In the Name of God. Isfahan University of Technology Department of Chemistry. Nuclear Fuel Cycle. By: Habib Soleimani Supervisor: Dr. Ghaziaskar. Contents:. Definitions Uranium and its compounds Uranium Mining and Milling Uranium Conversion Uranium enrichment Fuel fabrication
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
In the Name of God Isfahan University of Technology Department of Chemistry
Nuclear Fuel Cycle By: Habib Soleimani Supervisor: Dr. Ghaziaskar
Contents: • Definitions • Uranium and its compounds • Uranium Mining and Milling • Uranium Conversion • Uranium enrichment • Fuel fabrication • Spent fuel • Reprocessing
Definitions • Radioactivity ( radioactive decay ): 1 Becquerel= 1 decays/S 1 Curie= 37 billion decays/S • Half-life
Definitions • Radiation: particles: neutrons, alpha particles, and beta particles energy : waves of pure energy, such as gamma and X-rays.
Uranium • Uranium is present in the Earth’s crust at an average concentration of 2 ppm. Its natural abundance is equal that of Sn. • Acidic rocks with high silicate, such as granite, have higher than average concentrations of uranium. • sedimentary and basic rocks have lower than average concentrations . • Isotopes: U-233, U-234, U-235, U-236, U-237, U-238 and U-239 • Specific activity = 24.9 *103 Bq/g • All isotopes decay by emission of α-radiation with a radiation energy between 4.2 and 5.2 MeV.
Uranium Compounds • Uranium metal • Uranium dioxide ( UO2 ) • Thriuranium octaoxide ( U3O8 ) • Uranium tetrafluoride ( UF4 ) • Uranium hexafluoride ( UF6 )
Uranium metal • Uranium metal is heavy, silvery white, malleable, ductile, and softer than steel . • d = 19 g/cm3 , 1.6 times more dense than lead. • it is subject to surface oxidation. • Water attacks uranium metal slowly at room temperature and rapidly at higher temperatures. • Uranium metal powder or chips will ignite spontaneously in air at ambient temperature.
Uranium dioxide ( UO2 ) • It is an basic oxide. • Most commonly used as a nuclear reactor fuel. • It is a stable ceramic that can be heated almost to its melting point, 5,212°F (2,878°C), without serious mechanical deterioration . • It does not react with water to any significant level. • At ambient temperatures, UO2 will gradually convert to U3O8. • Particle density = 10.96 g/cm3 , bulk density = 2.0 - 5.0 g/cm3 • Uranium dioxide (UO2) will ignite spontaneously in heated air and burn brilliantly .
Thriuranium octaoxide ( U3O8 ) • It is an amphoteric oxide. • Triuranium octaoxide (U3O8) occurs naturally as the olive-green-colored mineral pitchblende. • In the presence of oxygen (O2), uranium dioxide (UO2) and uranium trioxide (UO3) are oxidized to U3O8. • It is generally considered for disposal purposes because, under normal environmental conditions, U3O8 is one of the most kinetically and thermodynamically stable forms of uranium. • It is insoluble in water • Particle density = 8.3 g/cm3 bulk density = 1.5 - 4.0 g/cm3
Uranium tetrafluoride ( UF4 ) • Uranium tetrafluoride (UF4) is a green crystalline solid. • m.p.= 1,760°F (96°C) • It is formed by the reaction UF6 + H2 in a vertical tube-type reactor or by the action HF+UO2 . • It is generally an intermediate in the conversion of UF6 to either uranium oxide (U3O8 or UO2) or uranium metal. • Uranium tetrafluoride (UF4) reacts slowly with moisture at ambient temperature, forming UO2 and HF, which are very corrosive. • Bulk density = 2.0 - 4.5 g/cm3.
Uranium hexafluoride ( UF6 ) • Uranium hexafluoride (UF6) is the chemical form of uranium that is used during the uranium enrichment process. • Within a reasonable range of temperature and pressure, it can be a solid, liquid, or gas. • Disadvantage: UF6+2H2O(g/l) 4HF(g)+UO2F2 • UF6 is not considered a preferred form for long-term storage or disposal because of its relative instability.
Uranium hexafluoride ( UF6 ) • UF6 is characterised by an unusually high vapour pressure for a solid. • UF6 is not flammable and is inert in dry air.
Mining • Excavation : Excavation may be underground and open pit mining . • In situ leaching (ISL) : oxygenatedacidic or basic groundwater is circulated through a very porous orebody to dissolve the uranium and bring it to the surface.
Milling • The ore is first crushed and ground to liberate mineral particles. • The amphoteric oxide is then leached with sulfuric acid ( Leaching): UO3(s) + 2H+(aq) UO22+(aq) + H2O UO22+(aq) + 3SO42-(aq) UO2(SO4)34-(aq) • The basic oxide is converted by a similar process to that of a water soluble UO2(CO3)34-(aq) ion.
Milling • Two methods are used to concentrate and purify the uranium: ion exchange and solvent extraction ( more common ). • solvent extraction : uses tertiary amines in an organic kerosene solvent in a continuous process: 2 R3N(org) + H2SO4(aq) (R3NH)2SO4(org) 2(R3NH)2SO4(org) + UO2(SO4)34-(aq) (R3NH)4UO2(SO4)3(org) + 2SO42-(aq)
Milling • The solvents are removed by evaporating in a vacuum . • Ammonium diuranate, (NH4)2U2O7 , is precipitated by adding ammonia to neutralize the solution. • Then (NH4)2U2O7 heat U3O8 (yellow cake)
Refining and converting U3O8 toUO3 U3O8+HNO3 UO2(NO3)2· 6H2O • Uranyl nitrate, UO2(NO3)2· 6H2O, is fed into a continuous solvent extraction process. The uranium is extracted into an organic phase (kerosene) with tributyl phosphate (TBP), and the impurities remain again in the aqueous phase. • Washing from kerosene with dilute nitric acid and concentrated by evaporation to pure UO2(NO3)2· 6H2O . • Then UO2(NO3)2· 6H2O heat UO3 (pure)
Converting UO3 to UF6 • The UO3 is reduced with hydrogen in a kiln: UO3(s) + H2(g) UO2(s) + H2O(g) then UO2(s) + 4HF(g) UF4(s) + 4H2O(g) • The tetrafluoride is then fed into a fluidized bedreactor and reacted with gaseous fluorineto obtain the hexafluoride: UF4(s) + F2(g) UF6(g)
Production of uranium metal • Uranium metal is produced by reducing the uranium tetrafluoride with either calcium or magnesium, both active group IIA metals that are excellent reducing agents. UF4(s) + 2Ca(s) U(s) + 2CaF2(s)
Enriched uranium grades • Highly Enriched Uranium ( HEU ): > 20% 235U 20% weapon-usable, 85% weapon-grade • Low Enriched Uranium ( LEU ): < 20% 235U 12% - 19.75% used in research reactors 3% - 5% used in Light Water Reactors • Slightly enriched Uranium ( SEU ): 0.9% - 2% 235U used in Heavy Water Reactors instead of natural uranium • Recovered Uranium ( R U ): recovered from spent fuel of Light Water Reactors
Enrichment Methods • Thermal Diffusion • Gaseous Diffusion • The Gas Centrifuge • Aerodynamic Process • Electromagnetic Isotope Separation( EMIS ) • Laser Processes • Chemical Methods • Plasma Separation
Laser Processes • Atomic Vapor Laser Isotope Separation (AVLIS) • Molecular Laser Isotope Separation (MLIS)
separation factor of asingle diffusion process step isdetermined as follows:
Separation factor & Separative power of centrifuge • M1, M2 ; Molecular weight of the molecules to be separated • R ; gas constant • D ; diffusion constant of the process gas • ρ ; density of the process gas • T ; temperature in degrees Kelvin • d ; diameter of the rotor • L ; length of the rotor • V ; circumferential velocity of the rotor
Design of enrichment plants • Several centrifuges are therefore operated in parallel in the separation stages of a centrifugecascade. • Centrifuge plants, are built of several operating units, which themselves consist of several cascades working in parallel. • Diffusion plants consist of a single large cascade with approximately 1,400 stages.
Fuel fabrication • Enriched UF6 is converted into uranium UO2 powder which is then processed into pellet form: UF6+H2 (g) UF4(S)+2HF(g) UF4(S)+H2O UO2(S)+2HF(g)
Fuel fabrication • The pellets are then fired in a high temperaturesintering furnace(with H2) to create hard, ceramic pellets of enriched uranium. • Fuel rods : corrosion resistant metal alloy ( zirconium ).