Patent Publication Number: US-2002005597-A1

Title: Dry recycle process for recovering UO2 scrap material

Description:
[0001] The present invention relates generally to the production of fissionable nuclear fuel comprising oxides of enriched uranium for use in nuclear reactors. More particularly, the invention describes a dry recycle process for reclaiming UO 2  scrap material.  
       BACKGROUND OF THE INVENTION  
       [0002] Fissionable fuel grade uranium oxides for service in power generating nuclear reactors are commonly produced from uranium hexafluoride. Generally, uranium hexafluoride is converted to uranium oxides for reactor fuel using a “wet process” in which the conversion reactions are carried out within an aqueous medium or liquid phase with the reactants in solution and/or as a solid suspension therein. Typically, this wet process comprises hydrolyzing uranium hexafluoride (UF 6 ) in water to form the hydrolysis product uranyl fluoride (UO 2 F 2 ), adding ammonium hydroxide to the uranyl fluoride to precipitate the uranyl fluoride as solid ammonium diuranate ((NH 4 ) 2 U 2 O 7 ). The precipitate is dewatered and calcined in a reducing atmosphere to produce an oxide of uranium (e.g. UO 2 ). This version of the wet process is frequently referred to as the “ADU” procedure, since it normally entails the formation of ammonium diuranate.  
       [0003] The uranium oxides commercially produced by such conventional methods comprise a fine relatively porous powder which is not suitable for use as such as fuel in a nuclear reactor. Typically, it is not a free-flowing relatively uniform-sized powder, but rather clumps and agglomerates to form particles of varying sizes, making it unsuitable to uniformly pack into units of an appropriate and consistent density. In view of this, the raw uranium oxide product derived from the chemical conversion process is normally processed through conventional powder refining procedures, such as milling and particle classification to provide an appropriate sizing of the powders. Such processing frequently includes blending of uranium oxide powders of different particle sizes or ranges and from different sources. The resulting processed powders are then press-molded into “green” or unfired pellets, which are subsequently sintered to fuse the discrete powder particles thereof into an integrated body having a unit density of 95-97% of theoretical (“TD”) of the oxide of uranium. These pellets are more suitable for utilization in the fuel system of a nuclear reactor.  
       [0004] During the foregoing chemical conversion process, UO 2  scrap materials such as sintered pellets, grinder swarf, press scrap, and calciner powder are produced. These materials are conventionally recycled. Usually, scrap UO 2  materials from the production facility are oxidized in a high-temperature furnace to produce U 3 O 8 , which is then reacted with nitric acid to produce uranyl nitrate solutions. ADU is precipitated from these solutions by addition of ammonium hydroxide. The ADU precipitate may or may not be dried before processing through the calciner in a hydrogen-reducing environment to produce UO 2  powder. This UO 2  powder has a low sinter density, generally less than 10.60 gm/cm 3  or 96.6% TD. In addition, sintered pellets produced from this UO 2  powder have a high open porosity, non-uniform microstructure and poor production yields, i.e. radial cracks and end flakes.  
       [0005] The prior nitric acid process suffers from the disadvantages that a plurality of steps are involved, along with difficult handling operations, such as dissolution of U 3 O 8  in nitric acid following oxidation of UO 2 , precipitation with ammonium hydroxide to form ADU, centrifugation and clarification and handling of wet sludge and liquid waste streams. There is a need for a simpler and less expensive process for recycling material, in particular rejected UO 2  sintered material, grinder swarf, press scrap and calciner powder. The present invention seeks to fill that need.  
       SUMMARY OF THE INVENTION  
       [0006] The present inventors have now discovered that it is possible to simplify the recycling and production of UO 2  without dissolving UO 2  in nitric acid. It is possible according to the present invention to produce high density UO 2  from scrap uranium-containing material at lower temperatures and under substantially dry conditions. The resulting high sinter density UO 2  may be used directly in the fabrication of fuel pellets or may be mixed with virgin UO 2  to adjust the isotopic content of the mixture to a desired level.  
       [0007] In accordance with one aspect in the present invention, there is provided a process for producing high sinter density UO 2  powder from uranium-containing scrap powder material, comprising oxidizing the uranium-containing scrap powder at low temperatures to produce U 3 O 8 , and reducing the resulting U 3 O 8  to produce UO 2  powder with high sinter density and high surface area.  
       [0008] In accordance with another aspect, the invention provides high sinter density UO 2  produced by the process of the invention.  
       [0009] In accordance with a yet further aspect, there is provided a fuel pellet fabricated from high sinter density UO 2  obtained according to the process of the invention.  
       [0010] In accordance with another aspect of the present invention, there is provided a composition comprising virgin UO 2  and UO 2  powder produced according to the process of the invention.  
       [0011] In accordance with a yet another aspect of the present invention, there is provided a fuel pellet produced using UO 2  powder obtained according to the present process, and/or produced using a composition of the invention comprising UO 2  admixed with virgin UO 2 .  
       [0012] The present invention provides a simpler and less expensive dry process for producing high sinter density UO 2  powder which meets sinter density, densification and porosity requirements for use in boiling water reactors. A uniform microstructure is also obtained. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013] The invention will now be described in more detail with respect to the accompanying drawings, in which:  
     [0014]FIG. 1 is a process flow diagram of the process of the invention;  
     [0015]FIG. 2 shows the sintering characteristics of U 3 O 8 ; and  
     [0016] FIGS.  3 A-C show the pellet results obtained using UO 2  produced according to the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     [0017] The process of the invention is generally described in FIG. 1. In the first step, uranium-containing scrap material is oxidized to form U 3 O 8 . The term “uranium-containing scrap material” as used herein means uranium-containing material, most usually in the form of UO 2 , obtained from rejected sintered pellets, i.e. pellets not suitable for use in fuel cladding, grinder swarf (dust generated by a sintered pellet grinder), press scrap and calciner powder. Typically, the uranium-containing scrap material is in powder form and has a surface area ranging from about 5.0-6.5 m 2 /gm, typically 5.2-6.0 m 2 /gm. The sinter density of the rejected material is generally in the region of 96.5 to 98.5% TD, and typically averages about 97.3% TD.  
     [0018] The oxidation step is generally performed in a conventional production furnace using boats that are pushed through the furnace according to ways well known in the art, or a rotary calciner/kiln, or a laboratory furnace, at low temperature. Optimally, a rotary calciner/kiln is employed because of the mixing mechanism of the material (via baffles inside the heating chamber) and uniformity of the radial temperature (i.e. temperature through the bed of powder). The temperature distribution in the production furnace is generally not as uniform and the material is not as well mixed throughout the gas stream, resulting in less uniform oxidation.  
     [0019] The term “low temperature” as used herein in connection with the oxidation step means a temperature which is high enough to achieve oxidation of the material without giving rise to sintering of the material. The “low temperature” is typically in the range of 300-500° C., more usually less than about 400° C., for example about 300° C. to 380° C.  
     [0020] Calcination includes a hydrogen reduction step, wherein hydrogen gas is passed over the oxidized material to produce reduced UO 2 . The calcination may be carried out in the same furnace as used for the oxidation or in a different furnace Calcination is typically carried out at temperatures higher than those used in the oxidation step, i.e. less than 800° C., more usually about 600° C. to 725° C.  
     [0021] Calcination is followed by a “homogeneous blend” step wherein the enrichment of the reduced UO 2  is determined. The isotopic content is determined using conventional techniques known to persons skilled in this art. The desired uranium content may be achieved by addition of virgin UO 2 , ADU powder or additional reclaimed powder.  
     [0022] A “mill slug and granulation” step then typically performed according to conventional procedures. Optionally, a further enrichment blend is carried out to achieve the desired uranium level by the addition of ADU powder or reclaimed UO 2  powder.  
     [0023]FIG. 2 shows the sintering characteristics of U 3 O 8  after 1 hour of exposure at each temperature. As the temperature increases from 400-700° C., the specific surface area of the U 3 O 8  decreases from values in the region of 7-14 m 2 /gm to 2 m 2 /gm. Between 350 and 380° C., the amount of U 3 O 8  sintering is small, as indicated by the high surface area measurements (7-14 m 2 /gm). Within this low temperature range (350-380° C. ), sintering is a function of the particle size of the ADU starting material At 700° C., the basic U 3 O 8  particle size has increased, as indicated by the low specific area measurements (2 m 2  /gm). At high temperatures, the particle size effect of the ADU starting material is not observed. Because the hydrogen reduction process occurs at temperatures below 800° C., UO 2  sintering does not occur in the present process.  
     [0024] The U 3 O 8  produced according to the first step of the process is substantially free of sintered product. As used herein, the term “substantially free” as used in connection with the absence of sintered product in the U 3 O 8  and/or the UO 2  means that the product contains less than 5 wt % of sintered product, generally less than 1 wt %, more usually 0.05-01 wt %.  
     [0025] The oxidation step is carried out by passing air or oxygen though the furnace or calciner and in contact with the uranium-containing scrap material. Generally the air/oxygen flow is in the region of 5 cc/min to 15 cc/min, more usually about 10 cc/min.  
     [0026] The low temperature oxidation step results in the formation of U 3 O 8  powder having a surface area generally in the range of about 5.8-7.5 m 2 /gm, more usually about 6.2-7.0 m 2 /gm. The increase in the surface area of the U 3 O 8  over that of the starting UO 2  (which is typically about 5-6.5 m 2 /gm) is due to the different crystal structure of the U 3 O 8 .  
     [0027] The reduction (calcination) step may be carried out in the same furnace or in a different furnace. Reduction is carried out by passing hydrogen gas through the furnace and into contact with the heated U 3 O 8 . The hydrogen gas may optionally be mixed with nitrogen for safety reasons. The hydrogen flow rate through the calciner is typically in the region of 225 to 490 scfh, usually about 300 scfh. The nitrogen flow is usually set at a rate that provides about 50 volume percent.  
     [0028] The reduction is carried out at a higher temperature than the oxidation step. The expression “higher temperature” as used herein in connection with the reduction step means a temperature which is high enough to obtain reduction of the U 3 O 8  but not sufficiently high to cause sintering of the resulting UO 2 . UO 2  sinters in the region of 800° C. The higher temperature used in the reduction step is generally less than 800° C., more usually in the region of 600-725° C. The resulting UO 2  is substantially free of UO   2    sintered product. Some particle fusion is observed due to U 3 O 8  sintering.  
     [0029] Reduction of the U 3 O 8  produces high sinter density UO 2  powder having a lower surface area than the starting UO 2 . Generally, the surface area of the resulting UO 2  is in the range of 3.5 and 5 m 2 /gm, more usually 3.5 to 4.5 m 2 /gm, for example 4.4 to 4.6 m 2 /gm. The sinter density of the high density UO 2  ranges from about 98.4 to 99.0% TD, and typically averages about 98.6% TD.  
     [0030] Examples of surface areas obtained according to the present invention using two calciners (calciners 1 and 2) are set forth below in Tables 1A and 1B.  
               TABLE 1A                          Calciner 1                             Rejected UO 2  Powder   U 3 O 8  Powder       Temp.       Surface Area m 2 /gm   Surface Area m 2 /gm   Calciner   Profile       Average: 5.6 m 2 /gm   Average: 6.3 m 2 /gm   Zone   %               5.6   6.2   1   300-400       6.1   7.0   2   300-400       5.5   6.5   3   300-400       5.7   6.6   4   300-400       5.6   6.4   5   300-400       5.2   5.9   6   400-500           6.3   7   400-500           5.8   8   400-500           6.3   9   300-500                         TABLE 1B                                     UO 2  Powder   UO 2  Powder           Temp. Profile   Surface Area m 2 /gm       Calciner Zone   ° C.   Average: 4.4 m 2 /gm               1   600-675   4.6       2   610-700   4.4       3   610-700   4.9       4   625-700   4.6       5   625-725   4.5       6   625-700   4.1       7   625-700   4.4       8   625-700   4.3       9   430-480   4.0               4.2               3.9                  
 
     [0031] In the above examples, each calciner has nine temperature zones. temperatures of zones 6-9 in the calciner 1 are important in ensuring correct surface area and sinter density properties are obtained in the final UO 2  powder. In Table 1A, the temperature in zones 6-9 is higher than that in zones 1-5. This acts to decrease the surface area of the U 3 O 8  thereby achieving the desired surface area range (3.5 to 5 m 2 /gm) for UO 2 . A higher temperature will produce a lower surface area U 3 O 8  and thereby a lower surface area UO 2 . In this way, by controlling the surface area of the U 3 O 8  by careful control of the temperature profile of the calciner, it is possible to control the surface area of the UO 2 .  
     [0032] The reduction step in calciner 2 is carried out at a higher temperature than for the oxidation step. The temperature is increased to about 600-725° C. along the length of the calciner, with the temperature at the end of the calciner (zone 9) dropping to about 430-480° C. At no time is the temperature allowed to exceed 800° C. to avoid sintering of the UO 2 .  
     [0033] The increase in temperature in the reduction step results in the formation of UO 2  particles having a surface area in the region of about 3.5 to 5.0 m 2 /gm. The UO 2  has a high sinter density, generally about 98-99% TD, more usually in the range of 98.3 to 98.9% TD.  
     [0034] The UO 2  powder produced according to the present invention may be used directly in the fabrication of fuel pellets if the isotopic U 235  content meets production needs. The isotopic content is determined using conventional techniques. Depending on what the desired uranium content of powder is, the UO 2  produced according to the invention may be blended with any one of “virgin” UO 2 , ADU powder or additional reclaimed UO 2  powder, in proportions of up to about 50% by weight to achieve the desired uranium content.  
     [0035] As used herein, the term “virgin UO 2 ” means UO 2  obtained by the hydrolysis of uranium hexafluoride followed by defluorination, and hydrogen reduction to produce UO 2  using a number of conventional processes.  
     [0036] The UO 2  produced according to the process of the invention may be subjected to milling, slugging, and granulation, according to conventional techniques. This may be performed prior to mixing with virgin UO 2 /ADU/reclaimed powder and may also be performed thereafter, as illustrated in FIG. 1. The resulting UO 2  powder is the pressed to form “green” fuel pellets using techniques well known in the art.  
     [0037] Recycle material contains a wide range of enrichments, i.e. percentage of U 235  isotope. Consequently, the material may be blended to produce a homogeneous batch of material to establish an average value. A portion of this homogeneous blend is then combined with virgin UO 2  to produce a number of different enrichment blends required production.  
     [0038] Referring to Table 2, there is shown the effect on particle size of times and temperatures on 9 samples of uranium-containing material.  
               TABLE 2                          Oxidation                                                 Oxidation   Oxidation   Particle                   Description   Temperature   Time   Size       Sample   Starting Material   ° C.   hrs   microns   Color   Phase               1   UO 2  Powder           1.7   Brown   UO 2             Base Case           1.5       2   UO 2  Powder   376       1.3   Brown   %-U 3 O 8                 400   5           Single                   4       3   UO 2  Powder   600       2.1   Green   %-U 3 O 8                 700   2           Single                   2       4   UO 2  Powder   376       2.7   Black   %-U 3 O 8  +               400   5           %-U 3 O 8                 600               700   4               900                   2                   1       5   UO 2  Powder   1000        9.1   Black   %-U 3 O 8  +                   4           U 3 O 8         6   UO 2  Powder   1000        6.6   Black               Milled   4                   30 mins       7   Sintered Pellet   375       4.2   Black   %-U 3 O 8                     3       8   Sintered Pellet   Milled   15 mins       Brown   UO 2         9   Sintered Pellet   Milled   15 mins               376           Green   %-U 3 O 8  +                   10           UO 2  (OH) 2                    
 
     [0039] As shown in Table 2, UO 2  exposed within the temperature range of 375 and 400° C. for nine hours shows a smaller particle size than the initial material. When higher temperatures are used in the region of 600-700° C., the size increases significantly, even if the oxidation time is less. The residence time in the production furnace for sintered material ranges from about 2-3 hours, averaging about 2.7 hours. In the rotary calciner/kiln, the powder in the production unit has a residence time of 90 minutes. These results indicate that temperature plays a more important role in the sintering of U 3 O 8  than residence time.  
     [0040] The particle size in Table 2 was determined using light scattering measurements. Consequently, these measurements determine the actual size of the sintered particle which is composed of many small particles. The surface area (m 2  m 2 /gm) discussed above is inversely proportional to particle size, and is more representative of the basic particle size that has sintered or fused together to form the sintered particles measured by light scattering. Based on the results shown in FIG. 2 and Table 2, it appears that as the oxidation temperature increases, both the basic particle size and the number of particles that sinter together increase.  
     [0041] By careful choice of the oxidizing conditions, it is possible, according to the present invention, to minimize particle size and maximize specific surface area of the powder without causing sintering of U 3 O 8  or UO 2 .  
     [0042] Following reduction to produce the high density UO 2 , the powder is milled, slugged and granulated. The UO 2  powder is then blended, if desired, to achieve the desired isotopic content. This blended UO 2  powder is incrementally added to virgin UO 2 , reclaimed powder, or mixed blends of virgin UO 2  and reclaimed powder, in proportions of up to 25 wt % with sintered scrap material and up to 50 wt % with powder scrap material.  
     [0043] Two additional steps may if desired be performed, namely mill, slug and granulation and homogenous blending. These operations improve the ceramic quality of the powder by raising the sintered density and improving the microblending of the powder.  
     [0044] The invention permits improved production rates of recycled UO 2 . It has been found that rejected UO 2  powder can be recycled at rates up to 60 to 80 kgs/hr using the process of the invention.  
     EXAMPLES  
     [0045] The invention will now be further described with reference to the following examples.  
     Example 1  
     [0046] Results of Dry Recycle Process  
     [0047] Oxidation of recycle sintered material at temperatures between 350° C. to 400° C. was performed according to the invention to give U 3 O 8 . The particles sizes were determined using a microtrac particle analyzer. This analyzer utilizes a dual system to measure both forward-scattered and side-scattered light. Particle sizes between 0.12 and 42.2 microns were obtained.  
     [0048] The oxidized sintered material in the form of U 3 O 8  had the following particle size distribution.  
                           TABLE                                      Average Particle Size               (Microns)   90% less (Microns)                       3.5   7.2           4.0   8.3                      
 
     [0049] Hydrogen reduction of U 3 O 8  was performed in a calciner to form UO 2  at the lower temperature profile than with virgin UO 2  as shown below. The data presented below was obtained using an ADU feed. However, the process is not limited to ADU as a feed, and works with powder obtained from a nitrate process or a process that reacts UF 6  with steam (referred to as the “dry conversion process”).  
                       TABLE                           ADU Feed   Dry Recycle Feed       Zone   Temperature ° C.   Temperature ° C.                  1   750   375       2   765   375       3   750   400       4   770   500       5   770   550       6   750   635       7   700   550       8   700   500                  
 
     [0050]                           TABLE                                   Average Particle Size   90% Less than           (Microns)   (Microns)                                                    5.4   9.8           4.7   8.8           5.2   9.4           5.1   10.0                        
     [0051] The UO 2  particles are larger than the U 3 O 8  particles, indicating some sintering of U 3 O 8  occurred during hydrogen reduction calcination Process, and also prior to complete reduction to UO 2 . Blending of the reclaimed material in different proportions with virgin ADU material provides good microstructure and sinter density.  
     Example 2  
     [0052] UO 2  powder produced according to the invention was added to ADU, reclaimed (UCON), and crossblends of ADU and reclaimed (UCON) powder. UCON powder is a powder produced by oxidizing rejected material and dissolving in nitric acid, followed by ADU precipitation (see U.S. Pat. No. 5,514,06 incorporated herein by reference). UCON powder also includes scrap material that is oxidized, dissolved in nitric acid, purified by solvent extraction to produce uranyl nitrate, followed by ADU precipitation. Solvent extraction removes a significant amount of cation and anion impurities.  
     [0053] The following proportions were investigated:  
     [0054] 1, 2, 3, 5, 10, 20 and 25 wt %.  
     [0055]FIG. 3 shows the results. It will be seen that all percentages gave porosities that were well below the specification of 0.25%. The pellet densification was also well below the specification requirements. Initial tests using dry conversion powder also gave good results. Reclaim (UCON) and crossblends of ADU and reclaim (UCON) met the sinter density specification with up to 25% of the dry recycled powder. ADU powder was limited to 3% of the recycled material.  
     [0056] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.