1. 1 CHEM 312: Lecture 12 Part 1 Uranium Chemistry and the Fuel Cycle Readings: Uranium chapter: §http://radchem.nevada.edu/classes/r dch710/files/uranium.pdf Chemistry in the fuel cycle §Uranium àSolution Chemistry àSeparation àFluorination and enrichment àMetal Focus on chemistry in the fuel cycle §Speciation (chemical form) §Oxidation state §Ionic radius and molecular size Utilization of fission process to create heat §Heat used to turn turbine and produce electricity Requires fissile isotopes § 233 U, 235 U, 239 Pu §Need in sufficient concentration and geometry 233 U and 239 Pu can be created in neutron flux 235 U in nature §Need isotope enrichment §Ratios of isotopes established à234: 0.005±0.001, 68.9 a à235: 0.720±0.001, 7.04E8 a à238: 99.275±0.002, 4.5E9 a Fission properties of uranium §Defined importance of element and future investigations §Identified by Hahn in 1937 §200 MeV/fission §2.5 neutrons

2. 2 U Fuel Cycle Chemistry Overview Uranium acid-leach Extraction and conversion Understand fundamental chemistry of uranium and its applications to the nuclear fuel cycle

3. 3 Fuel Fabrication Enriched UF 6 UO 2 Calcination, Reduction Tubes Pellet Control 40-60°C Fuel Fabrication Other species for fuel nitrides, carbides Other actinides: Pu, Th

4. 4 Uranium chemistry Uranium solution chemistry Separation and enrichment of U Uranium separation from ore §Solvent extraction §Ion exchange Separation of uranium isotopes §Gas centrifuge §Laser 200 minerals contain uranium §Bulk are U(VI) minerals àU(IV) as oxides, phosphates, silicates §Classification based on polymerization of coordination polyhedra §Mineral deposits based on major anion Pyrochlore §A 1-2 B 2 O 6 X 0-1 àA=Na, Ca, Mn, Fe 2+, Sr,Sb, Cs, Ba, Ln, Bi, Th, U àB= Ti, Nb, Ta àU(V) may be present when synthesized under reducing conditions *From XANES spectroscopy *Goes to B site Uraninite with oxidation

5. 5 Uranium solution chemistry overview Strong Lewis acid, Hard electron acceptor §F - >>Cl - >Br -  I - §Same trend for O and N group à based on electrostatic force as dominant factor Hydrolysis behavior §U(IV)>U(VI)>>>U(III)>U(V) U(III) and U(V) §No data in solution àBase information on lanthanide or pentavalent actinides Uranyl(VI) most stable oxidation state in solution §Uranyl(V) and U(IV) can also be in solution àU(V) prone to disproportionation §Stability based on pH and ligands §Redox rate is limited by change in species àMaking or breaking yl oxygens *UO 2 2+ +4H + +2e -  U 4+ +2H 2 O 5f electrons have strong influence on actinide chemistry §For uranyl, f-orbital overlap provide bonding

6. 6 Uranium chemical bonding: oxidation states Tri- and tetravalent U mainly related to organometallic compounds §Cp 3 UCO and Cp 3 UCO + àCp=cyclopentadiene *5f CO  backbonding  Metal electrons to  of ligands *Decreases upon oxidation to U(IV) Uranyl(V) and (VI) compounds §yl ions in aqueous systems unique for actinides àVO 2 +, MoO 2 2+, WO 2 2+ *Oxygen atoms are cis to maximize (p  )  M(d  ) àLinear MO 2 2+ known for compounds of Tc, Re, Ru, Os *Aquo structures unknown §Short U=O bond distance of 1.75 Å for hexavalent, longer for pentavalent àSmaller effective charge on pentavalent U §Multiple bond characteristics, 1  and 2 with  characteristics

7. 7 Uranium solution chemistry Trivalent uranium §Very few studies of U(III) in solution §No structural information àComparisons with trivalent actinides and lanthanides Tetravalent uranium §Forms in very strong acid àRequires >0.5 M acid to prevent hydrolysis àElectrolysis of U(VI) solutions *Complexation can drive oxidation §Coordination studied by XAFS àCoordination number 9±1 *Not well defined àU-O distance 2.42 Å §O exchange examined by NMR Pentavalent uranium §Extremely narrow range of existence §Prepared by reduction of UO 2 2+ with Zn or H 2 or dissolution of UCl 5 in water àU(V) is not stable but slowly oxidizes under suitable conditions §No experimental information on structure §Quantum mechanical predictions

8. 8 Hexavalent Uranium Large number of compounds prepared §Crystallization §Hydrothermal Determination of hydrolysis constants from spectroscopic and titration §Determine if polymeric species form §Polynuclear species present except at lowest concentration Hexavalent uranium as uranyl in solution

9. 9 Uranyl chemical bonding Uranyl (UO 2 2+ ) linear molecule Bonding molecular orbitals   g 2  u 2  g 4  u 4 àOrder of HOMO is unclear *  g <  u <  g <<  u proposed  Gap for  based on 6p orbitals interactions  5f  and 5f  LUMO §Bonding orbitals O 2p characteristics §Non bonding, antibonding 5f and 6d §Isoelectronic with UN 2 Pentavalent has electron in non-bonding orbital

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11. 11 Uranyl chemical bonding yl oxygens force formal charge on U below 6 §Net charge 2.43 for UO 2 (H 2 O) 5 2+, 3.2 for fluoride systems àNet negative 0.43 on oxygens àLewis bases *Can vary with ligand in equatorial plane *Responsible for cation-cation interaction *O=U=O- - -M *Pentavalent U yl oxygens more basic Small changes in U=O bond distance with variation in equatoral ligand Small changes in IR and Raman frequencies §Lower frequency for pentavalent U §Weaker bond

12. 12 Uranium speciation Speciation variation with uranium concentration §Hydrolysis as example §Precipitation at higher concentration àChange in polymeric uranium species concentration CHESS Calculation

13. 13 Uranium purification from ores: Using U chemistry in the fuel cycle Preconcentration of ore §Based on density of ore Leaching to extract uranium into aqueous phase §Calcination prior to leaching àRemoval of carbonaceous or sulfur compounds àDestruction of hydrated species (clay minerals) Removal or uranium from aqueous phase §Ion exchange §Solvent extraction §Precipitation §Acid solution leaching *Sulfuric (pH 1.5) ØU(VI) soluble in sulfuric ØAnionic sulfate species ØOxidizing conditions may be needed ØMnO 2 ØPrecipitation of Fe at pH 3.8 §Carbonate leaching àFormation of soluble anionic carbonate species *UO 2 (CO 3 ) 3 4- àPrecipitation of most metal ions in alkali solutions àBicarbonate prevents precipitation of Na 2 U 2 O 7 *Formation of Na 2 U 2 O 7 with further NaOH addition àGypsum and limestone in the host aquifers necessitates carbonate leaching

14. 14 Recovery of uranium from solutions Ion exchange §U(VI) anions in sulfate and carbonate solution àUO 2 (CO 3 ) 3 4- àUO 2 (SO 4 ) 3 4- §Load onto anion exchange, elute with acid or NaCl Solvent extraction §Continuous process §Not well suited for carbonate solutions §Extraction with alkyl phosphoric acid, secondary and tertiary alkylamines àChemistry similar to ion exchange conditions Chemical precipitation §Addition of base §Peroxide àWater wash, dissolve in nitric acid àUltimate formation of (NH 4 ) 2 U 2 O 7 (ammonium diuranate), yellowcake àheating to form U 3 O 8 or UO 3

15. 15 Uranium purification Tributyl phosphate (TBP) extraction §Based on formation of nitrate species §UO 2 (NO 3 ) x 2-x + (2-x)NO 3 - + 2TBP  UO 2 (NO 3 ) 2 (TBP) 2 §Process example of pulse column below

16. 16 Uranium enrichment Once separated, uranium needs to be enriched for nuclear fuel §Natural U is 0.7 % 235 U Different enrichment needs §3.5 % 235 U for light water reactors §> 90 % 235 U for submarine reactors § 235 U enrichment below 10 % cannot be used for a device àCritical mass decreases with increased enrichment §20 % 235 U critical mass for reflected device around 100 kg àLow enriched/high enriched uranium boundary

17. 17 Uranium enrichment Exploit different nuclear properties between U isotopes to achieve enrichment §Mass §Size §Shape §Nuclear magnetic moment §Angular momentum Massed based separations utilize volatile UF 6 § UF 6 formed from reaction of U compounds with F 2 at elevated temperature Colorless, volatile solid at room temperature §Density is 5.1 g/mL §Sublimes at normal atmosphere §Vapor pressure of 100 torr àOne atmosphere at 56.5 ºC O h point group §U-F bond distance of 2.00 Å

18. 18 Uranium Hexafluoride Very low viscosity §7 mPoise àWater =8.9 mPoise àUseful property for enrichment Self diffusion of 1.9E-5 cm 2 /s Reacts with water §UF 6 + 2H 2 O  UO 2 F 2 + 4HF Also reactive with some metals Does not react with Ni, Cu and Al §Material made from these elements need for enrichment

19. 19 Uranium Enrichment: Electromagnetic Separation Volatile U gas ionized §Atomic ions with charge +1 produced Ions accelerated in potential of kV §Provides equal kinetic energies §Overcomes large distribution based on thermal energies Ion in a magnetic field has circular path  Radius (  ) àm mass, v velocity, q ion charge, B magnetic field For V acceleration potential

20. 20 Uranium Enrichment: Electromagnetic Separation Radius of an ion is proportional to square root of mass §Higher mass, larger radius Requirements for electromagnetic separation process §Low beam intensities àHigh intensities have beam spreading *Around 0.5 cm for 50 cm radius §Limits rate of production §Low ion efficiency àLoss of material Caltrons used during Manhattan project

21. 21 Calutron Developed by Ernest Lawrence §Cal. U-tron High energy use §Iraqi Calutrons required about 1.5 MW each à90 total Manhattan Project §Alpha à4.67 m magnet à15% enrichment àSome issues with heat from beams àShimming of magnetic fields to increase yield §Beta àUse alpha output as feed *High recovery

22. 22 Gaseous Diffusion High proportion of world’s enriched U §95 % in 1978 §40 % in 2003 Separation based on thermal equilibrium §All molecules in a gas mixture have same average kinetic energy àlighter molecules have a higher velocity at same energy *E k =1/2 mv 2 For 235 UF 6 and 238 UF 6 § 235 UF 6 and is 0.429 % faster on average à why would UCl 6 be much more complicated for enrichment?

23. 23 Gaseous Diffusion 235 UF 6 impacts barrier more often Barrier properties §Resistant to corrosion by UF 6 à Ni and Al 2 O 3 §Hole diameter smaller than mean free path àPrevent gas collision within barrier §Permit permeability at low gas pressure àThin material Film type barrier §Pores created in non-porous membrane §Dissolution or etching Aggregate barrier §Pores are voids formed between particles in sintered barrier Composite barrier from film and aggregate

24. 24 Gaseous Diffusion Barrier usually in tubes §UF 6 introduced Gas control §Heater, cooler, compressor Gas seals Operate at temperature above 70 °C and pressures below 0.5 atmosphere R=relative isotopic abundance (N 235 /N 238 ) Quantifying behavior of an enrichment cell §q=R product /R tail § Ideal barrier, R product =R tail (352/349) 1/2 ; q= 1.00429

25. 25 Gaseous Diffusion Small enrichment in any given cell §q=1.00429 is best condition  Real barrier efficiency (  B )   B can be used to determine total barrier area for a given enrichment   B = 0.7 is an industry standard §Can be influenced by conditions §Pressure increase, mean free path decrease àIncrease in collision probability in pore §Increase in temperature leads to increase velocity àIncrease UF 6 reactivity Normal operation about 50 % of feed diffuses Gas compression releases heat that requires cooling §Large source of energy consumption Optimization of cells within cascades influences behavior of 234 U §q=1.00573 (352/348) 1/2 §Higher amounts of 234 U, characteristic of feed

26. 26 Gaseous Diffusion Simple cascade §Wasteful process §High enrichment at end discarded Countercurrent §Equal atoms condition, product enrichment equal to tails depletion Asymmetric countercurrent §Introduction of tails or product into nonconsecutive stage §Bundle cells into stages, decrease cells at higher enrichment

27. 27 Gaseous Diffusion Number of cells in each stage and balance of tails and product need to be considered Stages can be added to achieve changes in tailing depletion §Generally small levels of tails and product removed Separative work unit (SWU) §Energy expended as a function of amount of U processed and enriched degree per kg §3 % 235 U à3.8 SWU for 0.25 % tails à5.0 SWU for 0.15 % tails Determination of SWU §P product mass §W waste mass §F feedstock mass §x W waste assay §x P product assay §x F feedstock assay

28. 28 Gas centrifuge Centrifuge pushes heavier 238 UF 6 against wall with center having more 235 UF 6 §Heavier gas collected near top Density related to UF 6 pressure §Density minimum at center  m molecular mass, r radius and  angular velocity With different masses for the isotopes, p can be solved for each isotope

29. 29 Gas Centrifuge Total pressure is from partial pressure of each isotope §Partial pressure related to mass Single stage separation (q) §Increase with mass difference, angular velocity, and radius For 10 cm r and 1000 Hz, for UF 6 §q=1.26 Gas distribution in centrifuge

30. 30 Gas Centrifuge More complicated setup than diffusion §Acceleration pressures, 4E5 atmosphere from previous example §High speed requires balance §Limit resonance frequencies §High speed induces stress on materials àNeed high tensile strength *alloys of aluminum or titanium * maraging steel ØHeat treated martensitic steel *composites reinforced by certain glass, aramid, or carbon fibers

31. 31 Gas Centrifuge Gas extracted from center post with 3 concentric tubes §Product removed by top scoop §Tails removed by bottom scoop §Feed introduced in center Mass load limitations §UF 6 needs to be in the gas phase §Low center pressure à3.6E-4 atm for r = 10 cm Superior stage enrichment when compared to gaseous diffusion §Less power need compared to gaseous diffusion à1000 MW e needs 120 K SWU/year *Gas diffusion 9000 MJ/SWU *centrifuge 180 MJ/SWU Newer installations compare to diffusion §Tend to have no non-natural U isotopes

32. 32 Laser Isotope Separation Isotopic effect in atomic spectroscopy §Mass, shape, nuclear spin Observed in visible part of spectra Mass difference in IR region Effect is small compared to transition energies §1 in 1E5 for U species Use laser to tune to exact transition specie §Produces molecule in excited state Doppler limitations with method §Movement of molecules during excitation Signature from 234/238 ratio, both depleted

33. 33 Laser Isotope Separation 3 classes of laser isotope separations §Photochemical àReaction of excited state molecule §Atomic photoionization àIonization of excited state molecule §Photodissociation àDissociation of excited state molecule AVLIS §Atomic vapor laser isotope separation MLIS §Molecular laser isotope separation

34. 34 Laser isotope separation AVLIS §U metal vapor àHigh reactivity, high temperature àUses electron beam to produce vapor from metal sample Ionization potential 6.2 eV Multiple step ionization § 238 U absorption peak 502.74 nm § 235 U absorption peak 502.73 nm Deflection of ionized U by electromagnetic field

35. 35 Laser Isotope Separation MLIS (LANL method) SILEX (Separation of Isotopes by Laser Excitation) in Australia §Absorption by UF 6 §Initial IR excitation at 16 micron à 235 UF 6 in excited state §Selective excitation of 235 UF 6 §Ionization to 235 UF 5 §Formation of solid UF 5 (laser snow) §Solid enriched and use as feed to another excitation

36. 36 CHEM 312: Part 2 Lecture 12 Uranium Chemistry and the Fuel Cycle Readings: Uranium chapter: §http://radchem.nevada.edu/classes/r dch710/files/uranium.pdf Chemistry in the fuel cycle §Uranium àSolution Chemistry àSeparation àFluorination and enrichment àOxide àMetal Focus on chemistry in the fuel cycle §Speciation (chemical form) §Oxidation state §Ionic radius and molecular size Utilization of fission process to create heat §Heat used to turn turbine and produce electricity Requires fissile isotopes § 233 U, 235 U, 239 Pu §Need in sufficient concentration and geometry 233 U and 239 Pu can be created in neutron flux 235 U in nature §Need isotope enrichment §Ratios of isotopes established à234: 0.005±0.001, 68.9 a à235: 0.720±0.001, 7.04E8 a à238: 99.275±0.002, 4.5E9 a Fission properties of uranium §Defined importance of element and future investigations §Identified by Hahn in 1937 §200 MeV/fission §2.5 neutrons

37. 37 Nuclear Fuel: Uranium-oxygen system A number of binary uranium-oxygen compounds §UO àSolid UO unstable, NaCl structure àFrom UO 2 heated with U metal *Carbon promotes reaction, formation of UC §UO 2 àReduction of UO 3 or U 3 O 8 with H 2 from 800 ºC to 1100 ºC *CO, C, CH 4, or C 2 H 5 OH can be used as reductants àO 2 presence responsible for UO 2+x formation àLarge scale preparation *UO 4, (NH 4 ) 2 U 2 O 7, or (NH 4 ) 4 UO 2 (CO 3 ) 3 *Calcination in air at 400-500 ºC *H 2 at 650-800 ºC *UO 2 has high surface area

38. 38 Uranium-oxygen U 3 O 8 §From oxidation of UO 2 in air at 800 ºC   phase uranium coordinated to oxygen in pentagonal bipyrimid §  phase results from the heating of the  phase above 1350 ºC àSlow cooling

39. 39 Uranium-oxygen UO 3 §Seven phases can be prepared A phase (amorphous) àHeating in air at 400 ºC *UO 4. 2H 2 O, UO 2 C 2 O 4. 3H 2 O, or (HN 4 ) 4 UO 2 (CO 3 ) 3 ØPrefer to use compounds without N or C  -phase  Crystallization of A-phase at 485 ºC at 4 days §O-U-O-U-O chain with U surrounded by 6 O in a plane to the chain §Contains UO 2 2+  -phase §Ammonium diuranate or uranyl nitrate heated rapidly in air at 400-500 ºC  -phase prepared under O 2 6-10 atmosphere at 400-500 ºC

40. 40 Uranium-oxygen UO 3 hydrates §6 different hydrated UO 3 compounds UO 3. 2H 2 O §Anhydrous UO 3 exposed to water from 25-70 ºC  Heating resulting compound in air to 100 ºC forms  -UO 3. 0.8 H 2 O   -UO 2 (OH) 2 [  - UO 3. H 2 O] forms in hydrothermal experiments   -UO 3. H 2 O also forms

41. 41 Uranium-oxygen single crystals UO 2 from the melt of UO 2 powder §Arc melter used §Vapor deposition 2.0 ≤ U/O ≤ 2.375 §Fluorite structure Uranium oxides show range of structures §Some variation due to existence of UO 2 2+ in structure §Some layer structures

42. 42 UO 2 Heat Capacity Room temperature to 1000 K §Increase in heat capacity due to harmonic lattice vibrations àSmall contribution to thermal excitation of U 4+ localized electrons in crystal field 1000-1500 K §Thermal expansion induces anharmonic lattice vibration 1500-2670 K §Lattice and electronic defects

43. 43 Vaporization of UO 2 Above and below the melting point Number of gaseous species observed §U, UO, UO 2, UO 3, O, and O 2 àUse of mass spectrometer to determine partial pressure for each species àFor hypostiochiometric UO 2, partial pressure of UO increases to levels comparable to UO 2 àO 2 increases dramatically at O/U above 2

44. 44 Uranium oxide chemical properties Oxides dissolve in strong mineral acids §Valence does not change in HCl, H 2 SO 4, and H 3 PO 4 §Sintered pellets dissolve slowly in HNO 3 àRate increases with addition of NH 4 F, H 2 O 2, or carbonates *H 2 O 2 reaction ØUO 2 + at surface oxidized to UO 2 2+

45. 45 Solid solutions with UO 2 Solid solution §crystal structure unchanged by addition of another compound §mixture remains as single phase àThO 2 -UO 2 is a solid solution Solid solutions formed with group 2 elements, lanthanides, actinides, and some transition elements (Mn, Zr, Nb, Cd) §Distribution of metals on UO 2 fluorite- type cubic crystals based on stoichiometry Prepared by heating oxide mixture under reducing conditions from 1000 ºC to 2000 ºC §Powders mixed by co-precipitation or mechanical mixing of powders Written as M y U 1-y O 2+x §x is positive and negative

46. 46 Solid solutions with UO 2 Lattice parameter change in solid solution §Changes nearly linearly with increase in y and x àM y U 1-y O 2+x àEvaluate by change of lattice parameter with change in y *δa/δy Øa is lattice parameter in Å ØCan have both negative and positive values §δa/δy is large for metals with large ionic radii §δa/δx terms negative and between -0.11 to -0.3 àVaried if x is positive or negative

47. 47 Solid solutions of UO 2 Tetravalent M y U 1-y O 2+x §Zr solid solutions àLarge range of systems ày=0.35 highest value àMetastable at lower temperature §Th solid solution àContinuous solid solutions for 0≤y≤1 and x=0 àFor x>0, upper limit on solubility *y=0.45 at 1100 ºC to y=0.36 at 1500 ºC àAlso has variation with O 2 partial pressure *At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at 1500 ºC Tri and tetravalent M y U 1-y O 2+x §Cerium solid solutions àContinuous for y=0 to y=1 àFor x<0, solid solution restricted to y≤0.35 *Two phases (Ce,U)O 2 and (Ce,U)O 2-x àx<-0.04, y=0.1 to x<-0.24, y=0.7 à0≤x≤0.18, solid solution y<0.5 àAir oxidized hyperstoichiometric *y 0.56 to 1 at 1100 ºC *y 0.26-1.0 1550 ºC Tri and divalent §Reducing atmosphere àx is negative àfcc structure àMaximum values vary with metal ion §Oxidizing atmosphere àSolid solution can prevent formation of U 3 O 8 àSome systematics in trends *For Nd, when y is between 0.3 and 0.5, x = 0.5-y

48. 48 U-Zr oxide system

49. 49 Solid solution UO 2 Oxygen potential §Zr solid solution àLower than the UO 2+x system *x=0.05, y=0.3 Ø-270 kJ/mol for solid solution Ø-210 kJ/mol for UO 2+x §Th solid solution  Increase in  G with increasing y àCompared to UO 2 difference is small at y less than 0.1 §Ce solid solution àWide changes over y range due to different oxidation states àShape of the curve is similar to Pu system, but values differ *Higher  G for CeO 2-x compared to PuO 2-x

50. 50 Metallic Uranium Three different phase   phases àDominate at different temperatures Uranium is strongly electropositive §Cannot be prepared through H 2 reduction Metallic uranium preparation §UF 4 or UCl 4 with Ca or Mg §UO 2 with Ca §Electrodeposition from molten salt baths

51. 51 Metallic Uranium phases  -phase §Room temperature to 942 K §Orthorhombic §U-U distance 2.80 Å §Unique structure type  -phase §Exists between 668 and 775 ºC §Tetragonal unit cell  -phase §Formed above 775 ºC §bcc structure Metal has plastic character §Gamma phase soft, difficult fabrication §Beta phase brittle and hard Paramagnetic Temperature dependence of resistivity Alloyed with Mo, Nb, Nb-Zr, and Ti  -phase  ‐ phase U-U distances in layer (2.80±0.05) Å and between layers 3.26 Å

52. 52 Intermetallic compounds Wide range of intermetallic compounds and solid solutions in alpha and beta uranium §Hard and brittle transition metal compounds àU 6 X, X=Mn, Fe, Co, Ni §Noble metal compounds àRu, Rh, Pd *Of interests for reprocessing §Solid solutions with: àMo, Ti, Zr, Nb, and Pu

53. 53 Uranium-Aluminum Phase Diagram Uranium-Titanium Phase Diagram

54. 54 Chemical properties of uranium metal and alloys Reacts with most elements on periodic table §Corrosion by O 2, air, water vapor, CO, CO 2 Dissolves in HCl §Also forms hydrated UO 2 during dissolution Non-oxidizing acid results in slow dissolution §Sulfuric, phosphoric, HF Exothermic reaction with powered U metal and nitric Dissolves in base with addition of peroxide §peroxyuranates

55. 55 Review How is uranium chemistry linked with the fuel cycle What are the main oxidation states uranium Describe the uranium enrichment process §Mass based §Laser bases Understand the fundamental chemistry of uranium as it relates to: §Production §Solution chemistry §Speciation §Spectroscopy

56. 56 Questions What are the different types of conditions used for separation of U from ore What is the physical basis for enriching U by gas and laser methods? Describe the basic chemistry for the production of U metal What are the natural isotopes of uranium Describe the synthesis and properties of the uranium halides How is the O to U ratio for uranium oxides determined What are the trends in U solution chemistry What atomic orbitals form the molecular orbitals for UO 2 2+ What else could be used instead of 235 U as the fissile isotope in a reactor? Describe two processes for enriching uranium. Why does uranium need to be enriched?

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