide (U308). The plant 10 comprises a deconversion reactor 12 in which the uranium hexafluoride 6 (UF6), water vapor (H2O) and nitrogen (N2) are injected through the inlets 14. The reactor 12 is heated to a temperature of about 300 ° C by a heating jacket 16 and the internal pressure is about 1080 mbar. In the reactor 12 UO2F2 is formed in the form of powder and falls under the effect of gravity at a lower end 18 of the reactor 12, and a gas (HF + H2O) is extracted in the upper part of the reactor 12 and is treated by a set of filters 20. The lower end 18 of the reactor 12 opens into a first fixed structure 22 of the installation commonly called input box. The inlet box 22 comprises a worm 24 which conveys the UO2F2 powder from the reactor 12 to a rotary kiln 26. The rotary kiln 26 comprises a shell 28 rotatably mounted about its main axis A which is generally inclined towards the downstream to facilitate the forward movement of the UO2F2 powder. Levers (not shown) are arranged inside the shell 28 to stir the product during the rotation of the shell 28. The oven 26 also comprises means 30 for heating the shell 28 and a device 36 for producing vibrations in the ferrule 28 to prevent the powder from sticking to the walls of the shell 28. Inside the shell, the UO2F2 powder from the reactor 12 is converted into uranium sesquioxide powder (U3O8) by adding 7 of water vapor and hydrogen and under strong heat. The shell 28 opens into a second fixed structure 32 commonly called outlet box, which has an outlet channel 34 for discharging the uranium sesquioxide powder (U3O8) formed. The outlet box 34 also comprises injection means 38 for steam and hydrogen which circulate in the shell 28 against the current relative to the direction of flow of the powder in the shell 28. The box Inlet 22 has a tubular portion 40 coaxial with the ferrule 28 which opens into the rear end 28a of the ferrule 28.

Similarly, the outlet box 32 has a tubular portion 42 coaxial with the shell 28 in which the front end 28b of the shell 28 opens. Finally, sealing means 44 are provided between each end 28a, 28b of the shell 28 and the tubular portions 40, 42 of the input box 22 and the outlet box 32. As can be seen in more detail in FIG. 2, the heating means 30 of the ferrule 28 are made so as to heat a central portion 46 of the shell 28. This central portion 46 of the ferrule 28 here comprises, from upstream to downstream in the direction of flow of the product in the shell 28, consecutive sections, which are intended to be heated to different temperatures. A first upstream section 48 is intended to be heated to a temperature of about 750 ° C, so that the temperature inside the shell at the first section 48 is about 700 ° C.

A second section 50 is to be heated to a temperature of about 900 ° C so that the temperature inside the shell at the second section 50 is about 780 ° C. A third section 54 is intended to be heated to a temperature of about 780 ° C, so that the temperature inside the shell at the third section 54 is about 600 ° C. Thus, the first section 48 and the third section 54 are intended to be heated substantially at the same temperature of about 750 ° C. to 780 ° C. and the second section 50 is intended to be heated to a temperature of about 900 ° C. . The shell 28 is traversed by powder flowing from upstream to downstream.

While circulating in the central part of the ferrule 28, the powder is heated and a chemical reaction takes place between the powder and the ambient medium which comprises in particular water vapor and hydrogen. The chemical reaction is called the pyrohydrolysis reaction. The water vapor (H2O) and the hydrogen (H2) are injected counter to the direction of circulation of the powder in the shell 28 by the injection means 38 which pass through the front end 28b of the shell.

The injection means 58 open into a section 56 of the shell 28 which is located between the central portion 46 of the shell 28 and the front end 28b. A pyrohydrolysis reaction between the UO2F2 powder and the injected gases also takes place in this section 56, which will be referred to as the pyrohydrolysis section. The ferrule 28 also has elements on its external wall for its connection with other components of the installation 10. The upstream section 28a of the ferrule 28 comprises means 58 for connecting the ferrule 28 with the box. input 22 comprising in particular tread supports for guiding the ferrule 28 15 in rotation relative to the input box 22 and a ring gear driving the ferrule 28 in rotation about its main axis A. The upstream section 28a also comprises a connecting member 60 of the shell 28 to a secondary shell 20 (not shown) which participates in the sealing of the installation, to prevent any outward leakage of powder, a mounting flange 64 a sealing member (not shown) between the ferrule 28 and the secondary ferrule. The downstream section 28b of the ferrule 28 comprises connecting means 59 of the shell 28 with the outlet box 32 mainly comprising tread supports for guiding the rotating ferrule. The downstream section 28b also comprises a connecting member 60 of the shell 28 to a secondary shell, a flange 64 for mounting a sealing member and a connecting strip 62 of the shell 28 to the device 36 for producing vibration. The length of the shell 28 is about 10 meters and the diameter of the ferrule is about 800 mm. During the operation of the installation 10, the shell 28 is subject to many constraints.

Some constraints are thermal stresses, they come from the fact that the sections 48, 50, 54 of the central portion 46 of the ferrule 28 are heated by the heating means 30 at different elevated temperatures and other sections of the ferrule 28. are not heated to such temperatures. The thermal stresses also come from the fact that during maintenance phases on the ferrule 28 or more generally in the installation 10, the installation 10 is put out of service and the ferrule 28 is cooled to allow intervention. Then, at the commissioning of the installation 10, the shell is heated again. These successive cooling and heating phases of ferrule 28 result in different expansions in ferrule 28 which induce stresses therein. Other constraints on the shell 28 are mechanical and come from the device 36 for producing vibrations.

Other constraints come from the pressure inside the ferrule, which is about 11 1050hPa in normal use and whose value can reach 2000hPa in exceptional cases of use. Other constraints come from the treatment of the product, in particular because the atmosphere inside the ferrule comprises fluorinated compounds resulting from the pyrohydrolysis reaction, which are aggressive with respect to the material of ferrule 28. D Other stresses on the inner wall of ferrule 28 arise from the friction of the UO2F2 powder which causes erosion or friction corrosion. The aforementioned constraints are exerted on ferrule 28 differently depending on the sections of ferrule 28. Thus, for example, the second section 50 undergoes significant thermal stresses due to the high temperature of the ferrule 28. which it is heated and physico-chemical constraints from the pyrohydrolysis reaction that occurs within this section. The pyrohydrolysis section 56 is also subjected to physicochemical stresses because of the pyrohydrolysis reaction that occurs inside this section and is subjected to mechanical stresses because it is this pyrohydrolysis section 56 which carries the connecting band 62 to the vibration generating device 36. The assembly of the ferrule 28 is subjected to the friction of the powder in such a way that each 12 section of the ferrule 28 undergoes the erosion-corrosion phenomenon. The sections that are subjected to the most important stresses, that is to say the second section 50 of the central portion 46, as well as the pyrohydrolysis section 56 are made from thicker cylinders than the cylinders according to the state of the art. Here, the second section 50 of the central portion 46 is made from a cylinder at least 10 mm thick. The pyrohydrolysis section 56 is also made from a cylinder at least 10 mm thick.

The other sections of the shell 28 are made from cylinders approximately 6 mm thick. The ferrule 28 is thus made of several sections having different thicknesses, the sections which are subjected to significant stresses have a thickness of at least 10 millimeters and the stretches subjected to minor stresses have a lesser thickness, of approximately 6 millimeters.

These thicknesses are defined to ensure a lifetime to the ferrule 28 of about four years, while limiting the total cost of the shell 28. In fact, the fact of making some sections with a thickness of about 6 millimeters allows to limit the amount of material used and therefore reduce their overall cost. In order to limit the internal wear of ferrule 28, which is caused by the stresses described above, all the sections of ferrule 28 are made of a high-nickel content alloy.

The alloy used is the alloy designated HASTELLOY C-4 marketed by Haynes International Inc. This alloy has about 68% nickel, 16% molybdenum and 16% chromium. The sections are joined together by welding their adjacent ends. The shape of the end edges of two sections of different thicknesses is an important parameter for the life of the ferrule 25.

For this, the end edges of the cylinders are machined so that they comprise regular chamfers of a complementary angle of 70 to 90 degrees and a gap of the edges to be welded by 2 mm. The desired welding position is vertical.

The filler metal used during the welding phase of the sections also has an importance. The filler metal used is also a metal with a high nickel content, it comprises approximately 67.2% of nickel, 15.7% of molybdenum, 16.2% of chromium, 0.4% of iron and 0, 4% other metals. The welding process used is the TIG (Tungsten Inert Gas) process using a non-fuse electrode. The welds are performed in a semi-automatic mode under a neutral atmosphere to prevent instant oxidation upon fusion of the welded metal and preserve the electrode. The inert gas is Argon with a purity of 99.95%.

The shape of the weld bead, the depth of penetration, the welding speed are the important parameters to have a good assembly of two sections. The number of passes is determined by the maximum pass width, limited to 2.5 times the diameter of the filler wire. The temperature between two passes must not exceed 150 ° C. The filler wire is pre-baked at 250 ° C to dehydrate the protective sheath.

Claims (9)

REVENDICATIONS1. Plant (10) for the treatment of UO2F2 powder by converting it into U308 powder which has a cylindrical shell (28) rotatably mounted with respect to a fixed structure (22, 24), wherein the UO2F2 powder is circulated by Upstream downstream and is converted into U308 powder, means (30) for heating at least a first section (48, 54) of the ferrule (28) to a first temperature, heating means (30) at least a second portion (50) of the ferrule (28) at a second temperature higher than the first temperature of the injection means (38) of a gas stream comprising steam and water; hydrogen in a third section (56) of the ferrule (28) located downstream of the first section (48, 54) and the second section (50), such that said flow of gas flows countercurrently with respect to the powder flow, characterized in that said at least one second section (50) is made from a 25 cylinder at least 10 mm thick. 2. Installation (10) according to the preceding claim, characterized in that said third section (56) is made from a cylinder of at least 10 mm thick. 16 3. Installation (10) according to claim 1 or 2, characterized in that said at least a first section (48, 54) of the ferrule (28) is made from a cylinder of about 6 mm thick . 4. Installation (10) according to any one of the preceding claims, characterized in that the shell (28) comprises an upstream end section (28a) and a downstream section (28b) each of which is made from a cylinder at least 6 mm thick. 5. Installation (10) according to any one of the preceding claims, characterized in that the third section (56) comprises connecting means (62) of the shell (28) to vibration generating means (36) on the ferrule (28). 6. Installation (10) according to any one of the preceding claims, characterized in that each section (48, 50, 54, 56) of the ferrule (28) is made of a high-nickel alloy having about 68% nickel, 16% molybdenum and 16% chromium. 7. Installation (10) according to any one of the preceding claims, characterized in that the shell (28) comprises two first sections (48, 54) distributed on either side of said at least a second section (5 0). .30 17 8. Installation (10) according to the preceding claim, characterized in that the third section (56) is located downstream of said at least a first section (48, 54) and said at least a second section (50), according to the sense flow of the powder. 9. Installation (10) according to any one of the preceding claims, characterized in that two adjacent sections (48, 50, 52, 54) are joined together by welding and the filler material used is a high-grade metal. made of nickel which comprises approximately 67.2% of nickel, 15.7% of molybdenum, 16.2% of chromium and 0.4% of iron.