4/2003 Rev 2 I.4.9e – slide 1 of 100 Session I.4.9e Part I Review of Fundamentals Module 4Sources of Radiation Session 9eFuel Cycle - Enrichment IAEA Post Graduate Educational Course Radiation Protection and Safety of Radiation Sources

4/2003 Rev 2 I.4.9e – slide 2 of 100 Enrichment

4/2003 Rev 2 I.4.9e – slide 3 of 100 Why do Enrichment?  Enriched uranium not necessary for nuclear reactors - can improve the moderation (carbon, D 2 O) which allows the use of natural uranium (e.g., CANDU, Magnox)  Pure (enriched) isotopes offer enhanced properties and cleaner spectra for reactions

4/2003 Rev 2 I.4.9e – slide 4 of 100 Possible Isotope Separation Methods “Physical” - Distillation- Thermal - Distillation- Thermal - Ion Exchange- Solvent Extraction - Ion Exchange- Solvent Extraction - Barrier Diffusion- Centrifugation - Barrier Diffusion- Centrifugation - Nozzle Flow- Helical Flow - Nozzle Flow- Helical Flow“Chemical” - Chemical Exchange - Chemical Exchange - Ionization - Ionization - Laser/Light - Laser/Light

4/2003 Rev 2 I.4.9e – slide 5 of 100 Uranium Enrichment Methods  Currently, two main methods implemented commercially  Gaseous diffusion (GDP)  Gas centrifuge (GC)  Previous methods  Thermal diffusion  Electromagnetic ionization/Calutron  Future Methods (?) - Both laser based  AVLIS  Silex  In actual practice, theoretical enrichment values are rarely attained

4/2003 Rev 2 I.4.9e – slide 6 of 100 Other Enrichment Definitions  Enrichment Levels:  LEU = Low Enriched Uranium: assay < 10%  “IEU” = Intermediate Enriched Uranium (10% - 20%)  HEU = High Enriched Uranium: assay > 20% (usually focus on assay > 90%)  SWU = Separative Work Unit  measure of physical effort in separation (cost)  Enriched uranium is called Special Nuclear Material in the USA

4/2003 Rev 2 I.4.9e – slide 7 of 100 Gaseous Diffusion  Two enrichment processes:  Gaseous Diffusion  Gas Centrifuge

4/2003 Rev 2 I.4.9e – slide 8 of 100 Basic Theory of Gaseous Diffusion  Gaseous Diffusion uses molecular diffusion to separate the isotopes of uranium  Three basic requirements are needed  Combine Uranium with Fluorine to form Uranium hexafluoride (UF 6 )  Pass UF 6 through a porous membrane  Utilize the different molecular velocities of the two isotopes to achieve separation

4/2003 Rev 2 I.4.9e – slide 9 of 100  Enrichment of 235 U through one porous membrane (or barrier) is very minute  Thousands of passes are required to increase the enrichment of natural uranium (0.711%) to a usable assay of 4 or 5% for use in reactors Basic Theory of Gaseous Diffusion

4/2003 Rev 2 I.4.9e – slide 10 of 100 Eight-Step Program 1)UF 6 Feed Storage 2)Feed Supply Autoclave 3)Enrichment Cascade 4)Tails Condensation and Withdrawal 5)Tails Storage 6)Product Condensation and Withdrawal 7)Product Storage 8)Product Shipping

4/2003 Rev 2 I.4.9e – slide 11 of 100 Step 1 - UF 6 Feed Storage  Feed material typically arrives at a GDP via truck or rail  Normal feed (i.e., natural uranium) can be received in 2.5, 10, and 14-ton cylinders  UF 6 is in solid phase when being transported

4/2003 Rev 2 I.4.9e – slide 12 of 100 Cylinder Filled with Solid UF 6

4/2003 Rev 2 I.4.9e – slide 13 of 100 Step 2 - Feed Supply Autoclave  Used to heat a cylinder to liquid phase  Also acts as a containment  Includes many safety systems  Pressure  Temperature  Conductivity

4/2003 Rev 2 I.4.9e – slide 14 of 100 Typical Autoclave UF 6 is liquefied and homogenized Sample drawn in order to check chemical purity and isotopic concentration

4/2003 Rev 2 I.4.9e – slide 15 of 100 Phase Diagram of UF 6  Solid cylinder contents are heated to the liquid phase  Fed into cascade as UF 6 gas  Cylinder connected to cascade with a “pigtail”

4/2003 Rev 2 I.4.9e – slide 16 of 100 Step 3 - Enrichment Cascade  As mentioned earlier, the separation of 235 U from 238 U is accomplished by passing UF 6 through many hundreds of stages  UF 6 flows into the stage, where it comes in contact with the porous surface of a barrier  The barrier is what makes enrichment possible

4/2003 Rev 2 I.4.9e – slide 17 of 100 Gaseous Diffusion Enrichment of 235 U through one porous membrane (or barrier) is very minute

4/2003 Rev 2 I.4.9e – slide 18 of 100 Schematic of Diffusion Stage Some of the UF 6 (slightly more 235 U than 238 U) passes through to the low-pressure side of the converter High-pressure UF 6 gas stream passes through the barrier tubes

4/2003 Rev 2 I.4.9e – slide 19 of 100 Product1.0 kg UF 6 at 3.0% 235 U Feed Depleted Uranium5.5 kgs UF 6 at 0.3% 235 U 6.5 kgs UF 6 at 0.711% 235 U Shape of a Cascade

4/2003 Rev 2 I.4.9e – slide 20 of 100 SWU Cider Analogy  Apples represent feed quantities  Force represents Separative Work Unit or SWU (electricity/effort)  Cider represents product (LEU)  Peels/cores represent DU tails  Relationship is non-linear but approximately linear in LEU range

4/2003 Rev 2 I.4.9e – slide 21 of 100 SWU Cider Analogy Typical Feed (1 basket of apples) Waste (peels, cores, seeds, apples) Force(SWU) Less Feed More Force Feed (<1 basket) LessWaste MoreForce More Feed Less Force Feed (>1 basket) MoreWaste LessForce Product (1 liter cider) Same for all Three

4/2003 Rev 2 I.4.9e – slide 22 of 100 Length of the Cascade  Length is determined by the needed enrichment  5.0% enrichment requires many more stages than say 2.6% enrichment  The number of stages required for a given enrichment can be calculated

4/2003 Rev 2 I.4.9e – slide 23 of 100 Separation of 234 U  Natural uranium also consists of a small quantity of 234 U  Due to the process, 234 U gets enriched as well as 235 U  Separation factor for 234 U = 1.006

4/2003 Rev 2 I.4.9e – slide 24 of 100 Stages The entire cascade is composed of groups of stages

4/2003 Rev 2 I.4.9e – slide 25 of 100 Step 4 – Tails Condensation and Withdrawal  Tails are the depleted UF 6 stream  UF 6 is compressed and condensed into a liquid  Withdrawn into 10- or 14-ton cylinders  Cooled at ambient conditions until UF 6 is solid, taking at least 5 days  Typical assay of tails is between 0.2% and 0.4%

4/2003 Rev 2 I.4.9e – slide 26 of 100 Step 5 – Tails Storage  Stored on concrete storage yards  Stacked two high  Cylinder integrity is checked periodically  Large equipment used to move cylinders is limited to only moving solid cylinders

4/2003 Rev 2 I.4.9e – slide 27 of 100 Step 6 – Product Condensation and Withdrawal  Enriched UF 6 is removed from the cascade through heated piping, where it is compressed and cooled to make it a liquid  Put into 10-ton product cylinders  Filled cylinders are moved to a cool down area for solidification  Next, the gaseous contaminants are removed by “burping” them through chemical traps

4/2003 Rev 2 I.4.9e – slide 28 of 100 Assay and Accumulation  Assay in product cylinders can be determined by:  Automatic sampler during filling  In-line mass spectrometers  UF 6 can be held-up in accumulators  Allows uninterrupted filling of cylinders

4/2003 Rev 2 I.4.9e – slide 29 of 100 Step 7 – Product Storage  The 10-ton cylinders used for product withdrawal are not used for delivery  Transferred to 2.5-ton cylinders  Transferring allows the opportunity to confirm assay and purity of product

4/2003 Rev 2 I.4.9e – slide 30 of 100 Step 8 – Product Shipping  After a cylinder cools for 5 days, it can be shipped  Loaded into overpacks  Shipped via semi- truck

4/2003 Rev 2 I.4.9e – slide 31 of 100 Potential Hazards  Primary overall hazard is a major UF 6 release  Liquid cylinder drop is most credible  When UF 6 reacts with water, it forms hydrofluoric acid  Both corrosive and toxic

4/2003 Rev 2 I.4.9e – slide 32 of 100  UF 6 – Uranium Hexafluoride  HF – Hydrogen Fluoride  Cl 2 - Chlorine  NH 3 - Ammonia  ClF 3 – Chlorine Trifluoride Significant Hazards

4/2003 Rev 2 I.4.9e – slide 33 of 100 Gas Centrifuge  Two enrichment processes:  Gaseous Diffusion  Gas Centrifuge

4/2003 Rev 2 I.4.9e – slide 34 of 100 Gas Centrifuges (GC)  GC is a uranium enrichment process that uses a large number of rotating cylinders in series and parallel configurations to produce LEU suitable for commercial power reactor use  The approach is a hundred years old  Several large facilities overseas successfully and economically supply LEU using GC plants  No operating GC plants currently exist in the U.S., but there have been “many discussions.”

4/2003 Rev 2 I.4.9e – slide 35 of 100 Why Gas Centrifuge?  Large enrichment effect per stage  > 1.05 vs for GDP  More compact design  Reduced uranium inventories in cascades  Better energy efficiencies  < 5% of GDP energy typically stated  More rapidly achieves equilibrium/steady state  about a day instead of “weeks” for GDP

4/2003 Rev 2 I.4.9e – slide 36 of 100 Existing GC Plants and Capacities  Urenco Plants:  Capenhurst, UK: 2M SWU/yr, expanding to 2.5  Gronau, D: 1.3 M SWU/yr, expanding to ?  Almelo, ND: 1.5 M SWU/yr, expanding to 2.5  Russia/FSU  Several, 20 M SWU/yr  Japan  Several, circa 0.1 M SWU/yr, expanding to M (Rokkasho-Mura, northern Honshu)

4/2003 Rev 2 I.4.9e – slide 37 of 100 Gas Centrifuge Theory  Slight density difference between 235 UF 6 and 238 UF 6  Separated by centrifugal forces created by rotation (20,000+ rpm)  Enriched and depleted layers form  Improve separation with countercurrent flow from baffles and thermal means (heat bottom, cool top)  Improve separation and throughput with larger diameter and more height (DOE approach)  Remove by scoops etc.

4/2003 Rev 2 I.4.9e – slide 38 of 100 Gas Centrifuge

4/2003 Rev 2 I.4.9e – slide 39 of 100 Unique Characteristics of GCs  Rotors require high strength  Super strong maraging steels  Fiber (carbon) composites  Supercritical operation  rpm requires traversing natural harmonics, flexural nodes  Bearings/drives  must accommodate imperfections  vibration during acceleration/deceleration  drives must quickly traverse natural harmonics of the GC  Many GCs required in plant (10,000s)

4/2003 Rev 2 I.4.9e – slide 40 of 100 Gas Centrifuge (GC) Process  Main operations and hazards similar to GDP  UF 6 receive, store, desublime, “burp,” vaporize, product, tails, sample etc.  Only the enrichment part changes  GCs operate at lower pressure but higher separation  Requires fewer stages in series (circa 50)  Requires more stages in parallel to meet throughput  Thus, organized in series and parallel cascades

4/2003 Rev 2 I.4.9e – slide 41 of 100 Cascade Arrangement Note parallel and series arrangement

4/2003 Rev 2 I.4.9e – slide 42 of 100 Gas Centrifuge Hazards  Effect of high speed rotating equipment:  12” diameter rotor, at 350 m/sec edge speed  Circumference is 0.94 m (i.e., per revolution)  Result is 371 rev/sec = 22,300 rpm  82,000 g’s  Increases in diameter, height, and rpm improve enrichment and production but also increase hazards and consequences of failures  Compromises and trade-offs unavoidable and required

4/2003 Rev 2 I.4.9e – slide 43 of 100 Safety Comparison: GC vs GDP  Centrifuge  Lower pressures, less inventory, more isolation  Newer facility  Liquid UF 6 areas comparable  Conclude GC risk probably lower

4/2003 Rev 2 I.4.9e – slide 44 of 100 Atomic Vapor Laser Isotope Separation (AVLIS)

4/2003 Rev 2 I.4.9e – slide 45 of 100 AVLIS - Theory  Focus on AVLIS process  Uses uranium metal (actually U-Fe alloy - lower eutectic)  Melt metal at circa 2,300  C in a vacuum chamber (separator or pod) – (for example Electron-beam)  Forms U metal vapor “beam” between electrodes  Visible, UV lasers form U-235+ ions, condense on cathode  U-238 condenses on collector in back (and everywhere else!)

4/2003 Rev 2 I.4.9e – slide 46 of 100 AVLIS - Theory Enriched product on collector

4/2003 Rev 2 I.4.9e – slide 47 of 100 “The AVLIS Dilemmas”  Batch processing  Vacuum contradiction  high vacuum to avoid collisions, improve selectivity  low vacuum to improve throughput  Collector design/fabrication  ES&H Concerns  temperature extremes (hot or cold)  units/pods have to be opened more frequently than GDP  internal coatings of U “stuff” - U metal/ Ux deposits  intimate high energy equipment and lasers  dense phases and criticality  hot uranium is very corrosive and reactive

4/2003 Rev 2 I.4.9e – slide 48 of 100 AVLIS - Potential Hazards  Handling “molten” uranium  Mixing of high energy components and water  Unknown reliabilities - some analogues have high failure rates  Fire hazards from dye lasers, reactions generating hydrogen  Criticality due to dense phase - “nuggets” - and water  Maintenance

4/2003 Rev 2 I.4.9e – slide 49 of 100 SILEX Enrichment  SILEX - Separation of Isotopes by Laser Excitation  SILEX now known as GLE (Global Laser Enrichment)  In mid-2009, GEH (GE – Hitachi) submitted the final part of its licence application to NRC  If the licence application is successful and the decision to proceed is taken early in 2012, the GLE commercial production facility (at Wilmington, North Carolina) should be operational about 2014

4/2003 Rev 2 I.4.9e – slide 50 of 100 What is Depleted Uranium?  Definition:  Depleted uranium (DU) is uranium that contains less than the natural assay of uranium-235  Natural assay = approximately 0.712% U-235  “Normal DU” is around % U-235  DU comes as a “byproduct” - some say “waste” - from enrichment

4/2003 Rev 2 I.4.9e – slide 51 of 100 DU Background  Generated by every enrichment process  DU generation cannot be avoided  minimum of about 5:1 ratio to LEU product  ratio of 8-10:1 due to higher assays, more U feed  Perceived hazards  old and rusting containers  chemical - UF 6, F 2  liability for cleanup, accidents

4/2003 Rev 2 I.4.9e – slide 52 of 100 What are the potential hazards with DUF 6 ?  DU’s chemical toxicity greater than its radiotoxicity  heavy metal  ingestion equivalent to about 10 mSv dose can be fatal  DUF 6 readily reacts with atmospheric water vapor to form UO 2 F 2 and HF (both “bad”)  DUF 6 corrosive (+ HF effect)  DUF 6 reacts violently with most organic materials  For any DU disposition/plant alternative:  large quantities, liquid UF 6, high temps, airborne concern, fluorine/HF disposition

4/2003 Rev 2 I.4.9e – slide 53 of 100 DU Current Approach and Programs  Store as UF 6 solid in 48” cylinders (48G) outside  U.S./DOE: Inspect/Maintain/Paint cylinders  about 6 small leaks/leakers in 50,000 cylinders, over 40 years  DOE has title to most U.S. DUF 6  USEC payment for DOE to accept DUF 6  Overseas  Most countries store DU as DUF 6  Only France converts DUF 6 to DU 3 O 8 powder