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The Uranium Fuel Cycle

The Uranium Fuel Cycle. Robert Tsai November 21, 2006. Overview. Front end Mining Milling Enrichment / fuel fabrication Service period Back end Transport and storage Disposal (open fuel cycle) Reprocessing (closed fuel cycle). World reserves: 3.1 million tU. Open-pit mining: 30%

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The Uranium Fuel Cycle

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  1. The Uranium Fuel Cycle Robert Tsai November 21, 2006

  2. Overview • Front end • Mining • Milling • Enrichment / fuel fabrication • Service period • Back end • Transport and storage • Disposal (open fuel cycle) • Reprocessing (closed fuel cycle)

  3. World reserves: 3.1 million tU Open-pit mining: 30% Underground mining: 38% (55% in 1990) In situ leaching (ISL): 21% Mining Processes

  4. Milling – Uranium Extraction • Grinding (~100 microns) • Acid (H2SO4) or alkaline (Na2CO3 / NaHCO3) leach • Solid / liquid separation of slurry • Purification (simple or extensive) • Precipitation – diuranate salt (e.g. Na2U2O7) • Drying Uranium oxide concentrate (UOC) (predominantly U3O8)

  5. Milling – Uranium Conversion • Dissolving of U3O8 in HNO3 • Calcination (strong heating) → UO3 • Reduction with H2 → UO2 • Hydrofluorination (HF) → UF4 • Fluorination (F2) → UF6 • In most cases, end-use requires conversion to UF6 for enrichment • However, certain reactors (CANDU) can use “natural” UO2

  6. Enrichment • Natural uranium: 235U: 0.7%, 238U: 99.3% • Reactor-grade: 235U increased to 3-5% • Necessary to sustain fission chain reaction • Methods • Gas diffusion (GD) • High-speed gas centrifugation (GC) • 5% of power requirements for GD • Laser technology (still in development) • Afterward, UF6 converted back to UO2 for mechanical processing (fuel rods)

  7. Service Period • Fission process depletes fuel • 235U → 92Kr, 141Ba • 238U → 239U → 239Pu • PWRs and BWRs reloaded bet/1-2 years, 1/4-1/3 of assemblies replaced • Complicated optimization problem • Maximize core reactivity • Top priority to safety / operational limitations

  8. Transport and Storage • At Reactor (AR) storage • Handle intense radioactivity of freshly-discharged fuel • Wet (cooling ponds) vs. dry • Transport via heavily-shielded flasks • Away From Reactor (AFR) storage • Similar set-up to AR facilities • NRC has repeatedly confirmed storages’ safety, minimal environmental impact

  9. Disposal (Open Fuel Cycle) • No permanent disposal procedures have been implemented in the world • Plans for 2010? • Consensus: burial deep underground after period of interim storage • Safeguards: vitrification, corrosion-resistant canisters, constant monitoring (106 years) • U.S. waste: Yucca Mountain in Nevada

  10. Reprocessing (Closed Fuel Cycle) • Similar process as with fresh feed • Benefits • Resource conservation • Decreased waste load • Use of ex-military material • Uranium + plutonium → MOX fuel • Major challenge: build-up of by-products

  11. Conclusions • Front end • Well developed and understood process • Gas centrifugation: best available enrichment technology • Future of lasers questionable • Back end • Underground disposal viable but will always have critics • Reprocessing necessary but has limitations

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