Abstract:
A method of preparing an actinide nitride fuel for nuclear reactors is provided. The method comprises the steps of a) providing at least one actinide oxide and optionally zirconium oxide; b) mixing the oxide with a source of hydrogen fluoride for a period of time and at a temperature sufficient to convert the oxide to a fluoride salt; c) heating the fluoride salt to remove water; d) heating the fluoride salt in a nitrogen atmosphere for a period of time and at a temperature sufficient to convert the fluorides to nitrides; and e) heating the nitrides under vacuum and/or inert atmosphere for a period of time sufficient to convert the nitrides to mononitrides.

Description:
GOVERNMENT INTEREST 
     The United States Government, as represented by the Department of Energy, has a paid-up license in the invention described and may manufacture and/or use this invention for governmental purposes in accordance with the terms of Contract No. DE-AC52-06NA25396. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a low temperature process for preparing actinide nitrides, for use as fuel in nuclear reactors. 
     BACKGROUND OF THE INVENTION 
     With the increasing demand for energy and the pressing issues associated with CO2 emissions, nuclear power is once again becoming an attractive alternative for electrical production. The generation-IV reactors which are under development will run at much higher temperatures and with a much greater burn-up of the fuel than the current generation of nuclear reactors. A consequence of this higher working temperature and greater burn-up is a need for new fuel formulations. 
     There are many materials that may fit the requirements of the generation-IV reactors. Oxides are relatively simple to produce and are the current materials of choice for conventional reactors. Unfortunately, oxide fuels have several inherent limitations, which include a relatively low fissile density that reduces the breeding ratio and poor thermal conductivity that restricts the linear heating rate. Of the alternatives, nitrides represent the best combination of properties with the potential to solve these problems. 
     Uranium nitride has many favorable fuel properties, such as a high fissile density, a high melting point similar to that of oxide fuel and high thermal conductivity similar to that of metal fuel. While uranium nitride (UN) has many of the properties that would make it an excellent reactor fuel, it has failed to make the leap to practical systems due to the difficulty in its synthesis. In particular, the inclusion of carbon from the currently favored carbothermic reduction routes to UN is a major issue in the production of nitride fuels. The carbothermic synthesis relies on the conversion of the uranium carbide to the nitride at high temperatures. The high temperatures required for the UN production have unfortunate side effects in that the low vapor-pressure actinides, particularly americium, become volatile leading to serious contamination issues. 
     GB 1,186,630 discloses a process for the production of uranium nitride, plutonium nitride, or mixtures thereof, by reaction of uranium or plutonium tetrafluorides or trifluorides by high-temperature ammonolysis into the corresponding higher nitride or mixture of higher nitrides, which is then dissociated in vacuo into the corresponding mononitride or mixture of mononitrides. Optionally, zirconium nitride can be included in the admixture. 
     U.S. Pat. No. 3,953,355 discloses a process for the preparation of actinide nitrides from massive actinide metal, massive being a single piece of metal having a mass of 0.1 kg or more. The process involves partially hydriding the massive metal and simultaneously dehydriding and nitriding the dehydrided portion. The process is repeated until all of the massive metal is converted to a nitride. 
     U.S. Pat. No. 5,128,112 discloses a process of preparing an actinide nitride, phosphide, sulfide or oxide, by admixing an actinide organometallic precursor with a suitable solvent and a protic Lewis base selected from ammonia, phosphine, hydrogen sulfide and water, and heating the mixture until the actinide compound is formed. 
     B. N. Wani et al. report in  Fluorination of Oxides of Uranium and Thorium by Ammonium Hydrogen Fluoride. J. Fluorine Chem.  44 (1989) 177-185, that UO 2 , U 3 O 8  and ThO 2  were fluorinated by NH 4 HF 2  at room temperature to produce (NH 4 ) 4 UF 8 ·2H 2 O, (NH 4 ) 3 UO 2 F 5 ·H 2 O and (NH 4 ) 4 ThF 8 ·2H 2 O, respectively. 
     There remains a need to provide low temperature methods of preparing actinide nitrides for use as nuclear fuels. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a method of preparing an actinide nitride fuel for nuclear reactors comprising the steps of: a) providing at least one actinide oxide and optionally zirconium oxide; b) mixing the oxide or oxides with a source of hydrogen fluoride for a period of time and at a temperature sufficient to convert the oxide to a fluoride salt; c) heating the fluoride salt to remove water; d) further heating the fluoride salt in a nitrogen atmosphere for a period of time and at a temperature sufficient to convert the fluoride salt to nitride; and e) heating the nitride under vacuum and/or inert atmosphere for a period of time and at a temperature sufficient to convert the nitride to mononitride. 
     The present invention provides a process for producing actinide nitrides such as uranium nitride at temperatures of 1100° C. or less without carbon contamination. The method utilizes the simple conversion of oxides to fluorides and their subsequent conversion to nitrides. The process does not rely on any significant change in oxidation state of the uranium and hence does not employ carbothermic reduction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is further illustrated by the following drawing in which: 
         FIG. 1  is a line graph of x-ray diffraction data for UN 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word “about,” even if the term does not expressly appear. Any numerical range recited herein is intended to include all sub-ranges subsumed therein. Plural encompasses singular and vice versa. 
     Actinide series elements are the fifteen chemical elements that lie between actinium and lawrencium on the periodic table, with atomic numbers 89-103. Suitable actinides for use in nuclear fuels include, for example, thorium, uranium, plutonium, neptunium, americium and curium. Mixtures of two or more actinide oxides are preferred, such as mixtures of plutonium and uranium. Mixtures of three or more actinides can also be used, such as mixtures of plutonium, uranium and thorium. Especially preferred are mixtures of oxides of thorium, uranium, plutonium, neptunium, americium and curium; plutonium, neptunium, americium and curium; and uranium, plutonium, neptunium, americium and curium. In any of the above mixtures, the relative amount of each actinide may vary from 0.01-100% in the mixture. 
     Optionally, zirconium oxide can be added to the actinide oxide in amounts known in the art. Zirconium oxide is a well known stabilizer for nuclear fuels and is also used for cladding the fuel pellets. As used throughout the specification and claims, “oxide” or “oxides” refers to the at least one actinide oxide, which is optionally mixed with zirconium oxide. The process of the present invention thus produces at least one actinide mononitride, which is optionally mixed with zirconium mononitride. 
     The actinide oxide or oxides are mixed with a source of hydrogen fluoride for a period of time and at a temperature sufficient to convert the oxide to a fluoride salt, according to the following reaction (where “Act” denotes actinide):
 
ActO 2 +4NH 4 HF 2 →(NH 4 ) 4 ActF 8 ·2 H 2 O  (1)
 
     Suitable sources of hydrogen fluoride include, for example, ammonium bifluoride, ammonium fluoride and combinations of these. Preferably, the ammonium bifluoride and/or ammonium fluoride is enriched to at least 50%  15 N, with higher levels more preferred. The reaction of the oxide with hydrogen fluoride can be carried out at ambient temperature, at temperatures between 20°-30° C. As the temperature is increased to about 50° C. the salt is dried and water is driven off:
 
(NH 4 ) 4 ActF 8 ·2 H 2 O→(NH 4 ) 4 ActF 8 +2 H 2 O  (2)
 
     Following removal of water, fluoride salt is further heated in a nitrogen atmosphere for a period of time and at a temperature sufficient to convert the fluoride salt to nitride:
 
(NH 4 ) 4 ActF 8 +2NH 3 →ActN 2 +4NH 4 F+H 2   (3)
 
     Preferably, the nitrogen atmosphere is ammonia. 100% ammonia is preferable for rate optimization, however ammonia as a percentage in any inert carrier gas will work. The temperature used in this step of the process is between 600°-1000° C., more preferably 750°-850° C., and the period of time sufficient to complete the reaction is between 15 minutes to 3 hours, more preferably 30 minutes to 2 hours. 
     After conversion to the nitride, the at least one actinide nitride (and optionally zirconium nitride) is heated under vacuum and/or inert atmosphere for a period of time and at a temperature sufficient to convert the actinide nitride to a mononitride:
 
ActN 2 →ActN+½N 2 +H 2 +2H 2 O  (4)
 
     This is accomplished at temperatures between 1000°-1300° C., more preferably at temperatures between 1050°-1200° C. Complete 100% conversion to the mononitride occurs within ten hours, usually within 3-5 hours. 
     EXAMPLE 
     UO 2  was reacted with NH 4 HF 2  to give (NH 4 ) 4 UF 8  on a 50 g scale with a 10% excess of NH 4 HF 2 . Analysis of the bright green material indicates that it was mainly (NH 4 ) 4 UF 8  with small amounts of α-(NH 4 ) 2 UF 6  and γ-(NH 4 ) 2 UF 6 . 
     By washing the green material with large amounts of water, the reaction was completed and (NH 4 ) 4 UF 8 ·2H 2 O was obtained as the exclusive product, with no impurities detected. The hydrate was dried at 50° C. to give (NH 4 ) 4 UF 8  in quantitative yield (100% yield). 
     When the (NH 4 ) 4 UF 8  was heated to 800° C. under NH 3 , a quantitative conversion from (NH 4 ) 4 UF 8  to a uranium nitride, which analyzed as UN 2 , occurred. 
     For the UN 2 , x-ray diffraction patterns (shown in  FIG. 1 ) can be indexed as cubic, with a=5.305034(30) Å. The Rietveld Refinement (see Table 3, below) confirmed the product as UN 2  with a small impurity (0.7%) consisting of UO 2 . While UN 2  has been previously characterized, it is one of the rarer uranium nitrides with very few reports describing its preparation or properties. 
     The EXAFS spectrum also confirmed the known UN 2  structure and lattice parameters and are presented in Table 2. 
     An examination of the thermal stability of UN 2  using thermogravimetric analysis (TGA) indicated that a significant weight loss occurs at 1000° C. Heating the UN 2  to 1100° C. resulted in the complete conversion to UN within 2 hours. The materials obtained for the thermal decomposition of UN 2  was cubic UN. The EXAFS spectrum fits the x-ray data and confirms the presence of the simple UN. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Experimental Powder X-ray Diffraction Pattern of Uranium Dinitride 
               
             
          
           
               
                   
                   
                   
                 calculated 
                 experimental 
               
               
                 hkl 
                 d(A) 
                 20 
                 intensity 
                 intensity 
               
               
                   
               
             
          
           
               
                 111 
                 3.06379(2) 
                 29.12313 
                 100 
                 100 
               
               
                 002 
                 2.65332 
                 33.75363 
                 37.2 
                 44.68 
               
               
                 022 
                 1.87618 
                 48.48097 
                 38.2 
                 40.16 
               
               
                 311 
                 1.60001 
                 57.55807 
                 34.2 
                 27.57 
               
               
                 222 
                 1.53189 
                 60.37619 
                 7.8 
                 6.92 
               
               
                 004 
                 1.32666 
                 70.98974 
                 4.7 
                 4.75 
               
               
                 331 
                 1.21743 
                 78.50319 
                 11.1 
                 10.22 
               
               
                 042 
                 1.1866 
                 80.95721 
                 8.4 
                 8.59 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Comparison of XRD and EXAFS Spectroscopy 
               
               
                 Results on Uranium Dinitride 
               
             
          
           
               
                   
                   
                 EXAFS 
                   
                 XRD* 
                   
               
               
                   
                 Bond 
                 R ± 0.02 Å 
                 C.N ± 20% 
                 R ± 0.002 Å 
                 C.N. 
               
               
                   
                   
               
             
          
           
               
                   
                 U-N1 
                 2.28 
                 7.5 
                 2.299 
                 8 
               
               
                   
                 U-U1 
                 3.73 
                 12 
                 3.754 
                 12 
               
               
                   
                 U-N2 
                 4.38 
                 24 
                 4.402 
                 24 
               
               
                   
                   
               
             
          
         
       
     
                                                                                                                                                                                                           TABLE 3               Rietveld Refinement, Experimental versus Calculated Fit                                R-Values            Rexp: 0.83   Rwp: 9.19   Rp: 7.10   GOF: 11.07       Rexp′: 1.41   Rwp′: 15.58   Rp′: 16.25   DW: 0.45                    Quantitative Analysis—Rietveld            Phase 1: “LaB6 SRM 660a”   50.231(91)%       Phase 2: “UN2 Fm-3m”   49.380(91)%       Phase 3: “Uraninite C”   0.389(78)%               Background       Chebychev polynomial,   9884(14)       Coefficient 0       1   −6544(24)       2   3848(21)       3   −1595(21)       4   710(17)       5   −179(16)               Instrument            Primary radius (mm)   240       Secondary radius (mm)   240       Receiving slit width (mm)   0.0809(99)       Divergence angle (°)   1.000(16)       Full Axial Convolution        Filament Length (mm)   10        Sample Length (mm)   20(11)        Receiving Slit Length (mm)   30(15)        Primary Sollers (°)   2.3        Secondary Sollers (°)   2.3       Tube_Tails             Source Width (mm)   0.092(55)        Z1 (mm)   −0.04(38)        Z2 (mm)   0.345(31)        Fraction   0.121(43)               Corrections            Zero Error   0.0516(20)       Specimen displacement   0.0690(45)       LP Factor   0               Structure 1            Phase name   LaB6 SRM 660a       R-Bragg   2.240       Spacegroup   Pm-3m       Scale   0.07313(20)       Cell Mass   203.778       Cell Volume (A{circumflex over ( )}3)   71.83047       Wt %—Rietveld   50.231(91)       Crystallie Size        Cry Size Lorentzian (nm)   307.8(34)       Crystal Linear Absorption Coeff. (1/cm)   1124.705       Crystal Density (g/cm{circumflex over ( )}3)   4.711               PVII peak type            FWHM = a + b/Cos(Th) + c Tan(Th)              a   0.0013(86)         b   0.0009(91)         c   0.0068(33)            Exponent m = 0.6 + ma + mb/Cos(Th) + mc/Tan(Th)              ma   20(570)         mb   0(430)         mc   5(79)               Lattice parameters        a(Å)   4.1569000            Site  Np  x    y      z  Atom Occ  Beq               La1  1  0.00000  0.00000  0.00000  La 1  0.7038       B1   6  0.19587  0.50000  0.50000  B 1  0.65(13)               Structure 2            Phase name   UN2 Fm-3m       R-Bragg,   6.256       Spacegroup   Fm-3m       Scale   0.006623(12)       Cell Mass   1064.164       Cell Volume (Å{circumflex over ( )}3)   149.3016(26)       Wt %—Rietveld   49.380(91)               Crystallite Size        Cry Size Lorentzian (nm)   207.1(16)       Crystal Linear Absorption Coeff. (1/cm)   3267.166(56)       Crystal Density (g/cm{circumflex over ( )}3)   11.83568(20)                    PVII peak type       FWHM = a + b/Cos(Th) + c Tan(Th)              a   0.0001(60)         b   0.0001(66)         c   0.0299(38)            Exponent m = 0.6 + ma + mb/Cos(Th) + mc/Tan(Th)              ma   0.0(17)         mb   0.05(59)         mc   1.70(96)       Lattice parameters         a (Å)   5.305034(30)            Site  Np  x    y      z  Atom Occ  Beq               U1  4  0.00000  0.00000  0.00000   U + 6 1  1       N1  8  0.25000  0.25000  0.25000 N 1  3.52(21)       Structure 3            Phase name   Uraninte C       R-Bragg   2.184       Spacegroup   Fm-3m       Scale   0.0000472(95)       Cell Mass   1080.105       Cell Volume (Å{circumflex over ( )}3)   162.38(24)       Wt %—Rietveld   0.389(78)       Crystallite Size        Cry Size Lorentzian (nm)   0(3600000)       Crystal Linear Absorption Coeff. (1/cm)   3010.5(45)       Crystal Density (g/cm{circumflex over ( )}3)   11.045(16)               PV_TCHZ peak type        U   1.2(39)        V   0.4(26)        W   −0.09(45)        Z   0        X   0.0(12)        Y   0               Lattice parameters        a (Å)   5.4556(27)            Site  Np  x    y      z  Atom Occ  Beq               U1  4  0.00000  0.00000  0.00000   U + 4 1  1       O1  8  0.25000  0.25000  0.25000  O−2 1  1               Very weak unknown peaks at dhkl × 4.403 Å, 5.486 Å, 2.496 Å.            
Very weak unknown peaks at dhkl=4.403 Å, 5.486 Å, 2.496 Å.
 
     Whereas particular embodiments of this invention have been described above for purpose of illustration, it will be evident to those skilled in the art the numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.